Introduction

• Electronics is defined as the science of the motion of charges in a gas, vacuum, or semiconductor.

• Today, electronics generally involves transistors and transistor circuits

• Microelectronics refers to integrated circuit (IC) technology, which can produce a circuit with multimillions of components on a single piece of semiconductor material.

History of Semiconductors

1821: Thomas Seebeck discovered semiconductor properties of PbS1833: Michael Faraday reported on conductivity temperature dependence of semiconductors1875: Werner von Siemens invented a selenium photometer1878: Alexander Graham Bell used this device for wireless optical communications1907 Round demonstrated the first LED (using SiC)1940 Russell Ohl discovered a p-n junction diode

History of Semiconductors

• Russell Ohl – Inventor of a p-n junction (1940)

• In 1939, vacuum tubes were state of the art in radio equipment. Most scientists agreed tubes were the future for radio and telephones everywhere.

• Russell Ohl didn't agree. He kept right on studying crystals, occasionally having to fight Bell Labs administration to let him do it.

1947: Bardeen, Brattain, and Shockley discovered a Bipolar Junction transistor

History of Semiconductors

First Transistor, 1947

The Nobel Prize in Physics 1956

First Integrated Circuit is Invented by Jack Kilby : 1958

Intel’s 1.7 Billion Transistor Chip 2004

Brief History

• In December 1947, the first transistor was demonstrated at Bell Telephone Laboratories by William Shockley, John Bardeen and Walter Brattain.

• From then until 1959, the transistor was available only as a discrete device, so the fabrication of circuits required that the transistor terminals be soldered directly to the terminals of other components.

• In September 1958, Jack Kilby of Texas Instruments demonstrated the first integrated circuit fabricated in germanium. At about the same time, Robert Noyce of Fairchild Semiconductor introduced the integrated circuit in silicon.

• 1954, Chapin, Fuller, and Pearson developed a solar cell.

• 1958, John Kilby, invented the Integrated Circuit (IC).

• 1958, Leo Esaki discovered a tunnel diode (Esaki diode).

• 1960, Kahng and Atalla demonstrated the first MOSFET.

• 1962, three groups headed by Hall, Nathan, and Quist demonstrated a semiconductor laser.

• 1963, Gunn discovered microwave oscillations in GaAs and InP (Ridley-Watkins-Hilsum-Gunn effect).

• 1963, Wanlass and Sah introduced CMOS technology

Brief History

• The development of the IC continued at a rapid rate through the 1960s, using primarily bipolar transistor technology.

• Since then, the metal-oxide-semiconductor field-effect transistor (MOSFET) and MOS integrated circuit technology have emerged as a dominant force, especially in digital integrated circuits.

• Device size continues to shrink and the number of devices fabricated on a single chip continues to increase at a rapid rate.

• Today, an IC can contain arithmetic, logic, and memory functions on a single semiconductor chip.

• The primary example of this type of integrated circuit is the microprocessor.

Brief History

Passive and Active Devices

• In a passive electrical device, the time average power delivered to the device over an infinite time period is always greater than or equal to zero.

• Resistors, capacitors, and inductors, are examples of passive devices.

• Active devices, such as dc power supplies, batteries, and ac signal generators, are capable of supplying particular types of power.

• Transistors are also considered to be active devices in that they are capable of supplying more signal power to a load than they receive.

Electronic Circuits

Schematic of an electronic circuit with two input signals: the dc power supply input, and the signal input

An atom is composed of :• Nucleus (which contains positively charged protons and

neutral neutrons)• Electrons (which are negatively charged and that orbit the

nucleus)

Atomic Structure

Valence Electrons

• Electrons are distributed in various shells at different distances from nucleus

• Electron energy increases as shell radius increases.• Electrons in the outermost shell are called valence

electrons• Elements in the period table are grouped according to the

number of valence electrons• The valence electrons are shared between atoms, forming

what are called covalent bonds

At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.

