superconductor and semiconductors
Post on 23-Feb-2018
239 Views
Preview:
TRANSCRIPT
-
7/24/2019 Superconductor and Semiconductors
1/45
Lecture notes on
ElectricalEngineering
Materials
Semiconductor Devices
Magnetic Properties of Materials
Optical Properties of Materials
Dielectric Properties
Introduction to IC Fabrication
2014
Keshav Raj Sigdel
2014-09-17Kathmandu University
-
7/24/2019 Superconductor and Semiconductors
2/45
Electrical Engineering Materials
EEEG 207
P a g e1
Semiconductor Devices
Bipolar Junction Transistor
A transistor is a semiconductor device used to amplify the signal. When a third doped element is
added to the crystal diode it becomes a transistor. Transistors are of two types (i) n-p-n transistor
and (ii) p-n-p transistor.
A transistor transfers a signal from low resistance to a high resistance. The prefix trans means
the signal transfer property of the device while istor classifies it as a solid state element (i.e.
resistor).
So transfer of resistor is a transistor.
Transfer + resistor =transistor
There are three regions and two junctions in a transistor.
(i) Emitter: The left side of transistor which supplies the charge carrier (i.e. electrons or
holes) is called emitter. It is highly doped and moderate in size. The emitter is always
forward bias with respect to base. In first p-n-p transistor hole to base and the second
n-p-n transistor it transfer electron to base.
(ii)Base: The middle section of the transistor which is lightly doped and very thin in size.
The base emitter junction is forward biased, allowing low resistance for emitter
circuit. The base collector junction is reversed biased and provides high resistance in
the collector circuit.
(iii)Collector: The section in the right side which collects the charge is called collector. It is
moderately doped and larger in size. The collector is always reverse biased. In p-n-p
it receives hole that flow in the output circuit. In n-p-n transistor it receives electron.
P Pn
ForwardReverse
Fig (i) PNP
n np
ForwardReverse
Fig (ii) NPN
E C
B
E C
B
-
7/24/2019 Superconductor and Semiconductors
3/45
Electrical Engineering Materials
EEEG 207
P a g e2
The Bipolar Junction Transistor (BJT) operates by the injection and collection of minority
carriers. Since the action of both electrons and holes is important in this device, it is called a
bipolar transistor.
The two operations of a transistor are amplification and switching.
Current amplification factor
The ratio of change in output current to the change in input current of a transistor is called
current amplification factor.
Common Base Common Emitter Common Collector
C
E
I
I
C
B
I
I
E
B
I
I
Also, E C BI I I and C B CEOI I I
Where 1CEO CBOI I is the leakage current (Collector emitter current with open
base).
Note: p-n-p (pointed in) & n-p-n (not pointed in)
Load line
Consider a common emitter n-p-n transistor circuit where no signal is applied only dc source are
present. The output characteristics are shown by figure.
From thisCC CE C C
CE CC C C
V V I R
V V I R
As VCCandRCare fixed values therefore it is first degree equation and can be represented by a
straight line on output characteristics. This is known as dc load line.
No signal
IB
IC
IE
+ -
RCVCE
+-
VBB VCC
IB
IC
IE
+ -
RCVCE
+-
VBB VCC
Fig (i)
Cot off region
Saturation region
VCE
B
A
VCC0
Load line
Q
Active regionCC
C
V
R
IC
IB=5A
IB=10A
IB=15A
IB=20A
IB=0A
Fig (ii)
-
7/24/2019 Superconductor and Semiconductors
4/45
Electrical Engineering Materials
EEEG 207
P a g e3
When IC= 0 Maximum VCE= VCC
When VCE= 0 MaximumIC= VCC/RC. By joining these two points load line is obtained.
Cutoff region: The region below IB=0 is called cutoff region. In this region both junctions are
reverse bias.
Saturation region: The region left to the curve is saturation region. In this region both junctions
are forward bias.
Active region: The region where the parallel curves are drawn is called active region. In this
region emitter base junction is forward bias and emitter collector junction is reverse bias. The
transistor will function normally in this region.
The zero signal value of ICand VCEare known as the operating point. The point where the load
line and characteristics curve intersect and satisfy the zero signal value of IC and VCE .(i.e. Q-
point.) It is also called operating point because the variation ofICand VCEtake place about thispoint when signal is applied. It is also called quiescent (silent) point or Q-point.
Minority carrier distribution and terminal current
Assumptions
(i) Holes diffuse from emitter to collector; drift is negligible in the base region.
(ii)Emitter injection efficiency is 1.
(iii)Collector saturation is negligible.
(iv) Active part of base and two junctions have same cross section.
(v) All currents and voltages are steady state.
p n pIE IC
IB
+VEB- -VCB+
Ep
Cp Wb
xnFig: Simplified p-n-p transistor geometry
-
7/24/2019 Superconductor and Semiconductors
5/45
Electrical Engineering Materials
EEEG 207
P a g e4
Since injected holes are assumed to flow emitter to collector by diffusion. Neglecting
recombination in the two depletion regions, the hole current entering the base at emitter junction
isIEand hole current leaving the base at collector is IC. The base width between two depletion
regions is wband uniform cross section area isA.
The excess hole concentration at the edge of the emitter depletion region is
1EBqV
kTE np p e
(1)
And the excess hole concentration at the collector side of the base is
1CBqV
kTC np p e
(2)
Since the emitter base junction is forward bias EBkT
V q and collector junction is strongly
reversed bias 0CBV . Then
EBqVkT
E np p e (3)
C np p (4)
Also from continuity equation
2
2 2
n n
n p
d p x p xdx L
(5)
The solution of this equation is
1 2n n
p p
x xL L
np x C e C e
(6)
Where Lp is the diffusion length of holes in the base region. To solve this equation we use
boundary condition to found constants C1and C2.
1 20n Ep x p C C (7)
1 2b b
p p
W WL L
n b CP x W p C e C e
(8)
Solving equations (7) and (8) we get
-
7/24/2019 Superconductor and Semiconductors
6/45
Electrical Engineering Materials
EEEG 207
P a g e5
1
b
p
b b
p p
WL
C E
W WL L
p p eC
e e
(9)
& 2
b
p
b b
p p
WL
E C
W WL L
p e pC
e e
(10)
By putting these values of C1 and C2 in equation (6) and 0Cp for strongly reversed bias
junction
b n b n
p p p p
b b
p p
W x W xL L L L
n E W WL L
e e e ep x p
e e
i.e. excess hole distribution
1 2n n
p p
x xL L
n E Ep x M p e M p e
Where 1 1&
b b
p p
b b b b
p p p p
W WL L
W W W W L L L L
e eM M
e e e e
Here, the excess electron concentration in the p+emitter is shown to decay exponentially to zero
corresponding to a long diode. This is due to the minority carrier electron diffusion length is
often shorter than the thin emitter region at high emitter doping level. Othrewise, the narrow
diode expressions must be used in the emitter region.
Fig: Distribution of injected carrier in active region
np x
nx
1 EM p
2 EM p
1
n
px
L
EM p e
2
n
p
xL
EM p e
p Straight line approximation
bW
-
7/24/2019 Superconductor and Semiconductors
7/45
Electrical Engineering Materials
EEEG 207
P a g e6
Evaluation of terminal current
We have solution of excess hole distribution in the base region
1 2n n
p p
x xL L
np x C e C e
(1)
The emitter and collector current are obtained from the gradient of hole concentration at each
depletion region edge.
