Electronics The Seventh and Eighth and Lectures

Eighth weekهـ - 1436/ 12 / 28

هـ 1437/ 1 / 1

السلمي / سمر أ

Chapter Two: Junction Diode Physical Electronics Forward-biased and Reverse- biased Junction

depletion region and applying bias, and Diffusion and Drift currentsDrive excess of minority carrier in Forward-biased and Reverse- biased

Junction Mathematical description of excess of minority carrier in Forward-biased

and Reverse- biased JunctionDrive diffusion current in forward-biased and reverse- biased JunctionI – V characteristic of forward-biased and reverse-biased JunctionDiode ModelsPn Junction’s mission(half and full wave rectifiers, filters, regulator)Zener diode

Ohmic Contact Metal–Oxide–Semiconductor Contact (MOS) Structure , Effect of voltageEnergy levels forms in MOS in different bias

Outline for today

Time of Periodic Exams

The first periodic exam in / 1 / 1437 - 1312هـ , Please everyone attend In her

group

The second homework

I put the second homework in my website in the university homework Due

Thursday 2 / 1/ 1437 H in my mailbox in Faculty of Physics Department , I will

not accept any homework after that , but if you could not come to university you

should sent it to me by email in the same day

Forward-biased and Reverse- biased JunctionOur discussion until now is about equilibrium condition (The absence of power supply (battery) between the parties of diode).

But what about the condition (the existence of power supply between the parties of diode?How the power supply connect between the parties of the diode ?? What happens to contact potential increased or decreased?What will happened to diffusion and drift currents in the junction?

In the condition of power supply existence, so that positive terminal of it connect with p-type side and negative terminal of it connect with n-type side; this condition called forward bias. however, If the two terminals reverse, the condition called reverse bias as in the figure blow.

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Forward-biased Junction (depletion region and applying bias)

In forward bias, we notice that the outer field (forward voltage) opposes built-in

field created by the space charges in the depletion region. Therefore, the number

of donor and acceptor ions reduces; thus depletion region width decreases. The

potential barrier is lower in this condition than in equilibrium condition; in

addition voltage difference across the diode decreases (( V0 –Vf )) as in the figure.

Also we notice that Fermi level will not be at the same energy level in the two

types as in equilibrium condition.

Forward-biased Junction (Diffusion and Drift)

This decreasing of voltage difference across the diode in forward bias affect the

diffusion current ( due to injection of holes in p-type and electrons in n-type ) so,

we obtain a huge flow of diffusion currents of the electrons and holes compared

with it in equilibrium condition ; also due to climbing of electron easily to new

voltage difference level .

However, drift current is not effected by decreasing of voltage difference or

length of potential barrier due to not effecting of minority carrier which remain

pulled inside the diffusion current at the edge of the depletion region . Therefore,

drift current in this condition remains as in equilibrium condition

Reverse-biased Junction (depletion region and applying bias)

In reverse bias, we notice that the outer field (reverse voltage ) in the same

direction of built-in field created by the space charges in the depletion region.

Therefore, the number of donor and acceptor ions rise; thus depletion region

width increases. The potential barrier is higher in this condition than in

equilibrium condition; in addition voltage difference across the diode increases

((V0 + Vr )) as in the figure. Also we notice that Fermi level will not be at the

same energy level in the two types as in equilibrium condition

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Reverse-biased Junction (Diffusion and Drift)

In reverse bias which opposite of forward bias, the increasing of voltage

difference across in the diode affect negatively to diffusion current . There shall

be a few flow of diffusion currents of the electrons and holes transported to the

other end compared with it in equilibrium condition ; also due to climbing of

electron difficulty to new voltage difference level .

Similar to forward bias , drift current is not effected by increasing of voltage

difference or length of potential barrier. Therefore, drift current in this condition

remains as in equilibrium condition

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Forward-biased and Reverse- biased Junction

Look at increases of direction of electron and hole diffusion in forward bias and decreases in reverse bias and drift as the same as in equilibrium condition.