Valence Electrons

a portion of the periodic table

Semiconductor Materials

Elemental CompoundSemiconductors Semiconductors1. Si Silicon 1. GaAs Gallium arsenide2. Ge Germanium 2. GaP Gallium phosphide 3. AlP Aluminum phosphide 4. AlAs Aluminum arsenide 5. InP Indium phosphide

Elemental/Compound Semiconductors

• Silicon (Si) and Germanium (Ge) are in group IV, and are elemental semiconductors

• Galium arsenide (GaAs) is a group III-V compound semiconductors

Silicon Crystal

• At 0°K, each electron is in its lowest possible energy state, and each covalent bounding position is filled.

• If a small electric field is applied, the electrons will not move silicon is an insulator

Silicon Atom Diagram at 0°K

Two-dimensional representation of single crystal silicon At T = 0 K; all valence electrons are bound to the silicon atoms by covalent bonding

Silicon Atom Diagram at Ambiant Temp

The breaking of covalent bond for T>0°K creating an electron in the conduction band and a positively charged “empty state”

Intrinsic Silicon

• If the temperature increases, the valence electrons will gain some thermal energy, and breaks free from the covalent bond

• It leaves a positively charged hole• In order to break from the covalent bond, a valence electron

must gain a minimun energy Eg: Bandgap energy

Energy band

• The energy Eν is the maximum energy of the valence energy band. • The energy Ec is the minimum

energy of the conduction energy band. • The bandgap energy Eg is the

difference between Ec and Eν • The region between these two

energies is called the forbidden bandgap. • Electrons cannot exist within

the forbidden bandgap.

a) Vertical scale is electron energy & horizontal scale is distance through the semiconductor, although these scales are normally not explicitly shown.

Energy band diagram

b)The generation process of creating an electron in the conduction band and the positively charged “empty state” in the valence band

a) Vertical scale is electron energy & horizontal scale is distance through the semiconductor, although these scales are normally not explicitly shown.

Energy band diagram

b)The generation process of creating an electron in the conduction band and the positively charged “empty state” in the valence band

The energy gap decreases with the increase in temperature and is given by EG( T) = EG0-Twhere = a constant, (depends on material nature)

b = 3.60 X 10-4 for silicon ; b = 2.23 X 10-4 for germanium ;

For germanium EG0 = 0.785eV; at 0°KFor silicon EG0 = 1.21eV; at 0°KFor germanium, EG(T) = 0.785 - 2.23 X 10-4T; & At room temperature (300°K), EG = 0.72 ev.For silicon, EG(T) = 1.21 - 3.60 X 10-4T; & At room temperature (300°K), EG= 1.1 ev.

Energy gap

Energy-band structure of (a) an insulator, (b) a semiconductor, and (c) a metal.

Insulators/Conductors

• Materials that have large bandgap energies (in the range of 3 to 6 electron-volts (eV)) are insulators, because at room temperature, essentially no free electron exists in the material

• Materials that contain very large number of free electrons at room temperature are conductors

• In a metal, the conduction band is partially filled. These electron can move easily in the material and conduct heat and electricity (Conductors).

Semiconductors

• Most electronic devices are fabricated by using semiconductor materials along with conductors and insulators.

• Silicon is by far the most common semiconductor material used for semiconductor devices and integrated circuits.

• Other semiconductor materials are used for specialized applications.

• In a semi-conductor at 0 k the conduction band is empty and valance band is full. The band-gap is small enough that at room temperature some electrons move to the conduction band and material conduct electricity.

• In a semiconductor, the bandgap energy is in the order of 1 eV. The net flow of free electrons causes a current.

• In a semiconductor, two types of charged particles contribute to the current: the negatively charged electrons and the positively charged holes

Semiconductors

An electron–volt is the energy of an electron that has been accelerated through a potential difference of 1 volt, and 1 eV = 1.6 × 10−19 joules.

In semiconductors, two types of charged particles contribute to the current:

• the negatively charged free electron

• the positively charged hole.

• Two charge carrying particles (free electrons and holes) are formed along with a new electron-hole pair.

• µn = mobility of the free electron (-ve charge carrying particle)

• µp = mobility of the hole (+ve charge carrying particle)

Semiconductor

• CarriersA free electron is negative charge and a hole is positive charge. Both of them can move in the crystal structure. They can conduct electric circuit.