We have n
p n p
n
d p xI x qAD
dx
(2)
At 0nx hole component of emitter current is
2 10p
EP p np
D
I I x qA C CL (3)
Similarly collector reverse saturation current IC is made up of entirely of holes entering the
collector depletion region from the base is
2 1b b
p p
W WL Lp
C p n b
p
DI I x W qA C e C e
L
(4)
Substituting the value of C1and C2we get
2b b
p p
b b
p p
W WL L
E C
p
EP W WL Lp
p e e pD
I qAL
e e
ctnh csch & csch ctnhp pb b b b
EP E C C E C
p p p p p p
D DW W W WI qA p p I qA p p
L L L L L L
If E EPI I for amplification factor 1 , then the base current is
ctn h csch
tanh2
p b bB E C E C
p p p
p bE C
p p
D W WI I I qA p p
L L L
D WqA p p
L L
-
7/24/2019 Superconductor and Semiconductors
8/45
Electrical Engineering Materials
EEEG 207
P a g e7
Field Effect Transistor
A field effect transistor is a three terminal semiconductor device in which the output current is
controlled by the applied electric field. The current in a FET is carried only by one type of
majority charge carriers electrons or holes. It is also called unipolar transistor. The current in the
device from source to drain can be controlled by the application of an electric potential (fortransverse electric field) introduced by gate, the device is known as field effect transistor (FET).
The field effect transistors are of two types:
(i) Junction field effect transistor (JFET)
(ii) Metal oxide semiconductor field effect transistor (MOSFET)
Construction
D
S
G
Fig: Symbol of n-channelD
S
G
Fig: Symbol of p-channel
p p
n
c
Chanel
Depletion
Drain (D)
Gate (G)
Source (S)
Fig: n-channel
p p
n
n
Drain (D)
Gate G
Source (S)
Fig: n-channel JFET
n
n
p
p
Drain (D)
Gate G
Source (S)
Fig: p-channel JFET
-
7/24/2019 Superconductor and Semiconductors
9/45
Electrical Engineering Materials
EEEG 207
P a g e8
A JFET consists of a p-type or n-type silicon bar containing two p-n junctions as shown in
figure. The bar forms the conducting channel for the charge carriers. If the bar is of n-type, it is
called n-channel JFET and if the bar is of p-type it is called p-channel JFET. The two p-n
junctions forming diodes are connected internally and a common terminal called gate. Other
terminals are source and drain taken out from the bar. Thus the FET consists of essentially three
terminals.
(i) Source (S): It is the terminal through which majority carriers inter the bar. Since carriers
come from it is called source.
(ii)Drain (D): It is the terminal through which majority carriers (electrons) leave the bar. The
drain to source voltage VDSdrives the drain current ID.
(iii)Gate (G): There are two internally connected heavily doped regions which form two p-n
junctions. The gate source voltage VGSreverse biases the gate.
(iv)
Channel (C): It is the space between two gates through which majority carriers pass from
source to drain when VDSis applied.
Working principle of JFET
It consists of n-type silicon bar the upper contact is known as drain (D) and lower contact as the
source (S). A current is established in the bar by an applied voltage 0-30 volts with negative
terminal to the source electrode. The electrons which are majority carriers in n-type bar leave the
bar through the drain electrode. The conventional current IDenters the bar at D.
p p
n
n
D
S
VGG
ID
VDD
VDS
VGS
-
7/24/2019 Superconductor and Semiconductors
10/45
Electrical Engineering Materials
EEEG 207
P a g e9
Two small regions of polarity opposite to that of crystal are created near the center at opposite
side of a bar. These two leads are called gate. The region between the gates is called channel.
If p-n junction is reversed biased, width of depletion zone (region) is increased and greater the
reverse bias voltage. Thus by applying the reverse bias the depletion layer becomes wider and
cross sectional area of channel is decreased. This will increase the resistivity and decrease theflow of current. If the varying reverse biased voltage is applied to FET. The current flowing in
source drain circuit is varied in inverse proportion. The biasing voltage is considered as electric
field. Hence strength of resistivity can be determined by electric field. From this concept we use
the name junction field effect transistor.
Characteristics of JFET
The external batteries VDD and VGG are connected in drain and gate respectively such that the
gate source junction is reversed biased. The characteristics may be obtained by investigating how
the current is flowing between the drain and source. The drain current varies with the drain tosource voltage for fix value of VGS.
Let VGS be initially fixed to zero at the initial position 0, VDS is zero. The thickness of the
depletion region around p-n junction is uniform. If VDSis positive the depletion region are thin at
low VDSvalues, the drain current increases with the voltage as shown by the curve OA. In this
case the depletion layer is thicker towards the drain side as the voltage V DSis further increased,
the p-n junction at the gate becomes more reversed biased. The current is now force to flow in a
channel which increases lightly.
At this stage, the increasing current due to channel is narrowing and the curve practically
horizontal AB. The drain voltage at which the cross sectional area of the channel becomes
minimum is called the pinch-off voltage. The FET is said to be in pinch-off region AB. When
VDDis further increased current IDincreases rapidly due to avalanche multiplication of electron
caused by breaking of covalent bond of Si atom in the depletion region between gate and drain.
The voltage at which break down occurs is denoted by VDGO.
VGG
VDD
RVDS
VGS
ID
0
A
B
C
VP VDS=VDGO VDS
Pinch off
(Saturation region Break down regionOhmic region
-
7/24/2019 Superconductor and Semiconductors
11/45
Electrical Engineering Materials
EEEG 207
P a g e10
Metal oxide semiconductor field effect transistor (MOSFET)
Metal oxide semiconductor field effect transistor is an important semiconductor device and is
widely used in many circuit applications. The input impedance of a MOSFET is much more than
that of JFET because of very small gate leakage current. The MOSFET can be used in any of the
circuits covered for the JFET.
Construction
It is similar to JFET except with the following modifications
(i) There is only a single p-region. This region is called substrate.
(ii)A thin layer of silicon oxide is deposited over the left side of the channel. A metallic gate
is deposited over oxide layer. As silicon dioxide is an insulator, therefore gate is
insulated from the channel. From this reason MOSFET is sometimes called insulated
gate FET.
(iii)Like JFET, a MOSFET has three terminals viz. source, gate and drain.
D
S
G
Fig: Symbol
Substratep
n
n
Drain (D)
Gate (G)
Source (S)
Fig: n-channel MOSFET
Oxide layer
Substrate
-
7/24/2019 Superconductor and Semiconductors
12/45
Electrical Engineering Materials
EEEG 207
P a g e11
Working principle of MOSFET
Instead of gate diode as in JFET, here gate is formed as a small capacitor. One plate of this
capacitor is the gate and the other plate is the channel with metal oxide as the dielectric. When
negative voltage is applied to the gate, electrons accumulate on it. Those electrons repeal the
conduction band electrons in the n-channel. Therefore, lesser number of conduction electrons is
made available for current conduction through the channel. The greater is the negative voltage on
the gate, the lesser the current conduction from source to drain. If the gate is given positive
voltage, more electrons are made available in the n-channel. Consequently, the current from
source to drain is increases.
Phototransistor
The phototransistor is a more sensitive semiconductor device than the p-n photo diode. The
phototransistor is usually connected a common emitter configuration with open base and
radiation is concentrated on the region near the collector junction JC as shown in figure. The
operation of this device can be understood if we recognize that the junction J Eis slightly forward
biased and the junction JC is reversed biased (i.e. the transistor is biased in the active region).
Assume first, that there is excitation of radiation. Under these circumstances minority carriers are
generated thermally and the electrons crossing from the base to the collector as well as the holes
crossing from the collector to the base, constitute the reverse saturation current ICO.
