1.
UNIT I
PN JUNCTION DEVICES
1) PN junction diode –structure, operation and V-I characteristics,
diffusion and transition capacitance
2) Rectifiers – Half Wave and Full Wave Rectifier,
3) Display devices- LED, Laser diodes,
4) Zener diode characteristics- Zener Reverse characteristics –
Zener as regulator

2.
Types of Diodes

3.
PN junction diode
• If we take P type semiconductor or N type semi conductor separately,
it is of little use
• If we join a piece of P type material with an N type material such that
crystal structure remains continuous at the boundary, a PN junction is
formed
• The p-n junction diode is made from the semiconductor materials
such as silicon, germanium, and gallium arsenide.
• The p-n junction diodes made from silicon semiconductors works at
higher temperature when compared with the p-n junction diodes
made from germanium semiconductors. Hence silicon is more
preferred

4.
PN Diode

5.
Formation of PN Junction
• Joining n-type material with p-type material causes excess electrons
in the n-type material to diffuse to the p-type side and excess holes
from the p-type material to diffuse to the n-type side.
• Movement of electrons to the p-type side exposes positive ions in the
n-type side while movement of holes to the n-type side exposes
negative ions in the p-type side, resulting in an electron field at the
junction and forming the depletion region.
• A voltage results from the electric field formed at the junction.

6.
Operation of Diode
• The n side will have large number of electrons and very few holes (due to thermal
excitation) whereas the p side will have high concentration of holes and very few
electrons. Due to this a process called diffusion takes place. In this process free
electrons from the n side will diffuse (spread) into the p side and combine with
holes present there, leaving a positive immobile (not moveable) ion in the n side.
Hence few atoms on the p side are converted into negative ions.
• Similarly few atoms on the n-side will get converted to positive ions.
• Due to this large number of positive ions and negative ions will accumulate on
the n-side and p-side respectively. This region so formed is called as depletion
region.
• Due to the presence of these positive and negative ions a static electric
field called as "barrier potential" is created across the p-n junction of the diode. It
is called as "barrier potential" because it acts as a barrier and opposes the flow of
positive and negative ions across the junction.

7.
Biasing of Diode
• Applying an external DC voltage to the diode is called
as biasing.
• If the p-side (anode) is connected to the positive
terminal of the supply and the n-side (cathode) to the
negative terminal of the supply, the diode is said to be
forward biased.
• If the n-side is connected to the positive terminal of
the supply and the p-side to the negative terminal of
the supply, the diode is said to be reversed biased.

8.
Types of Biasing

9.
Zero Biased PN Junction Diode
1)No external energy source is applied
2)A natural Potential Barrier is developed across a depletion layer which is
approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt
for germanium diodes.

10.
Reverse Biased PN Junction Diode
1)thickness of the depletion region increases
2)the diode acts like an open circuit blocking any current flow,
(only a very small leakage current).

11.
Forward Biased PN Junction Diode
1)The thickness of the depletion region reduces
2)The diode acts like a short circuit allowing full current
to flow

12.
V-I characteristics of Diode

13.
Diode Current
Diode current equation expresses the relationship between the current flowing through the diode
as a function of the voltage applied across it. Mathematically it is given as
where:
I = the net current flowing through the diode;
I0 = "dark saturation current", the diode leakage current density in the absence
of light;
V = applied voltage across the terminals of the diode;
q = absolute value of electron charge;
k = Boltzmann's constant; and
T = absolute temperature (K)
n= between 1 and 2, ideality factor
.

14.
Diode Current (contd.)
• In forward biased condition, there will a large amount of
current flow through the diode. Thus the diode current
equation (equation 1) becomes
• On the other hand, if the diode is reverse biased, then the
exponential term in equation (1) becomes negligible. Thus we
have

15.
Diode junction capacitances
• Transition capacitance (CT)
• Diffusion capacitance (CD)

16.
Transition Capacitance
1. When P-N junction is reverse biased the depletion region act as an insulator
or as a dielectric medium and the p-type an N-type region have low
resistance and act as the plates.
2. Thus this P-N junction can be considered as a parallel plate capacitor.
3. This junction capacitance is called as space charge capacitance or transition
capacitance and is denoted as CT .
4. Since reverse bias causes the majority charge carriers to move away from
the junction , so the thickness of the depletion region denoted as W
increases with the increase in reverse bias voltage.