• RecombinationSome free electrons filling the holes results in the disappearance of free electrons and holes.

• Thermal equilibriumAt a certain temperature, the recombination rate is equal to the ionization rate. So the concentration of the carriers is able to be calculated.

• In a pure semiconductor: no. of holes = no. of electrons i.e. n=p

• The concentration of electrons and holes directly influence the magnitde of the current

• In an intrinsic semiconductor (a single crystal semiconductor) the densities of holes and electrons are equal.

Carrier concentration for n typeThermal equilibrium equation

Electric neutral equation

200 inn npn

Dnn Npn 00

Carrier concentration for p typeThermal equilibrium equation

Electric neutral equation

200 ipp nnp

App Nnp 00

Because the majority is much great than the minority, we can get the approximate equations shown below:

for n type for p type

D

in

Dno

Nnp

Nn2

0

A

ip

Ap

Nnn

Np2

0

0

• Carrier concentration in thermal equilibrium

• At room temperature(T=300K)

carriers/cm3

inpn

kTEi

GeBTn 32

10105.1 in

B: constant related to specific semiconductor materialEg: Bandgap energy (eV)T: Temperature (°K)K: Boltzman Constant in eV/°K

• ni has a strong function of temperature.

• The high the temperature is, the dramatically great the carrier concentration is.• At room temperature only one of every billion atoms is

ionized.• Silicon’s conductivity is between that of conductors and

insulators. • Actually the characteristic of intrinsic silicon approaches

to insulators.

Semiconductor Constants

• Electron density in the conduction band.

• NC= 2.86 X 1019cm-3 for silicon and 4.7 X 1017cm-3 for gallium arsenide.

• NV= 2.66 X 1019cm-3 for silicon and 7 X 1018cm-3 for gallium arsenide

Where n is in cm-3and N(E) is density of states in (cm3-eV)

N(E) = γ(E – Ec)1/2

The probability that an electron occupies and electronic state with energy E is given by Fermi-Dirac distribution.Fermi function f(E) is :

f(E) = 1/(1+e(E-Ef)/kT)

Fermi function

Fermi distribution function F(E) versus (E–EF) for various temperatures.

Mass Action Law

n = p : number of electrons in CB = number of holes in VB

This is due to the fact that when an electron makes a transition to the Conduction Band, it leaves a hole behind in Valance Band, having a bipolar (two carrier) conduction and the number of holes and electrons are equal.

n.p = ni2

This equation is called as mass-action law.

Intrinsic semiconductor. (a) Schematic band diagram. (b) Density of states. (c) Fermi distribution function. (d) Carrier concentration.

n-Type semiconductor. (a) Schematic band diagram. (b) Density of states. (c) Fermi distribution function (d) Carrier concentration. Note

that np= ni2.

• Doped semiconductors are materials in which carriers of one kind predominate.

• Only two types of doped semiconductors are available.• Conductivity of doped semiconductor is much greater

than the one of intrinsic semiconductor.• The pn junction is formed by doped semiconductor.

Extrinsic Semiconductor / Doping

Extrinsic Semiconductor / Doping

• The electron or hole concentration can be greatly increased by adding controlled amounts of certain impurities

• For silicon, it is desirable to use impurities from the group III and V.

• The phosphorus (group V) atom is called donor impurity because it donates an electron that is free to move

• The boron (group III) has accepted a valence electron (or donated a hole), it is therefore called acceptor impurity

• An N-type semiconductor can be created by adding phosphorus or arsenic

N-Type Semiconductor

Each dopant atom donates a free electron and is thus called a donor.

A silicon crystal doped by a pentavalent element.

The doped semiconductor becomes n type.

A two-dimensional representation of the silicon crystal showing the movement of the positively charged “empty state”

• In the figure, it appear as if a positive charge is moving through the semiconductor

• This positively charged imaginary “particle” is called a hole.

P-Type Semiconductor

A silicon crystal doped with a trivalent impurity.

Each dopant atom gives rise to a hole, and the semiconductor becomes p type.