The collector current is given by
1 0C CO BI I I
p
n
n
D
S
ID
VDS
G
VGG
+
-
+
-
-
7/24/2019 Superconductor and Semiconductors
13/45
Electrical Engineering Materials
EEEG 207
P a g e12
If the light is now turned on, additional minority carriers are photo generated, and these
contribute to the reverse saturation current in exactly the same manner as do the thermally
generated minority charges. If the component of the reverse saturation current due to the light is
designated IL, the total collector current is
1C CO LI I I
We note that, due to transistor action the current caused by the radiation is multiplied by the
large factor 1 .
Typical volt-ampere characteristics are shown in graph for an n-p-n planar phototransistor for
different values of illumination intensities. Note the similarity between this family of curves and
these for the CE transistor output characteristics with base current as a parameter. The current I C
is then increased by the term BI .
n
n
p
JC
JE
+
-VCE
C
E
IC
Radiation
Fig: Phototransistor
VCE0
IC (mA)
Fig: Output characteristics of the
MRD 450 n-p-n transistor
-
7/24/2019 Superconductor and Semiconductors
14/45
Electrical Engineering Materials
EEEG 207
P a g e13
Magnetic Properties of Materials
Introduction
Magnetism was observed as early at 800B.C. in a naturally occurring material called load stone.
In the modern concept, all materials, viz., metals, semiconductors and insulators are said toexhibit magnetism though of different nature. Materials in which a state of magnetism can be
induced are called magnetic materials. The magnetic properties of solids originate in the motion
of electrons and in the permanent magnetic moments of the atoms and electrons. The ability of
certain metals like iron, cobalt and nickel and some of their alloys and compounds to acquire
large permanent magnetic moments is of prime importance.
Intensity of Magnetization (I)
( )
( )
2
2
Magnetic moment MI
Volume V
m l m
a l a
Where m pole strength
2l effective length of magnet
a cross section area
Hence the intensity of magnetization is also defined as the magnetic pole strength per unit area of
cross section.
Magnetic Susceptibility m
m
Intensity of Magnetisation I
Maetic field H
It is pure number, sinceIandHhave the same units. Its value for vacuum is zero, because there
is no magnetization in vacuum.
We can classify the materials in terms of m .
If m is negative, the material is diamagnetic and the magnetic induction is weakened by the
presence of material.
If m is small positive, the material is paramagnetic and the magnetic induction is strengthened
by the presence of material.
-
7/24/2019 Superconductor and Semiconductors
15/45
Electrical Engineering Materials
EEEG 207
P a g e14
Ifm
is large positive, the material is ferromagnetic. However in ferromagnetic materials I is not
accurately proportional to H and so m is not constant.
Magnetic Permeability
Let us consider a relation 0B H I
From the definition of susceptibility
mI H
0
0 1
m
m
B H H
H
If we write 0 1 m then we have B H
The constant is called the magnetic permeability of the material. It may be defined as the ratio
of magnetic inductionBto the magnetizing fieldH.
For vacuum 0m and 0 . Hence magnetic induction in vacuum is 0 0B H
The ratio00
B
B
is called relative permeability ( r ). Obviously; 1r m .
We may also clarify magnetic materials in terms of relative permeability r .
1
1
1
r
r
r
Diamagnetic
Paramagnetic
Ferromagnetic
Diamagnetic Materials
Those substances which when placed in magnetizing field are magnetized feebly in the opposite
direction of applied field. This property is found in the substance whose outermost orbit has aneven number of electrons. Since the electrons have spins opposite to each other, the net magnetic
moment of each atom is zero. If these materials are brought close to the pole of a powerful
electromagnet they are repelled away from a magnet.
Diamagnetic materials have small negative susceptibility; relative permeability is less than unity.
For example: Bismuth, Antimony, gold, water, alcohol, quartz, hydrogen etc.
-
7/24/2019 Superconductor and Semiconductors
16/45
Electrical Engineering Materials
EEEG 207
P a g e15
Paramagnetic Materials
Those substances which when placed in magnetizing field are magnetized feebly in the direction
of magnetizing field are called paramagnetic substances. This property is found in the substance
whose outermost orbit has an odd number of electrons. If these substances are brought close to
the pole of a powerful electromagnet they get attracted towards the magnet.
Paramagnetic materials have small positive susceptibility; relative permeability is little greater
than unity.
For example: Platinum, aluminum, chromium, manganese, copper sulphate, liquid oxygen,
solutions of salt of irons and nickel.
Ferromagnetic Materials
Those substances which when placed in magnetizing field are strongly magnetized in the
directions of magnetizing field are called ferromagnetic substances. This property is found in thesubstances which are generally like paramagnetic materials. If these substances are brought close
to the pole of a powerful electromagnet they are strongly attracted towards the magnet.
Ferromagnetic materials have large positive susceptibility; relative permeability is much greater
than unity (few thousands).
For example: Iron, Nickel, Cobalt, gadolinium, and their alloys.
Anti-ferromagnetic Materials
Anti-ferromagnetic materials are crystalline materials. In these materials, the dipole moments ofneighboring dipoles are equal and opposite in orientation so that the net magnetization vanishes.
If they are placed in the magnetic field, they are feebly magnetized in the direction of field. Such
materials are called anti-ferromagnetic materials.
Susceptibility of those materials varies with temperature. It increases in temperature and reaches
a maximum at a particular temperature called Neel temperature (TN). Above this temperature
these materials behave like paramagnetic materials.
For example: MnO, FeO, CaO, NiO, MnO4, MnS etc.
Ferri Magnetic Materials
If the spin of the atoms are such that there is a net magnetic moment in one direction, the
materials are called ferri magnetic materials.
For example: ferrites i.e. Fe2O3
-
7/24/2019 Superconductor and Semiconductors
17/45
Electrical Engineering Materials
EEEG 207
P a g e16
Classification of magnetic materials based on atomic dipoles
Those atoms which have permanent magnetic dipole moment is absent is called diamagnetic.
These atoms which have permanent magnetic dipole exists even in the absence of any external
field may be paramagnetic, ferromagnetic, anti-ferromagnetic and ferrimagnetic depending on
the interaction between the individual dipole.
Thus if the interaction between the atomic permanent dipole moments is zero or negligible then
the material will be paramagnetic.
Fig: Schematic illustration of paramagnetic
arrangement of spin.
If a dipole interacts in such a manner that they tend to line up in parallel, the material will be
ferromagnetic.
Fig: Schematic illustration of ferromagneticarrangement of spin.
If the neighboring dipoles tends to line up so that they are anti-parallel, the material is anti-
ferromagnetic or ferrimagnetic depending on magnitudes of dipoles on the two sub-lattices as
indicated schematically for a one dimensional mode.
Fig: Schematic illustration of anti-ferromagnetic
arrangement of spin.
Fig: Schematic illustration of ferrimagnetic
arrangement of spin.
Classification Permanent dipoles Interactions between neighboring dipoles
Diamagnetic No -----
Paramagnetic Yes negligible
Ferromagnetic Yes Parallel orientation
Anti-ferromagnetic Yes Anti-parallel orientation of equal moment
Ferrimagnetic Yes Anti-parallel orientation of unequal moment
-
7/24/2019 Superconductor and Semiconductors
18/45
Electrical Engineering Materials
EEEG 207
P a g e17
Atomic Magnetic Moment
An electron revolving in an orbit about the nucleus of an atom is a minute current loop and
produces a magnetic field. Thus it behaves like a magnetic dipole.
Let us consider an electron of mass m and charge e moving with speed v in a circular Bohr orbitof radius r as shown in figure.
It constitute a current of magnitude
eI
T
where Tis the orbital period of electron.