17.
Diode Resistances
• Ideal diode-> Forward Bias-Zero resistance, Reverse Bias-
Infinite resistance
• There are four resistances
1. DC or Static resistance
2. AC or Dynamic resistance
3. Average AC resistance
4. Reverse resistance

18.
Resistor Resistance Graph Diode Resistance Graph

19.
Derivation of Dynamic Resistance
𝐼 = 𝐼0 𝑒 ൗ𝑣
𝜂𝑣 𝑇 − 1
𝑑𝐼
𝑑𝑣
=
𝑑
𝑑𝑣
𝐼0 𝑒 ൗ𝑣
𝜂𝑣 𝑇 − 1
=
𝐼0 𝑒 ൗ𝑣
𝜂𝑣 𝑇
𝜂𝑣 𝑇
=
𝐼+𝐼0
𝜂𝑣 𝑇
=
𝐼
𝜂𝑣 𝑇
𝑟𝑓 =
𝑑𝑣
𝑑𝐼
=
𝜂𝑣 𝑇
𝐼
=
26 𝑚𝑉
𝐼

20.
Transition Capacitance
• The amount of capacitance changed with increase in voltage is called transition capacitance.
• The transition capacitance is also known as depletion region capacitance, junction
capacitance or barrier capacitance.
• Transition capacitance is denoted as CT.
• The change of capacitance at the depletion region can be defined as the change in electric
charge per change in voltage.
• CT = dQ / dV Where,
CT = Transition capacitance
dQ = Change in electric charge
dV = Change in voltage
• The transition capacitance can be mathematically written as,
CT = ε A / W Where,
ε = Permittivity of the semiconductor
A = Area of plates or p-type and n-type regions
W = Width of depletion region

21.
Diffusion capacitance (CD)
1. When the junction is forward biased, a capacitance comes into play , that is
known as diffusion capacitance denoted as CD. It is much greater than the
transition capacitance.
2. During forward bias, the potential barrier is reduced. The charge carriers
moves away from the junction and recombine.
3. The density of the charge carriers is high near the junction and reduces or
decays as the distance increases.
4. The change in charge with respect to applied voltage results in capacitance
called as diffusion capacitance.
5. Diffusion capacitance is directly proportional to the electric current or
applied voltage.

22.
Diffusion capacitance (CD) (contd.)
• When the width of depletion region decreases, the diffusion
capacitance increases.
• The formula for diffusion capacitance is CD = τ ID / η VT , where τ is the
mean life time of the charge carrier, ID is the diode current and VT is
the applied forward voltage, and η is generation recombination factor.
• The diffusion capacitance is directly proportional to the diode current.
In forward biased CD >> CT . And thus CT can be neglected.
• The diffusion capacitance value will be in the range of nano farads
(nF) to micro farads (μF).

23.
Example:
• For an asymmetrical silicon diode, let the mean life time of the holes
be 10ns, and 𝜂=1. if the forward current is 0.1mA, determine the
diffusion capacitance
• Ans= 38.5pF

24.
Display devices- LED

25.
Display devices- Laser diodes
• Laser Diode is a semiconductor device similar to a light-emitting
diode (LED).
• It uses p-n junction to emit coherent light in which all the waves are
at the same frequency and phase.
• This coherent light is produced by the laser diode using a process
termed as “Light Amplification by Stimulated Emission of Radiation”,
which is abbreviated as LASER.
• Since a p-n junction is used to produce laser light, this device is
named as a laser diode.

26.
Laser diodes

27.
Laser diodes