Class 12 th Physics
Priyanka Jakhar
Physics Lecturer
GGIC Vijay Nagar
Ghaziabad U.P.
Semiconductor Part 3

Alloy Method :--The most common method of making p
n junction is called Alloying. An alloyed junction is made
from an n-type of semiconductor made of germanium or
silicon, by melting of the pellet of trivalent indium placed on
either side. The whole arrangement or system is heated to
about 500 degree Celsius. The indium is absorbed into the
germanium or silicon to produce a p region and hence a p n
junction is formed
Diffusion Method :-- Another method to form a p
n Junction is known as Diffusion. In this process, the
semiconductor wafers of one conductivity either p-
type or n-type are placed in a vessel that contains an
oxide of the impurity to be added. The combination is
passed slowly through a furnace with a controlled
temperature of 800⁰C to 1200⁰C depending upon the
type of junction. At such a high temperature, a gas of
impurity atoms diffuses into the semiconductor
material and forms a thin layer of opposite
conductivity. Thus, resulting in the formation of the p
n junction.
p n junction formation

Vapour deposited junction :--
Suppose we have to grow a layer of n-Si on p-Si. The p-Si wafer is kept in an atmosphere of
silane plus Phosphorus vapours . On cracking of silane at higher temperatures , a fresh layer of
n- Si grows on p-SI giving the PN junction.
p-n Junction Diode:--
When a small quantity of trivalent impurity is
added in one region of a single pure
semiconductor crystal and a small quantity of
pentavalent impurity is added, to other region of
semiconductor crystal these regions become p-
type and n-type semiconductors respectively. or
Consider a thin p-type silicon semiconductor wafer.
Convert a part of the p-type semiconductor into n-type
silicon semiconductor by adding a small quantity of
pentavalent impurity
A p–n junction is a junction formed by joining p-type and
n-type semiconductors together in very close contact.
The boundary between p-type and n-type

If metallic connecting wires are attached from the outer side of p-region and n-
region, a two terminal semiconducting device is obtained. This device is known
as p- n junction diode or semiconducting diode.
when an n-type semiconductor is joined with the p-type semiconductor, a p-n
junction is formed. The region where the p-type and n-type semiconductors are
joined is called p-n junction. It is also defined as the boundary between p-type and
n-type semiconductor. This p-n junction forms a most popular semiconductor
device known as diode.
The holes are the majority carriers in the p-type semiconductor and electrons are the
majority carriers in the n-type semiconductor
A p-n junction is the basic building block of many semiconductor devices like diodes, transistor
etc.
The credit of discovery of the p-n junction goes to American physicist Russel Ohi
of Bell Laboratories.
Diffusion :--The process by which, charge carriers (electrons or holes) in a
semiconductor moves from a region of higher concentration to a region of lower
concentration is called diffusion.
Diffusion process occurs in a semiconductor that is non-uniformly doped.

Diffusion current :--Current produced due to motion of charge carriers from a region of higher
concentration to a region of lower concentration is called diffusion current.
The region in which more number of electrons is present is called higher concentration region and
the region in which less number of electrons is present is called lower concentration region.
Both drift and diffusion current occurs in semiconductor devices.
Diffusion current occurs without an external voltage or electric field applied.
Diffusion current does not occur in a conductor.
The direction of diffusion current is same or opposite to that of the drift current.
Depletion layer:-- After the diffusion of majority charge
carriers across the junction, a very thin layer is formed on both
sides of the junction in which there is no mobile charge carrier
(hole or electron). This region is known as depletion layer or
depletion region or space charge region .
(1) In a normal semiconductor diode, the width (W) of depletion
layer is of the order of 𝟏𝟎−𝟔m i.e., 1 micron.
(2) In the depletion layer n-region has positively charged donor
ions and p-region has negatively charged acceptor ions.These
ions are immobile.
(3) One surface of depletion layer is positively charged and other
surface is negatively charged. As a result of it electric field 𝑬𝒊 is
developed inside the depletion region whose direction is from