Now2 2 r
T
v
And so2
evI
r
From electromagnetic theory, the magnitude of orbital magnetic
dipole moment l
for a currentI in a loop of areaAis
lIA
and its direction is perpendicular to the plane of the orbit as shown. Substituting the value of Ifrom above and taking 2A r . We get
2
2 2l
ev evr r
r
(1)
Because the electron has negative charge, its magnetic dipole moment l
is opposite in direction
to its orbital angular momentum L
whose magnitude is given by
L mvr (2)
Dividing equation (1) by (2) we get
2
l e
L m
(3)
L
v
e
r
e
-
7/24/2019 Superconductor and Semiconductors
19/45
Electrical Engineering Materials
EEEG 207
P a g e18
Thus the ratio of the magnitudel
of the orbital magnetic dipole moment to the magnitude Lof
the orbital angular momentum for the electron is constant, independent of the details of the orbit.
This constant is called the gyromagnetic ratio for the electron.
We can write equation (3) in vector form
2l
eL
m
The minus sign means that l
is in opposite direction ofL
. The unit of electromagnetic moment
is ampere-m2or joule/Tesla.
Bohr Magneton
From wave mechanics, the permitted scalar values of L
are given by
12
hL l l
where lis the orbital quantum number. Therefore the magnitude of the orbital magnetic moment
of the electron is
14
l
ehl l
m
The quantity4
eh
mforms a natural unit for the measurement of atomic magnetic dipole moments,
and is called the Bohr magneton, denoted byB
.
Thus, 1l Bl l
where
19 34
31
24 2
1.6 10 6.6 10
4 4 3.14 9.1 10
9.28 10
B
C Jseh
m Kg
Amper m
-
7/24/2019 Superconductor and Semiconductors
20/45
Electrical Engineering Materials
EEEG 207
P a g e19
Electron spin and magnetic moment
An electron not only revolves on a circular orbit around the positive nucleus but also rotates
around an axis of its own. The magnetic moment associated with spinning of electron is called
spin magnetic moment s .
If we consider the simple case of an electronic charge being spread over a spherical volume, then
the electron spin would cause different charge elements of this sphere to form closed currents.
This will result a net spin magnetic moment. This net magnetic moment, obviously, would
depend upon the detailed structure of the electron and its charge distribution. It can be shown
that s is connected with spin angular momentum Sas
2s e Sm
Where the coefficient is known as the spin gyromagnetic ratio and depends on the structure of
the spinning particles and its charge distribution. The experimental value of for an electron is
-2.0024. Here the negative sign indicates that s is in a direction opposite to that of S.
Since2
hS
for an electron,
19 34
31
24 2
2 4
1.6 10 6.6 10
2.0024 8 3.14 9.1 10
9.4 10
s
hem
C Js
Kg
Amper m
Thus the magnetic moments due to the spin and the orbital motions of an electron are of the same
order of magnitude. It should be noted that spin and s are intrinsic properties of an electron and
exist even for a stationary electron (L=0).
Since the magnitude of the spin magnetic moment is always same, the external field can only
change its direction. If the electron spin moments are free to orient themselves, they will orient
themselves in the direction of the applied field B. Thus, paramagnetic is the result of spinmagnetic moments.
-
7/24/2019 Superconductor and Semiconductors
21/45
Electrical Engineering Materials
EEEG 207
P a g e20
Magnetic moment due to nuclear spin
In addition to electronic contribution, nuclear spin also contributes to magnetic moment of
atoms. By analogy with Bohr magneton, the nuclear magneton arises due to spin of the nucleus.
It is given by
4n
p
eh
m
where pm represents the mass of the proton. By putting the known values, we get
27 25.05 10n Amper m
Obviously, the nuclear magnetic moments are smaller than those associated with electron.
Thus the permanent dipoles originate: (i) the orbital motion of the electron, (ii) electron spin, and
(iii) the nuclear spin.
Curie-Weiss law
The temperature dependence of many paramagnetic materials is governed by the experimentally
found Curie law, which states that the susceptibility m is inversely proportional to the absolute
temperature T.
1m m CT T
Where2
3
B
B
nC
k
is called Curie constant.
For many other substance, a more general relationship is observed, which is known as Curie-
Weiss law.
m
C
T
where is another constant that has the same unit as the temperature and may have positive as
well as negative values.
-
7/24/2019 Superconductor and Semiconductors
22/45
Electrical Engineering Materials
EEEG 207
P a g e21
Fig: Schematic representation of (a) the Curie law, (b) and (c) the Curie-Weiss law and (d) the
diamagnetic behavior is also shown for comparison.
Anti-ferromagnetic materials are paramagnet above Neel temperature TN. i.e. they obey there a
linear1
m
T f
law. Below TNhowever the inverse susceptibility may rise with decreasing
temperature. The extrapolation of paramagnetic line to1
0
yields a negative . Thus the
Curie-Weiss law needs to be modified for anti-ferromagnetic as
m
C C
T T
The Neel temperature is often below room temperature. Most anti-ferromagnetic are found
among ionic compounds. They are insulators or semiconductors. No practical application for
anti-ferromagnetic is known at this time.
Fig: Schematic representation of the temperature dependence of a poly crystalline anti-
ferromagnetic material law.
1
m
T
(d)Dia
(b)Ferro
(a)Para
(C)Anti-ferro
1
m
TTN
Paramagnetic
Anti-ferro
0
Curie-Weiss
-
7/24/2019 Superconductor and Semiconductors
23/45
Electrical Engineering Materials
EEEG 207
P a g e22
Hysteresis loss in magnetic materials
When a magnetic material is subjected to a gradually increasing magnetizing field, the intensity
of magnetization Iincrease with the increase in strength of magnetizing field Halong the pathOA. This curve is known as virgin or initial magnetization curve.
At H=H0 the intensity of magnetization assumes a steady value Imax. The magnetic material
cannot be magnetized more strongly than this and at this stage the material is said to have
reached the magnetic saturation limit.
Now if the magnetizing fieldHis gradually decreased the intensity of magnetization Iwill not
decrease the same path OA, but will decrease along the path AB such that whenHbecomes zero
Iwill not become zero but has a definite valueI=OB.
The value of intensity of magnetization of the magnetic material even when the magnetizingfield is reduced to zero is called its retentivity or remanence or residual magnetism.
Now if the direction of magnetizing field is reversed the intensity of magnetization takes along
the path BC till it become zero at C. Thus to reduce the residual magnetism to zero, a
magnetizing field is equal to the value OC has to be applied in reverse direction. The value of
reverse magnetizing field require to reduce the residual magnetism to zero is called the coercive
force or coercivity.
When the magnetizing field is further increased in reverse direction, the intensity of
magnetization increases along the path CD and acquires the magnetic saturation limit at point D.If the magnetizing fieldHis now reduced to zero, the intensity of magnetizationIfollow the path
DE. Finally if His increased in the original direction I follow the path EFA and a closed curve
ABCDEFA is obtained. This closed curve is known as hysteresis loop. On repeating the process
the same closed curve is obtained again and again but never the portion OA. It is seen that I
always lags behindH. This lagging ofIbehindHis called hysteresis.
-
7/24/2019 Superconductor and Semiconductors
24/45
Electrical Engineering Materials
EEEG 207
P a g e23
The shape of this loop varies from one material to another. Some ferrites have an almost
rectangular hysteresis loop. These ferrites are used in digital computers as magnetic information
storage device. The area of the loop represents energy loss (hysteresis loss) per unit volume
during one cycle of the periodic magnetization of the ferromagnetic materials. This energy loss is
in the form of heat. Therefore it is desirable that materials used in electronic generators, motors
and transformers should have a tall but narrow hysteresis loop for maximum losses.