Drive excess of minority carrier in Forward-biased and Reverse- biased Junction

In thermal equilibrium, we represent relations

However, forward and reverse bias represent relations as follows

The forward- biased voltage and reverse -biased voltage

In the case of biased, the low-level injection or carriers minority concentration is

very weak so will not affect the equilibrium majority carriers; thus we can

consider

Drive excess of minority carrier in Forward-biased and Reverse- biased Junction

We start by holes in non equilibrium condition

From relation

By substitute

We obtain

By subtracting pn0 from both sides, we get a excess of minority -carriers of holes

concentration in n-type

Drive excess of minority carrier in Forward-biased and Reverse- biased Junction

Similarly, electrons in non equilibrium condition

From relation

By substitute

We obtain

By subtracting np0 from both sides, we get a excess of minority -carriers of

electrons concentration in p-type

Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction

From previous lectures, we discussed about the equation for diffusion of minority

carriers

The solution for it is where A and B are constants

which can be found from boundary conditions (from the following figure a

semiconductor p-type variable concentration in one dimension)

(In the case of bias {non- equilibrium} we will deal of low-level injection)

Therefore, we assume that

And

Therefore, we obtain

Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction

Similar, in case of bias in junction

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Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction to Drive diffusion current

Similar, in case of bias in junction

Thus, the equation for diffusion of minority carriers in n –type & p- type is

By substituting to excess minority -carriers concentration from previous derivation

In focusing of borders depletion region, we obtain

Drive diffusion current in Forward-biased and Reverse- biased Junction

We notice that diffusion current variables in equilibrium condition (or the

absence of bias) while drift current does not change its status in equilibrium

condition .

To get the current density in the junction : first from p-type side (minority

electrons current density)

To focus in one dimension and to solving equations of diffusion of the minority

carriers

At borders of depletion region

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Drive diffusion current in Forward-biased and Reverse- biased Junction

from n-type side diffusion current density (minority holes current density)

To focus in one dimension and to solving equations of diffusion of the minority

carriers

At borders of depletion region

To get the total current density of junction must collect two current densities for

n-type and p-type, however, we should notice the direction of them

Drive diffusion current in Forward-biased and Reverse- biased Junction

From the figure

To calculate the current

Where the reverse saturation current is

Drive diffusion current in Forward-biased and Reverse- biased Junction

Figure illustrates the distribution of the electron and hole currents in the junction

in the forward bias

I – V Characteristic of Forward-biased and Reverse-biased Junction

You studied at electronic lab the relation between voltage and current of diode in

forward-biased and reverse-biased depending on electrical conductivity to

terminals of battery and diode as in the figure

We conduct first in forward bias then reverse bias.

In addition , a resistor is used to limit the forward current

to a value that will not overheat the diode and cause damage. This resistor called

dynamic resistance.

To calculate resistance value is inverted slope

dI/dV tangent to the curve. The value of

resistance in forward bias less than the value

in reverse bias Rf < Rb

I – V Characteristic of Forward-biased and Reverse-biased Junction

At forward bias circuit, we notice when increasing forward voltage, there is a flow in the

current (diffusion current here). After

the voltage value reaches to (barrier

potential), the forward current increase

rapidly (barrier potential for Si is 0.7v

and Ge is 0.3 v ) . From figure and

explanation, we notice that diode is not

linear which not depend on Ohm’s law

due to first quarter of graph.

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I – V Characteristic of Forward-biased and Reverse-biased Junction

At reverse bias circuit as fourth quarter of

graph , there is small value of voltage and

constant in relation between reverse voltage

and diffusion current (which called Reverse

Saturation Current). Keeping in mind, reverse

voltage has large value compared with forward

voltage to reached a particular voltage called

Avalanche Voltage. This is shown clearly when

we study a special type of diodes : Zener diode.

