The document discusses the P-N junction, which is formed at the interface between P-type and N-type semiconductors. It describes how doping the semiconductors with different impurities results in an excess or deficiency of electrons or holes. When the P and N-type materials are joined, charge carriers diffuse across the junction, leaving an electric field. This P-N junction exhibits rectifying behavior and has applications in diodes and transistors. The document also examines the I-V characteristics of a P-N junction diode under forward, reverse, and zero bias conditions.

1. P-N JUNCTION
PRESENTED BY- Asif Rahaman
ROLL NO: 34900721068
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING
2ND YEAR
COOCHBEHAR GOVERNMENT ENGINEERING COLLEGE

2. PN Junction
 PN junction definition: Using different doping
processes, the P-type semiconductor and the N-
type semiconductor are fabricated on the same
semiconductor (usually silicon or germanium)
substrate through diffusion, and a space charge
region is formed at their interface called a PN
junction. The PN junction has unidirectional
conductivity, which is a characteristic utilized by
many devices in electronic technology, such as
semiconductor diodes and bipolar transistors.

3. Principle of PN Junction
 N-type semiconductor: After doping pentavalent impurities (substances
with 5 electrons in the outermost layer of the nucleus, such as
phosphorus, arsenic, antimony, etc.) into the pure semiconductor, there
will be a large number of negatively charged electrons in the
semiconductor (because the outer layer of the semiconductor nucleus
generally has only 4 electrons, so it can be understood that when a
pentavalent element is doped, the number of electrons in the
semiconductor is more), and this kind of semiconductor with more
electrons is called an N-type semiconductor.
 P-type semiconductor: After doping trivalent impurities (such as boron,
aluminum, and gallium) into a pure semiconductor, there are fewer
electrons in the semiconductor, and a large number of holes (which can
be regarded as positive charges) will be generated, and this kind of
semiconductor with more holes is called a P-type semiconductor.

4. Principle of PN Junction

5. Formation of P-N Junction
 When the P-type semiconductor and the N-type semiconductor are joined
together, since the hole concentration in the P-type semiconductor is
high, and the electron concentration in the N-type semiconductor is high,
a diffusion movement will be formed, and the holes in the P-type
semiconductor will move towards its lower concentration. The electrons
of the N-type semiconductor will also diffuse to the place where its
concentration is low, thus diffusing to the P-type region. In this way, the
negative ions that cannot move freely remain in the P-type region, and
the positive ions that cannot move freely remain in the N-type region,
one positive and one negative, forming an internal electric field from left
to right inside the PN junction. This internal electric field basically
reflects the working characteristics of the PN junction. Another point to
note is that the PN junction is only partially charged, that is, the P-type
region is negatively charged, and the N-type region is positively charged,
but they are neutralized and the whole is neutral.

6. Formation of P-N Junction

7. Applications of P-N Junction Diode
 P-N junction diode can be used as a photodiode as the
diode is sensitive to the light when the configuration of the
diode is reverse-biased.
 It can be used as a solar cell.
 When the diode is forward-biased, it can be used in LED
lighting applications.
 It is used as rectifier in many electric circuits and as a
voltage-controlled oscillator in varactors

8. V-I Characteristics of P-N Junction Diode
 Volt-ampere (V-I) characteristics of
a pn junction or semiconductor
diode is the curve between voltage
across the junction and the current
through the circuit. Normally the
voltage is taken along the x-axis
and current along y-axis.The circuit
connection for determining the V-I
characteristics of a pn junction is
shown in the figure below.
 The characteristics can be
explained under three cases , such
as :
Zero bias
Forward bias
Reverse bias

9. V-I Characteristics of P-N Junction
Diode(cont.)
 Case-1 : Zero Bias
In zero bias condition , no
external voltage is applied
to the pn junction i.e the
circuit is open at K.Hence,
the potential barrier at the
junction does not permit
current flow.
Therefore, the circuit current
is zero at V=0 V.

10. V-I Characteristics of P-N Junction
Diode(cont.)
 Case-2 : 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 as shown in figure above.
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.

11. V-I Characteristics of P-N Junction
Diode(cont.)

12. V-I Characteristics of P-N Junction
Diode(cont.)
 Case-3 : 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
externalvoltage.
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 .

13. V-I Characteristics of P-N Junction
Diode(cont.)
 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.

14. V-I Characteristics of P-N Junction
Diode(cont.)

15. Drift current
Drift current in a p-n junction diode
In a p-n junction diode, electrons and holes are the minority charge carriers in the p-region and the
n-region, respectively. In an unbiased junction, due to the diffusion of charge carriers, the diffusion
current, which flows from the p to n region, is exactly balanced by the equal and opposite drift
current.In a biased p-n junction, the drift current is independent of the biasing, as the number of
minority carriers is independent of the biasing voltages. But as minority charge carriers can be
thermally generated, drift current is temperature dependent.
When an electric field is applied across the semiconductor material, the charge carriers attain a
certain drift velocity . This combined effect of movement of the charge carriers constitutes a current
known as "drift current". Drift current density due to the charge carriers such as free electrons and
holes is the current passing through a square centimeter area perpendicular to the direction of flow.
(i) Drift current density Jn, due to free electrons is given by:
(ii) Drift current density Jp, due to holes is given by:
Where: n - Number of free electrons per cubic centimeter.
p - Number of holes per cubic centimeter
– Mobility of electrons in
– Mobility of holes in
E – Applied Electric Field Intensity in V /cm
q – Charge of an electron = 1.6 × 10−19 coulomb.[1

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