Permanent magnets (hard magnetic materials) are device which retain their magnetic field
indefinitely i.e. coercivity and area of hysteresis loop are large.
Eddy current loss in magnetic materials
Fig: Solid transformer core with eddy current Iein a cross sectional area A.
The core loss is the energy which is dissipated in the form of heat with in the core of
electromagnetic devices when the core is subjected to an alternating magnetic field. Several
types of losses such as hysteresis and eddy current loss also happen.
Consider a transformer whose primary and secondary coils are wounded around the lags of a
rectangular iron yoke as shown in figure. An alternating electric current in the primary coil
causes an alternating magnetic flux in the core. This in turn induces the secondary coil an
alternating emf e proportional tod
dt
i.e.d dB
e e Adt dt
Concurrently, an alternating emf is induced within the core itself as shown in figure. This emf
gives rise to the eddy current Ie. The eddy current is larger, then larger the permeability , the
larger the conductivity of the iron core material, the higher the applied frequency and the
larger the cross-sectional areaAof the core.
eI
A
N
-
7/24/2019 Superconductor and Semiconductors
25/45
Electrical Engineering Materials
EEEG 207
P a g e24
In order to decrease the eddy current, first core can be made of an insulator in order to
decrease . Ferrites are thus effective but also expensive materials to build magnetic cores.
There are indeed used for high frequency applications. Secondly, the core can be manufactured
from iron powder where by each particle is covered by an insulating coating. However the
decrease in , in this case is at the expense in large decrease in . Thirdly, the most widely
applied method to reduce eddy currents is the utilization of cores made out of thin sheets which
are electrically insulated from each other. This way the cross-sectional area Ais reduced which
in turn reduce emf. These losses are however less than 1% of the total energy transferred.
Application Of magnetic materials
Electrical devices like power transformers, motors, generators, electromagnets etc use soft
magnetic materials. Electrical steels are use as core materials in them. For retaining magnetic
fields of permanent magnets, hard magnetic materials are used in fabrications.
Magnetic materials find significant use in the storage information. Credit cards are properly usedwhich also have magnetic strips. To store large quantities of information of low cost, computers
are usually backed up with magnetic disks. The recording head consisting of laminated
electromagnet is made soft ferric having 0.3m wide air gap. Here the data written by the
electrical signal generates a magnetic field across the gap with in the coil. Finally the stored
information is read using the same head, and an alternating emf is induced in the coil of the head
by moving tap or disk in the read or play back mode. This emf is amplified, filtered and fed to a
suitable output device (loudspeaker).
Fig: Schematic arrangement of recording (playback) head and magnetic tape. The gap width isexaggerated (Recording mode). The plastic substrate is about 25m thick.
Magnetic material is also used for making medical devices such as thin motors in implantablepumps and valves.
N
Magneticlayer
Substrate
Tapemotion
-
7/24/2019 Superconductor and Semiconductors
26/45
Electrical Engineering Materials
EEEG 207
P a g e25
****Notes****
Eddy Current- An induced electric current formed with in the body of a conductor in a varying
magnetic field. An eddy current is a current that is induced in the iron core (iron being a
conductor as well as having a high permeability). The current flows back and forth in the iron
core as the alternating current in the windings changes directions. Eddy current does not usefulwork. They cause the core to heat up. So the energy in these induced eddy currents is lost as heat.
But induced current is useful energy. It can be used to run a motor, computer etc. The eddy
current causes random motion of atoms in the iron core so we cant get at that energy as easily in
order to do useful work.
Optical Properties of Materials
Introduction
The interaction of light with the valance electrons of a material is responsible for optical
properties. The optical measurements that give the fullest information on the electronic systemare measurements of the reflectivity of light at normal incidence on single crystal.
Most recently, a number of optical devices such as lasers, photo detectors, waveguides, etc. have
gained considerable technological importance. They are used in communication, fiber optics,medical diagnostics, night viewing, solar applications, optical computing or for other
optoelectronic purposes.
Refractive index or n
In optics the refractive index or index of refraction n of a substance (optical medium) is a
dimensionless number that describes how light, or any other radiation, propagates through thatmedium. It is defined as
cn
v
Where cis the speed of light in vacuum and vis the speed of light in the substance. For example,the refractive index of water is 1.33, means that light travels 1.33 times as fast in vacuum as itdoes in water.
The historically first occurrence of the refractive index was in Snells law of refraction, n1sin1=n2sin2, where 1and 2are the angles of incidence of a ray crossing the interface between twomedia with refractive indices n1and n2.
Refractive index of materials varies with the wavelength. This is called dispersion; it causes thesplitting of white light in prisms and rainbows, and chromatic aberration in lenses. In opaque
-
7/24/2019 Superconductor and Semiconductors
27/45
Electrical Engineering Materials
EEEG 207
P a g e26
media, the refractive index is a complex number: while the real part describes refraction, theimaginary part accounts for absorption.
The concept of refractive index is widely used within the full electromagnetic spectrum, from x-
rays to radio waves. It can also be used with wave phenomena other than light (e.g., sound). In
this case the speed of sound is used instead of that of light and a reference medium other thanvacuum must be chosen.
Penetration depth (W)
Penetration depth is a measure of how deep light or any electromagnetic radiation can penetrate
into a material. It is defined as the depth at which the intensity of the radiation inside the materialfalls to 1/e (about 37%) of its original value at (or more properly, just beneath) the surface.
When electromagnetic radiation is incident on the surface of a material, it may be (partly)
reflected from that surface and there will be a field containing energy transmitted into the
material. This electromagnetic field interacts with the atoms and electrons inside the material.Depending on the nature of the material, the electromagnetic field might travel very far into thematerial, or may die out very quickly. For a given material, penetration depth will generally be a
function of wavelength.
The intensity of light sensitive device (such as photo detector)Iequals to the square of the field
strength .
2
0
2exp
wkI I z
c
where kis damping constant.
We define characteristics penetration depth (W) is that distance at which the intensity of lightwave travels through a material.
When 1
0
1Ie
I e
This definition yields2 4 4
c cW
wk fk k
The inverse of Wis called the exponential or attenuation or absorbance.
4 2k wk
c
It is measured in cm-1.
Reflection, Transmission, and Absorption
Reflection is the process by which electromagnetic radiation is returned either at the boundary
between two media (surface reflection) or at the interior of a medium (volume reflection),
whereas transmission is the passage of electromagnetic radiation through a medium. Both
processes can be accompanied by diffusion (also called scattering), which is the process of
deflecting a unidirectional beam into many directions. In this case, we speak about diffuse
-
7/24/2019 Superconductor and Semiconductors
28/45
Electrical Engineering Materials
EEEG 207
P a g e27
reflection and diffuse transmission. When no diffusion occurs, reflection or transmission of a
unidirectional beam results in a unidirectional beam according to the laws of geometrical optics.
In this case, we speak about regular reflection and regular transmission (or direct transmission).
Reflection, transmission and scattering leave the frequency of the radiation unchanged.
Absorption is the transformation of radiant power to another type of energy, usually heat, by
interaction with matter.
ReflectivityR
When light radiation passes from one medium into another having a different index of refraction,
some of the light is scattered at the interface between the two media even if both are transparent.