The current increase very rapidly after

Zener Breakdown or Breakdown voltage VBR

I – V Characteristic of Forward-biased and Reverse-biased Junction

the relation between diffusion

current and applied voltage

Forward-biased

Reverse-biased

is an exponential relation

Where I0 is reverse saturation current

I – V Characteristic of Forward-biased and Reverse-biased Junction

Temperature Effect : for forward-biased diode as temperature is increased, the

forward current increases for a given value of forward voltage. For a given value

of forward current, the forward voltage decrease,

as shown in figure. The blue curve is at room

temperature and red curve is elevated temperature

(300K + ΔT) notice that barrier potential decreases

as temperature increases. For reverse-biased diode

as temperature is increased, the reverse current

increases. However, there different between two

curves. Keep in mind that the reverse current

breakdown remains extremely small and can usually

be neglected

Diode Models

There are three models for diode

1-The Ideal Model:

This model is a simple switch. When the diode is forward-biased, it acts like a

closed (on) switch. When it is reverse –biased, it acts like a closed (off) switch.

the barrier potential, the forward dynamic resistance, and the reverse current are

all neglected in this model.

Diode Models2- The practical Model

The practical model adds the barrier potential to ideal model. When the diode is

forward-biased, it is equivalent to a closed switch in series with a small

equivalent voltage source equal to the barrier potential. When the diode is

reverse-biased , it is equivalent to an open switch just as ideal model because

barrier potential does not affect reverse bias.

Diode Models

3- The Complete Model:

This model consists of the barrier potential, the small forward dynamic

resistance, and the large internal reverse resistance.

Pn Junction’s mission

Of the most important uses and benefits of the pn junction is the rectifier

current from alternating current to direct current and you will study in detail that

in Electronics Lab. where junction connect to AC source (which current shape

wave or sine with time) to take advantage of the forward bias (passing current)

reverse bias (not passage current) are rectifier the alternating current.

There are two types of rectifier: half – wave rectifier and full wave rectifier.

In addition to rectifier process, the filter process came next. By putting

capacitor that will improve the current form of half wave or full wave into a form

close to a straight line by putting a number of capacitors

After rectifier and filter processes, regulator process came third (which we can

put Zener diode) that give straight current.

Three processes (rectifier , filter , regulator) to transformation from alternating

current to direct current

Half-Wave Rectifiers =

Half-Wave Rectifiers

To calculate average value of half-wave rectified output voltage

However, this calculation for ideal diode. So in practical diode, we take in the

account the barrier potential

AVAVG

Vp

0T

BRinPoutP VVV )()(

+

-

+

RL0

0.7 V

Vp(in)-

V

0

=V p(in)p(out) - 0.7V

P

AVGV

VPeriod

Area

Full-Wave Rectifiers

There two circle for full –wave rectifiers

1) A center- tapped full –wave rectifiers

2) A bridge full –wave rectifiers

center- tapped Full-Wave Rectifiers =

The Bridge Full-Wave Rectifiers =

Filters

By using capacitor

half or fall – wave is

filtered. By putting

number of capacitors,

the wave is filtered

more than before

Regulator

The final step to transformation from alternating current to direct current is

regulator. There are numbers of regulator devices. One of them is Zener diode

Zener diodeZener diode differs from normal pn junction in its designed to work in reverse breakdown area without the occurrence of any problems. Difference in Zener diode differential has more impurities in one of side of diode than the other as (n+p ) or ( p+n). In equilibrium condition Fermi level at the same energy level in n-type and p-type, but in non equilibrium condition opposite as we knew. However, in the case of reverse bias aligns the conduction band in the n-type and valence band in the p-type as if they are at the same energy level and thus the electron from the conduction band in the n-type moves (or tunneling) to the valence band in p-type as shown in the figure. Among the benefits of this diode is used in regulate voltage.