The flow of charge carriers, which is due to the applied voltage or electric field called drift current.
In a semiconductor, there are two types of charge carriers, they are electrons and holes. When the
voltage is applied to a semiconductor, the free electrons move towards the positive terminal of a
battery and holes move towards the negative terminal of a battery.
In a semiconductor, the electrons always try to move in a straight line towards the positive
terminal of the battery. But, due to continuous collision with the atoms they change the direction of
flow. Each time the electron strikes an atom it bounces back in a random direction. The applied
voltage does not stop the collision and random motion of electrons, but it causes the electrons to
drift towards the positive terminal.
Potential barrier :--The potential barrier in the PN-junction diode is the barrier
in which the charge requires additional force for crossing the region. In other
words, the barrier in which the charge carrier stopped by the obstructive force is
known as the potential
In equilibrium, the potential difference developed across the ends of a depletion
layer is known as barrier potential or potential barrier. It is represented by 𝑽 𝑩.
Barrier potential works as a barrier for motion (diffusion) of majority charge carriers.
Magnitude of 𝑽 𝑩 depends on the concentration of impurity mixed in the semiconductor and
temperature of the junction. At 25ºC temperature for p-n junction in silicon 𝑽 𝑩= 0 . 7 volt, for
p-n junction in germanium 𝑽 𝑩 = 0.3 volt. barrier.
Drift current :--

Barrier potential works as a barrier for
motion (diffusion) of majority charge
carriers. Magnitude of 𝑽 𝑩 depends on the
concentration of impurity mixed in the
semiconductor and temperature of the
junction.
At 25ºC temperature for p-n junction in
silicon 𝑽 𝑩 = 0 . 7 volt,
for p-n junction in germanium 𝑽 𝑩 = 0.3
volt.

Drift Current
Diffusion Current
The movement of charge carriers is because of the
applied electric field is known as drift current.
The diffusion current can be occurred because of the
diffusion in charge carriers.
It requires electrical energy for the process of drift
current.
Some amount of external energy is enough for the
process of diffusion current.
This current obeys Ohm’s Law. This current obeys Fick’s Law.
The direction of charge carriers in the semiconductor is
reverse to each other.
For charge carriers, the densities of diffusion are reverse
in symbol to each other.
The direction of the drift current, as well as the electric
field, will be the same.
The direction of this current can be decided by the
concentration of the carrier slope.
It depends on the permittivity It is independent of permittivity .
The direction of this current mainly depends on the
polarity of the applied electric field.
The direction of this current mainly depends on the
charge within the concentrations of carrier.
Difference between Drift Current and Diffusion Currents

Biasing conditions for the p-n Junction Diode
There are three biasing conditions for p-n junction diode and this is based on the voltage
applied
•Zero bias:-- There is no external voltage applied to the p-n junction diode. The p-n
Junction in which no external voltage is applied is called zero bias p-n junction. Zero
bias p-n Junction is also called as unbiased p-n junction.
•Forward bias:-- The positive terminal of the voltage potential is connected to the p-type
while the negative terminal is connected to the n-type.
•Reverse bias:-- The negative terminal of the voltage potential is connected to the p-type and
the positive is connected to the n-type.

Forward Biasing :
A junction diode is said to be forward-biased when the positive
terminal of the external battery is connected to the p-region and
the negative terminal to the n-region of the diode In this situation,
an external electric field E directed from p-region towards n-region
is set up in the diode.
The field E is much stronger than the opposing internal field 𝑬𝒊 .
Hence (positive) holes in the p-region, and electrons in the n-region
both move towards the junction (holes moving in the direction of E
and electrons opposite to E). These holes and electrons mutually
combine just near the junction and cease to exist.
For each electron-hole combination, a covalent bond breaks up in
the p-region near the positive terminal of the battery. the hole and
electron produced, the hole moves towards the junction, while the
electron enters the positive terminal of the battery through the
connecting wire.
Just at this moment, an electron is released from the negative
terminal of the battery and enters the n-region to replace the
electron lost by combining with a hole at the junction. Thus the
motion of majority-carriers (holes in p region and e in n
region)constitute a current across the junction.