The reflectivityRrepresents the fraction of the incident light that is reflected at the interface, or
0
RI
RI
whereI0andIRare the intensities of the incident and reflected beams respectively. If the light is
normal (or perpendicular) to the interface, then
2
2 1
2 1
n nR
n n
where n1and n2are the indices of refraction of the two media. If the incident light is normal if
the incident light is not normal to the interface, Rwill depend on the angle of incidence. When
light is transmitted from a vacuum or air into a solid, then
2
2
2
1
1
nR
n
Since the index of refraction of air is very nearly unity. Thus higher the index of refraction of the
solid greater is the reflectivity. For typical silicate glasses the reflectivity is approximately 0.05.
Just as the index of refraction of a solid depends on the wavelength of incident light and hence
reflectivity vary with wavelength.
Transmissivity T
Transmissivity is the ratio between the transmitted intensity ITand impinging light intensity I0.
Then 1 2
2
0 1 2
4TI n nT I n n
Notice that 1R T as conservation of energy.
For instance, when light passes from air 1 1n into glass 2 1.5n then 0.04 and 0.96R T .
i.e. most of the light is transmitted.
-
7/24/2019 Superconductor and Semiconductors
29/45
Electrical Engineering Materials
EEEG 207
P a g e28
Dielectric Properties
Introduction
A dielectric is an insulating material in which all the electrons are tightly bound to the nuclei of
the atoms and there are no free electrons available for the conduction of current. Therefore theelectrical conductivity of a dielectric is very low. The conductivity of an ideal dielectric is zero.
On the basis of band theory, the forbidden gap Egis very large in dielectrics. Materials such as
glass, polymers, mica, oil and paper are examples of dielectrics. They prevent flow of current
through them. Therefore they can be used for insulating purpose.
Dielectric Constant
It is found experimentally that the capacitance of a capacitor is increased if the space between its
plates is filled with a dielectric material. To understand this fact, Faraday took two identical
capacitors, one who was evacuated and the other was filled with dielectric material as shown in
the figure.
Then these two capacitors were charged with a battery of some potential difference. He found
that the charge on capacitor filled with a dielectric is larger than that of filled with air. If C0be
the capacitance in vacuum and C the capacitance when the space is filled with a dielectric
material then dielectric constant of the material
0
CK
C
Thus the dielectric constant of a material is the ratio of the capacitance of a given capacitor
completely filled with that material to the capacitance of the same capacitor in vacuum. In other
words, the ratio of permittivity of medium to the vacuum is also known as dielectric constant.
0
rK
This is also known as relative permittivity. It is found to be independent of the shape and size
and dimension of the capacitor.
Vacuum Dielectric Material
-
7/24/2019 Superconductor and Semiconductors
30/45
Electrical Engineering Materials
EEEG 207
P a g e29
Non Polar Dielectric
A non polar molecule is the one in which the center of gravity of the positive charge (protons)
and negative charge (electrons) coincide. So such molecules do not have any permanent dipole
moment. For example O2, N2andH2
Polar Dielectric
A polar molecule is the one in which the centre of gravity of positive charges is separated by
finite distance from that of negative charges. Unbalanced electric charges, using valance
electrons of such molecules result in dipole moment and orientation. Therefore those molecules
possess permanent electric dipole. For exampleN2O, H2OandHCL
Polarization of Dielectrics
When an electric field is applied to a dielectric material, it exerts a force on each charged particle
and charges is displaced in its orientation while the negative charges is displaced in opposite
direction as shown in figure (a). Consequently the center of positive and negative charges of each
atom displaced from their equilibrium positions. Such a molecule (or atoms) is then called as
induced electric dipole and this process is known as dielectric polarization.
0E
Fig (a)
Fig (c)
0E
pE
iq
iq
Dielectric slab
0E
Vacuum
Fig (b)
-
7/24/2019 Superconductor and Semiconductors
31/45
Electrical Engineering Materials
EEEG 207
P a g e30
We consider a parallel plate capacitor which has vacuum initially between its plates. When it is
charged with a battery, the electric field of strengthE0is set up between the plates of capacitor as
shown in figure (b). If and are the surface charge densities of the two plates of thecapacitor, then the electric field developed between the plate is given by
0
0
E
If now a slab of dielectric material is placed between the two plates of capacitor as shown in
figure (c), then it becomes electrical polarized. Hence its molecule becomes electric dipole
orientation in the direction of field. Because of this the center of positive and negative charges
gets displaced from each other. Therefore in the interior of dielectric as marked by dotted lines
these charges cancel. However the polarization charges on the opposite faces of dielectric slab
are not cancelled. These charges produce their own electric field Ep, which opposes the external
applied field E0. Under this situation, the net electric field in the dielectric is given by
0 pE E E
Polarization Density
The induced dipole moment developed per unit volume in a dielectric slab on placing it inside an
electric field is known as polarization density. It is denoted by the symbol P
. If p
is induced
dipole moment of individual atom and N is the number of atoms in a unit volume, the
polarization density as
P Np
The induced dipole moment of an individual atom is found to be proportional to the applied
electric field E
and is given by
0p E
q
q
iq
iq
Dielectric slab
AreaA AreaA
d
-
7/24/2019 Superconductor and Semiconductors
32/45
Electrical Engineering Materials
EEEG 207
P a g e31
Where is the proportionality constant also known as atomic polarizability.
Then 0P N E
Suppose A is the area of each plate of the capacitor, d is the separation between them. Then
volume of dielectric slab isAd. Sinceqiand +qiare the induced charges developed on the twofaces of the dielectric slab, the total dipole moment of the slab will be equal to qid. From the
definition of polarization density
i ip
q d qtotaldipolemomentP
volumeofslab Ad A
On placing the dielectric material between the plates of the capacitor, the reduced value of
electric field may be evaluated as
0
0 0 0 0
p p PE E
Or0
0
pE P
E P
The quantity 0E P
is of special significance and is known as electric displacement vector D
given by 0D E P
Relation between Dielectric constant and electric susceptibility
The polarization density of a dielectric is proportional to the effective value of electric field E
and is given by
0P E
Where is constant of proportionality and is known as susceptibility of dielectric material.
We have
00 0 0
0 0
0
0
1
1
1
EPE E E E E
E E
E
E
K
-
7/24/2019 Superconductor and Semiconductors
33/45
Electrical Engineering Materials
EEEG 207
P a g e32
Types of Polarization
Electronic Polarization
Under the action of an external field, the electron clouds of atoms are displaced with respect to
heavy fixed nuclei to a distance less than the dimension of atom. This is called electronic
polarization, which does not depend on temperature. The electronic polarization, is represented
as below
e eP N E
Ionic Polarization
This type of polarization occurs in ionic crystals for example in sodium chloride crystal. In the
presence of an external electric field, the positive and negative ions are displaced in opposite
directions until ionic bonding forces stop the process. This way the dipoles get induced. The
ionic polarization does not depend upon temperature.
Orientation Polarization
No field applied
Applied electric field E
E
Absence of field Presence of field
E
Absence of field Presence of field
Absence of field Presence of field
-
7/24/2019 Superconductor and Semiconductors
34/45
Electrical Engineering Materials
EEEG 207
P a g e33
This type of polarization is applicable in polar dielectrics. In the absence of an external electric
field, the permanent dipoles are oriented randomly such that they cancel the effects of each other.
When an electric field is applied, these dipoles tend to rotate and align in the direction of applied
field. This is known as orientation polarization which depends upon temperature.
In view all the polarizations; the total polarization is sum of electronic, ionic, and orientationpolarizations. This is given by
0e iP P P P
Dielectric Losses
When a dielectric material is placed in an alternating electric field, a part of energy is wasted,
which is known as dielectric loss i.e. the absorption of electrical energy by a dielectric material is
called dielectric losses.