Previously, we discussed about pn junction (a semiconductor type of n-type next to the other type p-type)  But what about another junction as semiconductor next to insulator or metalOhmic ContactThis contact consists metal and semiconductor (which follows Ohm's Law and does not have the ability to rectifier current and (current and voltage) characteristic is linear in both forward and reverse bias )

The scientific method to create This contact1- by increasing impurities in a semiconductor at the contact area so that the charge carriers crosses barrier2- by selecting a work function of metal (eΦ) close to work function of semiconductorwork function: the required energy to remove an electron from Fermi level or required energy for ionization metal

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Ohmic Contact

The left figure illustrates Fermi level and work function in the metal and

semiconductor (n-type) before contact. The right figure illustrates them when

configure junction of metal and semiconductor (n-type) in equilibrium condition

and how Fermi level aligning between metal and semiconductor (when there are

negative charges close to surface of metal, they attract positive charges in metal )

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Ohmic Contact

The left figure illustrates Fermi level and work function in the metal and

semiconductor (p-type) before contact. The right figure illustrates them when

configure junction of metal and semiconductor (p-type) in equilibrium condition

and how Fermi level aligning between metal and semiconductor (when there are

positive charges close to surface of metal, they attract negative charges in metal )

=

Metal–Oxide–Semiconductor Contact (MOS)

structure

In this contact, a thin layer of oxide is put on the surface of a semiconductor n-

type or p-type. Then, pole metallic (metal) is put above the surface of the oxide

layer . We should choose a good electrical insulation of oxide which has a large

energy gap and isolates the metal from the semiconductor which no passing

electrical current between them.

In thermal equilibrium condition

In the absence of application of the electric

field or the voltage, the Fermi and connection

and valence levels are horizontal and flat.

 When applying an electric field, there is

a bending in energy levels

Metal–Oxide–Semiconductor Contact (MOS)

Effect of voltage bias

According to the applied voltage on this contact, it will consist three different

situations such as what is shown in figure .

1- depletion 2- inversion 3- accumulation

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Metal–Oxide–Semiconductor Contact (MOS)

Effect of voltage bias (metal and n-type contact)

1 - Depletion :

When applying negative bias voltage at the surface of metal, a small amount of

negative charges is made. Then, the oxide layer prevent electric current from

passage to semiconductor . However, the electrons in substrate of semiconductor

n-type will be affected by these negative charges and moved away from the area

located under oxide and created the depletion region in semiconductor similar to

those that created in pn Junction

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Metal–Oxide–Semiconductor Contact (MOS)

Effect of voltage bias (metal and n-type contact)

2 - Inversion

When increasing a negative bias voltage on surface of metal, Instead of

expanding more of depletion region within the semiconductor, inversion status is

formed which holes gather next to the surface of the oxide. Those holes is the

minority carriers in the semiconductor n-type.

3 - Accumulation

When applying positive bias voltage at the surface of metal, negative majority

carriers attract and accumulate at the surface of the oxide in semiconductor n-

type.

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Metal–Oxide–Semiconductor Contact (MOS)

Energy levels forms in MOS in different bias (metal and p-type contact)

1- Depletion :

When applying positive bias voltage, Fermi level move down from its first

location in thermal equilibrium condition. Also,

straight bend at the energy level in oxide

and energy levels of the semiconductor p-type

move down near the interface of oxide.

In addition, electrons drop down in potential

well. We notice that the distribution of carriers

density of per unit area in semiconductor p-type

equal in the metal

=

Metal–Oxide–Semiconductor Contact (MOS)

Energy levels forms in MOS in different bias (metal and p-type contact) 2 - Inversion When increasing positive bias voltage more than threshold voltage VT ; the semiconductor inverse and electrons occupyinversion layer. Fermi level move more down from its first location in thermal equilibrium condition. Also, straight bend at the energylevel in oxide and energy levels of semiconductor p-type move more down near interface of oxide. In addition, electrons dropmore down in potential well. We notice that thedistribution of carriers density of per unit area in semiconductor p-type for maximum depletion region Wmax in addition to carrier of inversion layer Qn equal in the metal

=

Metal–Oxide–Semiconductor Contact (MOS)

Energy levels forms in MOS in different bias (metal and p-type contact)

3- Accumulation

When applying negative bias voltage, Fermi level move up from its first location

in thermal equilibrium condition. Also,

straight bend at the energy level in oxide

and energy levels of the semiconductor p-type

move up near the interface of oxide.

In addition, holes climb up in potential

well. We notice that the distribution of carriers

density of per unit area in semiconductor p-type

equal in the metal

=