This is called 'forward current (In addition to this large current, there is a small reverse current
due to the motion of minority-carriers, but it is almost negligible). The current in the external
circuit is carried by electrons only .
In forward-biased junction, the applied electric field E dominates the small barrier field Ei .As
a result, the majority-carriers (holes in p-region and electrons in n-region) are pulled towards
the Hence, the width of the depletion region decreases. It is due to this reason that the
junction diode offers a low resistance for the current to flow in forward bias.

Forward Bias :-- In forward biased condition , p-type of the pn
junction is connected to the positive terminal and n-type is
connected to the negative terminal of the external voltage.
This results in reduced potential barrier.
At some forward voltage i.e 0.7 V for Si and 0.3 V for Ge, the
potential barrier is almost eliminated and the current starts
flowing in the circuit.
Form this instant, the current increases with the increase in
forward voltage. Hence. a curve OB is obtained with forward
bias.
From the forward characteristics, it can be noted that at first
i.e. region OA , the current increases very slowly and the curve
is non-linear. It is because in this region the external voltage
applied to the pn junction is used in overcoming the potential
barrier.
However, once the external voltage exceeds the potential
barrier voltage, the potential barrier is eliminated and the pn
junction behaves as an ordinary conductor. Hence , the curve
AB rises very sharply with the increase in external voltage
and the curve is almost linear.

Knee voltage or threshold voltage :--The forward voltage beyond which the current
through the junction starts increasing rapidly with voltage is called knee voltage or threshold
voltage .If line AB is extended back it cuts the voltage access at potential barrier voltage .

Reverse biasing :-- A junction diode is said to be
reverse Biased when the positive terminal of the
external battery is connected to the n region and the
negative terminal of the battery is connected to the p
region of the diode. In this situation, the external field E
is directed from n region towards P region and thus aids
the internal battery barrier field 𝑬𝒊 .
Hence hole in the p region, and electrons in the n region
are both post away from the junction that is they
cannot combine at the junction. Thus, there is almost
no current flow due to flow of majority charge Carriers .
When the junction is Reverse biased, a very small
reverse current flow across the junction. This current is
carried by the few thermally generated minority
carriers electron in the P region and holes in the n
region which moves across the junction under the
applied electric field E .

The density of the minority charge Carriers depends upon the magnitude of thermal agitation,
the reverse current is very much temperature dependent and increases with increasing
temperature of the junction.
In reverse Biased junction, the apply field E supports the barrier field 𝑬𝒊 . as a result ,the
majority carriers holes in p region and electrons in n region are pushed away from the junction
. Hence ,the width of the depletion region increases. It is due to this reason that the junction
diode offers a high resistance for the current to flow in Reverse bias.

Reverse Bias :--In reverse bias condition , the p-type of the pn
junction is connected to the negative terminal and n-type is
connected to the positive terminal of the external voltage.
This results in increased potential barrier at the junction.
Hence, the junction resistance becomes very high and as a result
practically no current flows through the circuit.
However, a very small current of the order of μA , flows through
the circuit in practice. This is knows as reverse saturation
current(IS) and it is due to the minority carriers in the junction.
As we already know, there are few free electrons in p-type material
and few holes in n-type material. These free electrons in p-type
and holes in n-type are called minority carriers .
The reverse bias applied to the pn junction acts as forward bias to
there minority carriers and hence, small current flows in the
reverse direction.
If the applied reverse voltage is increased continuously, the
kinetic energy of the minority carriers may become high enough
to knock out electrons from the semiconductor atom.
At this stage breakdown of the junction may occur. This is
characterized by a sudden increase of reverse current and a
sudden fall of the resistance of barrier region. This may destroy
the junction permanently.

Breakdown voltage :--The maximum reverse bias voltage that can be applied to a p-n
diode is limited by breakdown. Breakdown is characterized by the rapid increase of the current
under reverse bias. The corresponding applied voltage is referred to as the breakdown voltage.
The breakdown voltage is a key parameter of power devices.