Let us consider an alternating electric field is applied to a dielectric material given by
0 cosE E t
where is the applied frequency.
Then there will be two cases
(i) For the electric displacement vector is in the phase with applied field
0 cosD D t
Then the electric displacement current is
0 sinD
J D tt
(1)
The energy loss in time period T. i.e. average energy loss is
2
0
0
0
2
T
T
J Edt
W J Edt
dt
2
0 0
0
sin cos2
0
D t E tdt
shows that the energy loss is zero when displacement vector is in phase with the applied field.
-
7/24/2019 Superconductor and Semiconductors
35/45
Electrical Engineering Materials
EEEG 207
P a g e34
(ii)When electric displacement vector is out of phase with applied field.
If 0 cosE E t
then 0 cosD D t
Where is the possible phase difference between E
andD
. So average energy
loss
0
0
T
T
J Edt
W
dt
But 0 sinD
J D tt
2
0 0
0
2
0 0
0
22
2
0 0
0
2
0 0
0 0
sin cos2
sin cos cos sin cos2
sin cos2
1 2sin
2 2
1sin
2
D t E tdt
D E t t tdt
D E tdt
D E
D E
(2)
If we consider electric field and electric displacement vector is complex the dielectric constant is
also complex.
0 0
0 0
0
0
cos sin
i t i
i t
D e D eD
E E e E
Di
E
So 00
cos sinD
i iE
(3)
Where and are the real and imaginary part of dielectric constant.
So from equation (3) equating the real and imaginary part we get
-
7/24/2019 Superconductor and Semiconductors
36/45
Electrical Engineering Materials
EEEG 207
P a g e35
0
0
cosD
E
and 0
0
sinD
E
So tan
(angle form)
This tan is known as the loss tangent. Since and are frequency dependent so is alsofrequency dependent.
0 0sinD E
So equation (2) becomes
0 0
20
2
0 0
1
2
12
2r
W E E
W E
W E
Thus the absorption of energy is proportional to the imaginary part of complex dielectric
constant. Whenever there is energy dissipated in the medium called dielectric losses.
Ferroelectric material
For the dielectric materials, the polarization is a linear function of applied field. The polarization
in these materials is not a unique function of the field strength. In particular these materialsexhibits hysteresis effects similar to those obtained in ferromagnetic material, they are therefore
ferroelectric material.
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response of
mechanical strain. The word piezoelectricity means electricity resulting from pressure.
The piezoelectric effect is the linear electromagnetically interaction between the mechanical and
electrical state in crystalline materials. It is found in useful applications such as the production
and detection of sound, generation of high voltages, electronic frequency generator etc.
-
7/24/2019 Superconductor and Semiconductors
37/45
Electrical Engineering Materials
EEEG 207
P a g e36
Introduction to Integrated Circuit Fabrication
Introduction
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or
a microchip) is a set of electronic circuits on one small plate ("chip") of semiconductor material,normally silicon. This can be made much smaller than a discrete circuit made from independent
components. An integrated circuit is one in which circuit components such as transistors, diodes,
resistors, capacitors etc are automatically part of semiconductor chip.
In the early 1960s, a new field of microelectronics was born primarily to meet the requirements
of the Military which wanted to reduce the size of electronic equipment to approximately one-
tenth of its then existing volume. This device for extreme reduction in the size of electronic
circuits has led to the development of microelectronic circuits called integrated circuits (ICs)
which are small that their actual construction is done by technicians using microscopes.
Integrated circuits are used in virtually all electronic equipment today and have revolutionized
the world of electrons. Computers, mobile phones, and other digital home appliances are now
inextricable parts of the structure of modern societies, made possible by the low cost of
producing integrated circuits.
ICs can be made very compact, having up to several billion transistors and other electronic
components in an area the size of a fingernail. The width of each conducting line in a circuit can
be made smaller and smaller as the technology advances; in 2008 it dropped below 100
nanometers and in future it is expected to be in the tens of nanometers and even more.
Advantages
1.
Increased reliability due to lesser number of connections.
2. Extremely small size due to the fabrication of various circuit elements in a single ship of
semiconductor materials.
3. Lesser weight
4.
Low power required.
5. Reduced cost
6. Suitability for small-signal operation
7. Easy replacement
Disadvantages
1. If any component of ICs goes out of order, the whole IC has to be replaced by a new one.
2. In an IC fabrication, it is neither convenient nor economical to fabricate capacitance
exceeding 30pF.
3. It is not possible to fabricate inductors and transformers.
4. It is not possible to produce high power ICs (greater than 10W).
-
7/24/2019 Superconductor and Semiconductors
38/45
Electrical Engineering Materials
EEEG 207
P a g e37
Types
Four basic types of constructions are employed in the manufacture of integrated circuits namely,
1. Monolithic 2. Thin-film 3. Thick-film 4. Hybrid
Since it combines both active (eg- diodes, transistors) and passive elements (eg- resistors,
capacitors) in a monolithic structure, the complete unit is called integrated circuits.
IC Terminology
1. Bonding- attachment of wires to an IC.
2. Chip- an extremely small part of silicon wafer on which IC is fabricated. One silicon
wafer of 2cm diameter may contain up to 1000IC chips.
3. Circuit probing- to check the proper electrical performance of each IC with the help of
probes.
4. Die- same as chip.
5. Diffusion- introduction of controlled small quantities of a material into the crystal
structure for modifying its electrical characteristics.
6. Diffusion mask- it is a glass plate with the circuit pattern drawn on it. Impurities can
diffuse through its light areas but not through its dark ones.
7. Encapsulation- putting a cap over the IC and sealing it in an inert atmosphere.
8. Epitaxy- physical placement of materials on a given surface.
9. Etching- removal of surface material from a chip by chemical means.
10.Metallization- providing ohmic contacts and inter connections by evaporating aluminum
over the chip.
11.Photoresist- a photo-sensitive emulsion which hardens when exposed to ultraviolet light.
12.Scribing- incising or cutting with a sharp point.
13.
Wafer- a thin slice of a semiconductor material either circular or rectangular in shape in
which a number of ICs are fabricated simultaneously.
-
7/24/2019 Superconductor and Semiconductors
39/45
Electrical Engineering Materials
EEEG 207
P a g e38
Making monolithic IC
A monolithic IC is one in which all circuit components and their interconnections are found on a
single thin wafer called the substrate.
The basic production processes for monolithic ICs are as follows
Step-1: P-substrate (Crystal growth)
This is the first step in making of an IC. A cylindrical p-type silicon crystal is grown having
typical dimensions 25cm long and 2.5cm diameter shown in figure (i).
The crystal is then cut by a diamond saw into many thin wafers like figure (ii), the typical
thickness of the wafer being 200m. One side of wafer is polished to get rid of surface
imperfections. This wafer is called the substrate. The ICs are produced in this wafer.
Step-2: Epitaxial growth of n-layer
The next step is to put the wafer in a diffusion furnace. A gas mixture of silicon atoms and
pentavalent atoms is passed over the wafers. This forms a thin layer of n-type semiconductor on
the heated surface of the substrate as shown figure. This layer is called the epitaxial layer and is
about 10m thick. It is in thin layer that the whole integrated circuit is formed.
Step-3: Oxidation
P-Substrate
n
200m
10m
Epitaxial layer
1m
P-Substrate
n
SiO2layer
200m
2.5cm
P-Substrate
(ii) P-type Silicon crystal
(i) P-type Silicon crystal
25cm
2.5cm
-
7/24/2019 Superconductor and Semiconductors
40/45
Electrical Engineering Materials
EEEG 207
P a g e39
In order to prevent the contamination of epitaxial layer, a thin SiO 2 layer about 1m thick is
deposited over the entire surface as shown in figure. This is achieved by passing pure oxygen
over the epitaxial layer. The oxygen atom combines with silicon atoms to form a layer of SiO2.
