Comprehensive Lecture on PN Junction Diodes and Their Applications in Electronics

• Lecture 14-22
Module 2:
Electronics and Communication Engineering
Dr. Suman Saha

Lecture 14,15
Module 2: PN JUNCTION DIODE
Dr. Suman Saha

Semiconductor Diodes
 Introduction
 Diodes
 Electrical Properties of Solids
 Semiconductors
 pn Junctions
 Semiconductor Diodes
 Special-Purpose Diodes
 Diode Circuits
 Load solution
 Applications

One application of diodes is in rectification
the example below shows a half-wave rectifier
In practice, no real diode has ideal characteristics
but semiconductor pn junctions make good
diodes
To understand such devices we need to look at
some properties of materials

Electrical Properties of Solids
Conductors
e.g. copper or aluminium
Insulators
e.g. polythene
Semiconductors
e.g. silicon or germanium
at very low temperatures these have properties
of insulators.
as the material warms up some electrons break
free and can move about, and it takes on the
properties of a conductor - albeit a poor one

Semiconductors
• Pure semiconductors
– thermal vibration results in some bonds being broken
generating free electrons which move about
– these leave behind holes which accept electrons from
adjacent atoms and therefore also move about
– electrons are negative charge carriers
– holes are positive charge carriers
• At room temperatures there are few charge
carriers
– pure semiconductors are poor conductors
– this is intrinsic conduction

N-Type Material
When extra valence electrons are introduced
into a material such as silicon an n-type material
is produced. The extra valence electrons are
introduced by putting impurities or dopant into
the silicon. The dopant used to create an n-type
material are Group V elements. The most
commonly used dopant from Group V are
arsenic, antimony and phosphorus.
The 2D diagram to the left shows the extra
electron that will be present when a Group V
dopant is introduced to a material such as
silicon. This extra electron is very mobile.
+4
+4
+5
+4
+4
+4
+4
+4
+4

P-Type Material
P-type material is produced when the dopant
that is introduced is from Group III. Group III
elements have only 3 valence electrons and
therefore there is an electron missing. This
creates a hole (h+), or a positive charge that
can move around in the material. Commonly
used Group III dopant are aluminum, boron, and
gallium.
The 2D diagram to the left shows the hole that
will be present when a Group III dopant is
introduced to a material such as silicon. This
hole is quite mobile in the same way the extra
electron is mobile in a n-type material.
+4
+4
+3
+4
+4
+4
+4
+4
+4

The dominant charge carriers in a doped semiconductor (e.g.
electrons in n-type material) are called majority charge
carriers. Other type are minority charge carriers
The overall doped material is electrically neutral

pn Junctions
When p-type and n-type materials
are joined this forms a pn junction
majority charge carriers
on each side diffuse across
the junction where they
combine with (and remove)
charge carriers of the
opposite polarity.
Hence, around the junction
there are few free charge
carriers and we have a
depletion layer (also
called a space-charge layer)

The diffusion of positive
charge in one direction
and negative charge in
the other produces a
charge imbalance
this results in a
potential barrier
across the junction
potential barrier

The barrier opposes the flow of
majority carriers and only a small
number have enough energy to
surmount it. This generates a
small diffusion current.
The barrier encourages the flow of
minority carriers and any that come
close to it will be swept over. This
generates a small drift
current.
For an isolated junction these two
currents must balance each other
and the net current is zero
Potential barrier

Biasing
Forward bias
• If the p-type side is made positive with respect to the
n-type side the height of the barrier is reduced.
• More majority charge carriers have sufficient energy to surmount it
the diffusion current therefore increases while the drift current
remains the same.
• There is thus a net current flow across the junction which increases
with the applied voltage.
Reverse bias
• If the p-type side is made negative with respect to the n-type side
the height of the barrier is increased, the number of majority
charge carriers that have sufficient energy to surmount it rapidly
decreases.
• The diffusion current therefore vanishes while the drift current
remains the same. Thus, the only current is a small leakage
current caused by the (approximately constant) drift current the
leakage current is usually negligible (a few nA)

Semiconductor Diodes
Forward and reverse currents

The Diode Transconductance Curve
• VD = Bias Voltage
• ID = Current through
Diode. ID is Negative for
Reverse Bias and
Positive for Forward Bias
• IS = Saturation Current
• VBR = Breakdown Voltage
• V = Barrier Potential
Voltage
VD
ID (mA)
(nA)
VBR
~V

IS

Forward and reverse currents
pn junction current is given approximately by
where I is the current, e is the electronic charge, V is the
applied voltage, k = 1.38 x 10-23 J/K is Boltzmann’s
constant. T is the absolute temperature and  (Greek
letter eta) is a constant in the range 1 to 2 determined by
the junction material for most purposes we can assume

= 1







 1
exp
ηkT
eV
I
I s

Thus
at room temperature e/kT ~ 40 V-1
If V > +0.1 V
If V < -0.1 V
IS is the reverse saturation current







