ELECTRONICS ENGINEERING ON PN JUNCTION

1. SEACOM ENGINEERING
COLLEGE
NAME OF THE EXAM : CONTINUOUS ASSESSMENT 1 (CA1)
PRESENTED BY : SUMAN SAFUI
UNIVERSITY ROLL NO :
BRANCH : MECHANICAL ENGINEERING SEM : 3rd
SUBJECT : BASIC ELECTRONICS ENGINEERING CODE : BS-BIO301

2. WORKING
PRINCIPLE
OF PN
JUNCTION
DIODE
INTRODUCTION : If a p-type and n-
type semiconductor are joined in
proper manner, a p-n junction is
formed. The junction is the border
where the p-type & n-type region
meet. A p-n junction is also called
junction diode

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. 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

8. 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
.

9. 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 curveAB rises very sharply with the increase in
external voltage and the curve is almost linear.

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

11. 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 .

12. 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.

13. V-I Characteristics of P-N Junction
Diode(cont.)
DIODE REVERSE BIAS

14. ADVANTAGES OF PN JUNCTION
➢ 1. Rectification: PN junctions allow current to flow in one direction (forward bias)
while blocking it in the opposite direction (reverse bias). This property makes
them ideal for rectifying alternating current (AC) to direct current (DC).
➢ 2.Switching: The ability to control current flow by biasing the junction makes PN
junctions highly effective as electronic switches. Applying a forward bias turns the
switch "on," allowing current to flow, while applying a reverse bias turns it "off,"
blocking current.
➢ 3.Voltage Regulation: PN junctions can be used to regulate voltage levels in
circuits. Zener diodes, for example, operate in reverse bias and break down at a
specific voltage, allowing excess voltage to be safely diverted.
➢ 4.ignal Conversion: PN junctions can be used to convert AC signals of different
frequencies and amplitudes. This is vital in radio communication, where diodes
demodulate radio waves to extract the underlying audio signal.

15. DISADVANTAGES OF PN JUNCTION
 1.Leakage Current: Even in reverse bias, a small current still flows due to
minority carriers (electrons in P-type and holes in N-type) diffusing across the
junction. This can be problematic for applications requiring very high isolation or
precise voltage control.
 2. Limited Reverse Voltage: Applying a high enough reverse voltage beyond the
breakdown voltage can cause a sudden and significant increase in
current, potentially damaging the diode permanently. This limits the voltage
handling capabilities of diodes.
 3. Temperature Dependence: The characteristics of PN junctions, such as leakage
current and turn-on voltage, are sensitive to temperature variations. This can
impact circuit performance and stability, especially in high-temperature
environments.
 4. Switching Delay: Due to the inherent capacitance of the depletion region, PN
junctions exhibit a slight delay in switching between on and off states. This can
limit their suitability for high-frequency applications.

16. 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

17. THANK YOU