Step-4: Photolithography
Selective removal of oxide is done by the process of photolithography. We cover the entire
surface of oxidized silicon with a photosensitive material called photoresist. Now portion ofphotoresist is remove away by subjecting ultraviolet radiation.
After removal of photoresist the oxide is etched. HCl solution etches the SiO2layer.
After this we remove the remaining photoresist from the rest of the portion.
Step-5: Diffusion
To dope through the window we usually do diffusion.
After diffusion we remove rest of oxide.
Step-5: Contact Metallization
Metallization needs selective deposition of metals over the p and n type i.e. contact outside.
P-Substrate
n
Photoresist
P-Substrate
n
UV radiations
P-Substrate
n
SiO2
P-Substrate
n
P-Substrate
n+
P-Substrate
n+
-
7/24/2019 Superconductor and Semiconductors
41/45
Electrical Engineering Materials
EEEG 207
P a g e40
Fabrication of components on monolithic IC
The notable feature of an IC is that it comprises a number of circuits elements inseparably
associated in a single small package to perform a complete electronic function. This differs from
discreet assembly where separately manufactured components are joined by wires. Some of the
circuit elements (e.g. diode, transistor, resistor, capacitor etc.) can be constructed in the IC form.
Diodes
P-Substrate
n
Exposed
(i)
P-Substrate
n
(ii)
P-Substrate
n
(iii)
P-Substrate
n
(iv)
Window
P-Substrate
n
(v)
p
P-Substrate
n
(vi)
p
P-Substrate
n
(vii)
p
1 2
1 2
(viii)
-
7/24/2019 Superconductor and Semiconductors
42/45
Electrical Engineering Materials
EEEG 207
P a g e41
Figure above shows how a diode is formed on a portion of substrate of monolithic IC. Part of
SiO2layer is etched off, exposing the epitaxial layer as shown in figure (i). The wafer is then put
into a furnace and trivalent atoms are diffused into the epitaxial layer. The trivalent atoms change
the exposed epitaxial layer from n-type material under SiO2layer as shown in figure (ii).
Next pure oxygen is passed over the wafer to from a complete SiO2layer as shown in figure (iii).A hole is then etched at the center of this layer; thus exposing the n-epitaxial layer as shown in
figure (iv). This hole in SiO2layer is called window. Now we pass trivalent atoms through the
window. The trivalent atoms diffuse into the epitaxial layer to form an island of p-type material
as shown in figure (v). The SiO2layer is again formed on a wafer by blowing pure oxygen over
the wafer as shown in figure (vi). Thus a p-n junction diode is formed on a substrate.
The last step is to attach the terminals. For this purpose, we etch the SiO2 layer at the desired
locations as shown in figure (vii). By depositing metal at these locations, we make electrical
contact with the anode and cathode of the integrated diode. Figure (viii) shows the electrical
circuit of the diode.
Transistors
Figure below shows how a transistor is formed on a portion of substrate of monolithic IC. Part of
SiO2layer is etched off, exposing the epitaxial layer as shown in figure (i). The wafer is then put
into a furnace and trivalent atoms are diffused into the epitaxial layer. The trivalent atoms change
the exposed epitaxial layer from n-type material under SiO2layer as shown in figure (ii).
Next pure oxygen is passed over the wafer to from a complete SiO2layer as shown in figure (iii).
A hole is then etched at the center of this layer; thus exposing the n-epitaxial layer as shown in
figure (iv). This hole in SiO2layer is called window. Now we pass trivalent atoms through the
window. The trivalent atoms diffuse into the epitaxial layer to form an island of p-type material
as shown in figure (v). The SiO2layer is again formed on a wafer by blowing pure oxygen over
the wafer as shown in figure (vi).
A window is now formed at the center of SiO2 layer, thus exposing the p-epitaxial layer as
shown in figure (vii). Then we pass pentavalent atoms through the window. The pentavalent
atoms diffuse into the epitaxial layer to form an island of n-type material as shown in figure
(viii). The SiO2layer is reformed over the wafer by passing pure oxygen as shown in figure (ix).
The terminals are processed by etching the SiO2layer at appropriate locations and depositing the
metal as these locations as shown in figure (x). In this way, we get the integrated transistor.
Figure (xi) shows the electrical circuit of a transistor.
-
7/24/2019 Superconductor and Semiconductors
43/45
Electrical Engineering Materials
EEEG 207
P a g e42
P-Substrate
n
Exposed
(i)
P-Substrate
n
(ii)
P-Substrate
n
(iii)
P-Substrate
n
(iv)
Window
P-Substrate
n
(v)
p
P-Substrate
n
(vi)
p
P-Substrate
n
(vii)
p
Window
P-Substrate
n
(viii)
p
n
P-Substrate
n
(ix)
p
n
P-Substrate
n
(x)
p
n
E BC
C
E
B
(xi)
-
7/24/2019 Superconductor and Semiconductors
44/45
Electrical Engineering Materials
EEEG 207
P a g e43
Resistors
Figure above shows how a resistor is formed on a portion of substrate of monolithic IC. Part of
SiO2layer is etched off, exposing the epitaxial layer as shown in figure (i). The wafer is then putinto a furnace and trivalent atoms are diffused into the epitaxial layer. The trivalent atoms change
the exposed epitaxial layer from n-type material under SiO2 layer as shown in figure (ii).Next
pure oxygen is passed over the wafer to from a complete SiO2layer as shown in figure (iii).
A window is now formed at the center of SiO2 layer, thus exposing the n-epitaxial layer as
shown in figure (iv). Then we diffuse a p-type material into the n-type area as shown in figure
P-Substrate
n
Exposed
(i)
P-Substrate
n
(ii)
P-Substrate
n
(iii)
P-Substrate
n
(iv)
Window
P-Substrate
n
(v)
p
P-Substrate
n
(vi)
p
P-Substrate
n
(vii)
p
1 2
21
(viii)
-
7/24/2019 Superconductor and Semiconductors
45/45
Electrical Engineering Materials
44
(v). The SiO2layer is re-formed over the wafer by passing pure oxygen as shown in figure (vi).
The terminals are processed by etching SiO2 layer at the two points above the p island and
depositing the metal at these locations as shown in figure (vii). In this way, we get an integrated
resistor. Figure (vii) shows the electrical circuit of a resistor.
The value of resistor is determined by the material, its length and area of cross section. The highresistance resistors are long and narrow while low-resistance resistors are short and greater the
cross section.
Capacitors
Figure above shows the process of fabricating a capacitor in the monolithic IC. The first step is
to diffuse an n-type material into the substrate which forms one plate of the capacitor as shown
in figure (i). Then SiO2 layer is reformed over the wafer by passing pure oxygen as shown in
figure (ii).The SiO2layer formed acts as the dielectric of the capacitor. The oxide layer is etched
and terminal 1 is added as shown in figure (iii). Next a large (compared to the electrode atterminal 1) metallic electrode is deposited on the SiO2 layer and forms the second plate of the
capacitor. The oxide layer is etched and terminal 2 is added. This gives an integrated capacitor.
The value of capacitor formed depends upon the dielectric constant of SiO2 layer, thickness of
SiO2layer and the area of cross section of the smaller of the two electrodes.
P-Substrate
n
Exposed
(i)
P-Substrate
n
(ii)
P-Substrate
n
(iii)
P-Substrate
n
(iv)
1 2
21
(v)
top related