 1
exp
kT
eV
I
I s
 
V
I
kT
eV
I
I s
s 40
exp
exp 







  s
s I
I
I 


 1
0

Turn-on and breakdown voltages for a silicon device
turn-on voltage of
about 0.5 V
conduction voltage
of about 0.7 V

Types of Diodes and Their Uses
PN Junction
Diodes:
Are used to allow current to flow in one direction while
blocking current flow in the opposite direction. The pn
junction diode is the typical diode that has been used in
the previous circuits.
A K
Schematic Symbol for a PN
Junction Diode
P n
Representative Structure for a
PN Junction Diode
Zener Diodes: Are specifically designed to operate under reverse
breakdown conditions. These diodes have a very
accurate and specific reverse breakdown voltage.
A K
Schematic Symbol for a Zener
Diode

Special-Purpose Diodes
• Light-emitting diodes
– discussed earlier when we looked at light
actuators

Light-Emitting
Diodes:
Light-emitting diodes are designed with a very large
band gap so movement of carriers across their
depletion region emits photons of light energy. Lower
band gap LEDs (Light-Emitting Diodes) emit infrared
radiation, while LEDs with higher band gap energy emit
visible light. Many stop lights are now starting to use
LEDs because they are extremely bright and last longer
than regular bulbs for a relatively low cost.
A K
Schematic Symbol for a Light-
Emitting Diode
The arrows in the LED
representation indicate
emitted light.

Photodiodes: While LEDs emit light, Photodiodes are sensitive to
received light. They are constructed so their pn
junction can be exposed to the outside through a
clear window or lens.
In Photoconductive mode the saturation current
increases in proportion to the intensity of the
received light. This type of diode is used in CD
players.
In Photovoltaic mode, when the pn junction is
exposed to a certain wavelength of light, the diode
generates voltage and can be used as an energy
source. This type of diode is used in the production
of solar power.
A K
A K
Schematic Symbols for
Photodiodes


• Zener diodes
– uses the relatively constant
reverse breakdown voltage
to produce a voltage
reference
– breakdown voltage is called
the Zener voltage, VZ
– output voltage of circuit
shown is equal to VZ despite
variations in input voltage V
– a resistor is used to limit
the current in the diode

• Schottky diodes
– formed by the junction between a layer of metal
(e.g. aluminium) and a semiconductor
– action relies only on majority charge carriers
– much faster in operation than a pn junction diode
– has a low forward voltage drop of about 0.25 V
– used in the design of high-speed logic gates

• Tunnel diodes
– high doping levels produce
a very thin depletion layer
which permits ‘tunnelling’
of charge carriers
– results in a characteristic
with a negative resistance
region
– used in high-frequency oscillators, where they can be
used to ‘cancel out’ resistance in passive components

• Varactor diodes
– a reversed-biased diode has two conducting regions
separated by an insulating depletion region
– this structure resembles a capacitor
– variations in the reverse-bias voltage change the
width of the depletion layer and hence the
capacitance
– this produces a voltage-dependent capacitor
– these are used in applications such as automatic
tuning circuits

Signal clamping
– a simple form of
signal conditioning
– circuits limit the
excursion of the
voltage waveform
– can use a
combination of
signal and Zener
diodes

Diode Circuits
Half-wave rectifier
• peak output voltage
is equal to the peak
input voltage minus
the conduction
voltage of the diode
• reservoir capacitor
used to produce a
steadier output

The application of half wave circuit are as follows
Signal demodulation purpose
Rectification of signal
Signal peak application
The disadvantage of half wave rectifier circuit are
Power Loss
Low input voltage
Output contains lot of ripples

Full wave rectifier software demonstration

Advantage :
The efficiency is double than half wave rectifier.
The ripple factor is low than half wave rectifier.
The output voltage and output power obtained in the full
wave rectifier are higher than that obtained in half wave
rectifier.
Disadvantage:
The only disadvantage of full wave rectifier is that they need
more circuit elements that of half wave rectifier that make it
costly.

• The ripple factor for bridge rectifier circuit
is
𝛾=.48

Advantage:
The efficiency is better than half wave rectifier circuit.
The DC output signal is smother.
In this case output signal is almost same with the input
signal.
Disadvantage:
It is a complex circuit rather than half and full wave circuit.
When more diodes are used more power loss occur.
In bridge rectifier two diodes connected in series conduct in
each half cycle, then voltage drop is higher in this case.