Total current density J with in the intrinsic semiconductor is given by J = Jn + Jp

= qnµnE + qpµpE

= (nµn + pµp)qE

= σE

σ is the conductivity of a semiconductor

The resistivity (ρ) of a semiconductor is the reciprocal of conductivity, i.e., ρ = 1/σ.

Conductivity of Semiconductor

• For pure (intrinsic) semiconductor, n = p = ni (intrinsic carrier concentration).

• Conductivity of an intrinsic semiconductor is

σi = niq(µn + µp).

Conductivity of N-and P-type semiconductors:

• For N-type semiconductors, as n>>p, then the conductivity, σ = q n µn

• For P-type semiconductors, as p>>n, then the conductivity, σ = q p µp

Conductivity of Semiconductor

There are two mechanisms by which holes and free electrons move through a silicon crystal.

Drift--- The carrier motion is generated by the electrical field across a piece of silicon. This motion will produce drift current.

Diffusion--- The carrier motion is generated by the different concentration of carrier in a piece of silicon. The diffused motion, usually carriers diffuse from high concentration to low concentration, will give rise to diffusion current.

Drift & Diffusion

Drift velocity and Current

• DriftDrift velocities

Drift current densities

Ev

Ev

ndrift

pdrift

n

p

Where µn , µp are the constants called mobility of holes and electrons respectively.

EqpJ

EqnEqnJ

pdriftp

nndriftn

)()(

wherepn m

qpm

qn

,

• Total drift current density

• Resistivity

EpnqJ pndrift ) ＋（

)(1

pn pnq ＋

• Resistivities for doped semiconductor

* Resistivities are inversely proportional to the concentration of doped impurities.

• Temperature coefficient(TC)TC for resistivity of doped semiconductor is positive due to negative TC of mobility

pA

nD

pnqN

qNpnq

1

1

)(1 For n type

For p type

• Resistivity for intrinsic semiconductor

* Resistivity is inversely proportional to the carrier concentration of intrinsic semiconductor.

• Temperature coefficient(TC)TC for resistivity of intrinsic semiconductor is negative due to positive TC of .

)(1

)(1

pnipn qnpnq

• Diffusion

A bar of intrinsic silicon (a) in which the hole concentration profile shown in (b) has been created along the x-axis by some unspecified mechanism.

Cont…

Diffusion currents only flow when there is a concentration difference for either the electrons or holes (or both).

driftdiffT

pndiffp

diffn

diff

ppdiffp

nndiffn

III

pDnDqAIII

dx

dpqADpqADI

dx

dnqADnqADI

Einstein Relationship

Einstein relationship exists between the carrier diffusivity and mobility:

Where VT is Thermal voltage.

At room temperature，

q

kTV

DDT

p

p

n

n

mvVT 25

Diffusion length (L)

• The average distance that on excess charge carrier can diffuse during its life time is called the diffusion length L

• Where D is the diffusion coefficient that may be related to the drift mobility, µ, through the Einstein relation as

DL

)/( qkTD

Diffusion length

• Lp is the average distance a hole will move before recombining.

• Ln is the average distance an electron will move before recombining.

ppp

nnn

DL

DL

Carrier life time

• The carrier life time is defined as the time for which, on average, a charge carrier will exist before recombination with a carrier of opposite charge.• It depends on the temperature and impurity concentration

in the semiconductor material.

p

n

pp

dt

dp

nn

dt

dn

0

0

Carrier life time

• The mean life times , of electrons and hole concentrations indicate the time required for the excessive electron and hole concentrations to return to their equilibrium values.

n p

Continuity Equation

Relating to the conservation of charge.

Rate of hole build up = increase of hole concentration in the volume - the recombination rate

dx

dp

dx

pdD

pp

dt

dppp

p

2

20

The equation of conservation of charge, or the continuity equation,

• Considering holes in the n-type material, the subscript n is added to P and p0.

• Also, since p is a function of both t and x, partial derivatives should be used. Making these changes, finally equation is,

x

p

x

pD

pp

t

p np

np

p

nonn

2

2

Where Ɛ is the electric field intensity within the volume.