Peak rectifier
use of a diode bridge
reduces the time for
which the capacitor
has to maintain the
output voltage and
thus reduced the
ripple voltage

Signal rectifier
used to demodulate full amplitude modulated signals
(full-AM), also known as an envelope detector found
in a wide range of radio receivers from crystal sets to
superheterodynes

Which of the following isn’t a type of
rectifier?
a) Precision Half-wave Rectifier
b) Bridge Rectifier
c) Peak Rectifier
d) None of the mentioned

For a half wave or full wave rectifier the
Peak Inverse Voltage of the rectifier is
always
a) Greater than the input voltage
b) Smaller than the input voltage
c) Equal to the input voltage
d) Greater than the input voltage for full
wave rectifier and smaller for the half wave
rectifier
bthe peak input voltage is smaller than the input
voltage due to the presence of diode(s). A single
diode reduces the output voltage by approximately
0.7V.

For a half-wave rectifier having diode
voltage VD and supply input of VI, the diode
conducts for π – 2Θ, where Θ is given by
a) tan-1
VD/VI
b) tan-1
VD/VI – VI
c) sin-1
VD/VI
d) sin-1
VD/VI – VI
cthe diode doesn’t conducts when
VD V
≥ I . Hence Θ = sin-1 (D/VI).

Lecture 16-
17
All shared the Nobel Prize in 1956
for this contribution.

Cut-off region:
Both emitter junction and collector
junction are in reversed bias i.e. The
transistor is off. There is no conduction
between the collector and the emitter
IB = 0, therefore IC = 0
Region of BJT Operation

BJT is on.
Collector current is proportional to and controlled by the
base current (IC=βIB) and relatively insensitive to VCE.
In this region the BJT can be an amplifier.

Saturation region:
The transistor is on.
The collector current varies very little with a change in the base
current in the saturation region.
The VCE is small, a few tenths of a volt.
The collector current is strongly dependent on VCE unlike in the
active region.
It is desirable to operate transistor switches in or near the
saturation region when in their on state.

Transistor connection
Common Emitter Configuration –
has both Current and Voltage Gain

In dc mode the levels of IC and IE
due to the majority carriers are related
by a quantity called alpha
Reverse saturation current
Common Base
The ac alpha is formally called the common-base ,
short-circuit , amplification factor

Common Base
has Voltage Gain
but no Current Gain
Common Base Voltage Gain
alpha =1, for practical devices alpha
typically extends from 0.90 to 0.998,

Input char. is the plot of the base current, IB, versus the base-emitter
voltage,VBE, for various values of the collector-emitter voltage,VCE
Ouput characteristic of the BJT is
the plot of the collector current, IC,
versus the collector-emitter voltage,
VCE, for various values of the base
current, IB as shown on the circuit
on the right.
Common Emitter

Common Emitter (CE) Amplifier Circuit
emitter current is given as IE = IC + IB.

In the dc mode the levels of IC and IB
are related by a quantity called beta
and defined by the following equation
For ac situations an ac beta is defined
as follow
The formal name for beta is common-emitter, forward-current,
amplification factor.

• The input is connected between base and emitter, while output
is connected between collector and emitter 
• Emitter is common to both input and output circuits. 
• The bias voltage applied are Vce and Vbe. 
• The emitter-base junction is forward biased and collector-
emitter junction is reverse biased. 
• The base current Ib flows in the input circuit and collector
current Ic flows sin the output circuit. 
• CE is commonly used because its current, Voltage, Power gain
are quite high nd output to input impedance ratio is moderate
• The rate of change in collector current to change in base
current is called amplification factor B. 
• The current gain in the common-emitter circuit is called BETA
(b). Beta is the relationship of collector current (output current)
to base current (input current).
• BE junction is forward biased while the CB junction is reverse
biased.

Bipolar Transistor Configurations

Q1Who invented BJT?
BJT was invented by W.H Brattin, Bardeen, and William Shockley.
Q2 What are the operating regions of BJT?
The operating regions of BJT are:
Forward active or active region
Reverse active or inverted region
Saturation
Cut-off
Q3 What are the applications of BJT?
Following are the applications of Bipolar Junction Transistor:
It is used as an amplifier
It is used as an oscillator
It is used as a demodulator
Q4 What happens if the transistor is not biased properly?
Following is the list of consequences if the transistor is not biased properly:
The work efficiency of the transistor reduces
There will be a distortion in the output signal
The operating point may shift
Transistor parameters will change
Q5 Why is there a maximum limit for the collector supply voltage for a transistor?
There is a maximum limit for the collector supply voltage for a transistor because when the collector current is increased
rapidly there are chances of transistor getting damaged. To avoid this, the voltage in the collector should have a maximum
limit.

Reference
1. Solid state electronic devices by Ben G.streetman and
S.K Banerjee
2. Lecture notes: Sec. 3 Sedra & Smith
3. Electronics devices and Circuit Theory By Robert L.
Boylestad
4. Principle of electronic material and devices by S.O.
Kasap.
5. Working Principle of MOSFET and MOSFET
Characteristics https://www.electrical4u.com/mosfet-
working-principle-of-p-channel-n-channelmosfet/

Comprehensive Lecture on PN Junction Diodes and Their Applications in Electronics