UNIT I - PN JUNCTION DEVICES

PN junction diode –structure, operation and V-I characteristics, diffusion

and transition capacitance – Clipping & Clamping circuits - Rectifiers – Half
Wave and Full Wave Rectifier– Display devices- LED, Laser diodes, Zener
diode characteristics- Zener diode Reverse characteristics – Zener diode as
regulator.

1. With a neat diagram explain the working of a PN junction diode

in forward bias and reverse bias and show the effects of
temperature on its VI characteristics
 A PN junction is formed from a piece of semiconductor (Ge or
Si) by diffusing p-type material (Acceptor impurity Atoms) to
one half side and N type material to (Donar Impurity Atoms)
other half side. The plane dividing the two zones is known as
'Junction'.
 The P-region of the semiconductor contains a large number of holes
and N region, contains a large number of electrons. A PN junction just
immediately formed is shown in Fig.

When PN junction is formed, there is a tendency for the electrons in the N-region
to diffuse into the p- region, and holes from P-region to N-region. This process is
called diffusion. While crossing the junction, the electrons and holes recombines
with each other, leaving the immobile ions in the neighborhood of the junction
neutralized as shown in Fig.

 These immobile + ve and –ve ions, set up a potential across the

junction. This potential is called potential barrier or junction barrier.
Due to the potential barrier no further diffusion of electrons and
holes takes place across the junction.
 Potential barrier is defined as a potential difference built up across
the PN junction which restricts further movement of charge carriers
across the junction. The potential barrier for a silicon PN junction is
about 0.7 volt, whereas for Germanium PN junction is approximately
0.3 volt.
Symbol of Diode:
 The symbol of PN junction diode is shown in Fig. The P-type and N-
type regions are referred to as Anode and Cathode respectively. The
arrowhead shows the conventional direction of current flow when
the diode is forward biased.

Working of PN Junction Diode:

Forward Bias:

 When the positive terminal of the external battery is connected to

the P-region and negative terminal to the N-region, the PN junction
 is said to be forward biased as shown in Fig.

 When the junction is forward biased, the holes in the p-region are
repelled by the positive terminal of the battery and are forced to move
towards the junction. similarly, the electrons in the N-region are
repelled by the negative terminal of the battery and are forced to move
towards the-junction.
 This reduces the width of the depletion layer and barrier potential. If
the applied voltage is greater than the potential barrier vr, then the
majority carriers namely holes in P-region and electrons in N-region,
cross the barrier. During crossing some of the charges get neutralized
the remaining charges after crossing, reach the other side and
constitute current in the forward direction. The PN junction offers very
low resistance under forward biased condition.
 Since the barrier potential is very small (nearly 0.7 V for silicon and
0.3 V for Germanium junction), a small forward voltage is enough to
completely eliminate the barrier. once the potential barrier is
eliminated by the forward voltage, a large current start flowing through
the PN junction.
 Reverse Bias:

 When the positive terminal of the external battery is connected to the

N-region and negative terminal to the p-region, the PN junction is
said to be reverse biased. When the junction is reverse biased, the
holes in the P-region are attracted by the negative terminal of the
battery. Similarly, the electrons in the N-region are attracted by the
positive terminal of the external battery. This increases the width
of the depletion layer and barrier potential (Vs).
 The increased barrier potential makes it very difficult for the
majority carriers to diffuse across the junction. Thus, there is no
current due to majority carriers in a reverse biased PN junction. In
other words, the PN junction offers very high resistance under
reverse biased condition.
 In a reverse biased PN junction, a small amount of current (in µA)
flows through the junction because of minority carriers. ( i.e.,
electrons in the P-region and holes in the N region). The reverse
current is small because the number of majority carrier in both
regions is small.
V-l characteristics of PN-Junction Diode:

 A graph between the voltage applied across the PN junction

and the current flowing through the junction is called the V-I
characteristics of PN junction diode. Fig. shows the V-I
characteristics of PN junction diode.
Forward Characteristics:
 Fig. (a) shows the circuit arrangement for drawing the forward
V-I characteristics of PN junction diode. To apply a forward
bias, the +ve terminal of the battery is connected to Anode (A)
and the negative terminal of the battery is connected to Cathode
(K). Now, when supply voltage is increased the circuit current
increases very slowly and the curve is nonlinear (region-OA).
 The slow rise in current in this region is because the external
applied voltage is used to overcome the barrier potential (0.7 V
for Si; 0.3V for Ge ) of the PN junction' However once the
potential barrier is eliminated and the external supply voltage is
 increased further, the current flowing through the PN junction
diode increases rapidly (region AB). This region of the curve is
almost linear. The applied voltage should not be increased
beyond a certain safe limit, otherwise the diode will burnout.
 The forward voltage at which the current through the PN junction
starts increasing rapidly is called by knee voltage. It is denoted by
the letter VB.

Reverse Characteristics:
 Fig (b) shows the circuit arrangement for drawing the reverse
V-I characteristics of PN junction diode. To apply a reverse bias,
the +ve terminal of the battery is connected to cathode (K) and
- ve terminal of the battery is connected to anode (A).
 Under this condition the potential buried at the junction is
increased. Therefore, the junction resistance becomes very high
and practically no. current flows through the circuit. However,
in actual practice, a very small current (of the order of µA) flows
in the circuit. This current is called reverse current and is due
to minority carriers. It is also called as reverse saturation current
(I). The reverse current increases slightly with the increase in
reverse bias supply voltage.
 If the reverse voltage is increased continuously at one state
(marked by point C on the reverse characteristics) breakdown
of junction occurs and the resistance of the barrier regions falls
suddenly. Consequently, the reverse current increases rapidly
(as shown by the curve CD in the current) to a large value. This
may destroy the junction permanently. The reverse voltage at
which the PN junction breaks is called as break down voltage.

2. Explain Diffusion capacitance and transition capacitance?

Diffusion capacitance 𝐂𝐃: Definition:
 The junction behaves like a capacitor. The capacitance, which
exists in a forward-biased junction is called a diffusion or
storage capacitance
 The diffusion capacitance arises due to the arrangement of
minority carrier density. And its value is much larger than the
depletion layer capacitance.
Explanation:
 When forward bias voltage is applied to the p-n junction diode,
electrons (majority carriers) in the n-region will move into the
p-region and recombines with the holes.
 In the similar way, holes in the p-region will move into the n-
region and recombines with electrons. As a result, the width of
depletion region decreases.
 The electrons (majority carriers) which cross the depletion
region and enter into the p-region will become minority carriers
of the p-region similarly; the holes (majority carriers) which
cross the depletion region and enter into the n-region will
become minority carriers of the n-region.
 A large number of charge carriers, which try to move into
another region will be accumulated near the depletion region
before they recombine with the majority carriers. As a result, a
large amount of charge is stored at both sides of the depletion
region.
 Diffusion capacitance occurs in a forward biased p-n junction
diode. Diffusion capacitance is also sometimes referred as
storage capacitance. It is denoted as CD.
 The accumulation of holes in the n-region and electrons in the
p-region is separated by a very thin depletion region or
 depletion layer.
 This depletion region acts like dielectric or insulator of the
capacitor and charge stored at both sides of the depletion layer
acts like conducting plates of the capacitor.
 Diffusion capacitance is directly proportional to the electric
current or applied voltage. If large electric current flows
through the diode, a large amount of charge is accumulated near
the depletion layer. As a result, large diffusion capacitance
occurs.
 In the similar way, if small electric current flows through the
diode, only a small amount of charge is accumulated near
the depletion layer. As a result, small diffusion capacitance
occurs.
 When the width of depletion region decreases, the diffusion
capacitance increases. The diffusion capacitance value will be
in the range of nano farads (nF) to micro farads (μF).
 The formula for diffusion capacitance is CD = dQ / dV
Where, CD = Diffusion capacitance dQ = Change in number of
minority carriers stored outside the depletion region dV = Change
in voltage applied across diode

Transition capacitance (CT)

 The conducting plates or electrodes of the capacitor are good
conductors of electricity. Therefore, they easily allow electric
 current through them.

 On the other hand, dielectric material or medium is poor conductor

of electricity. Therefore, it does not allow electric current through it.
However, it efficiently allows electric field.

The capacitors store electric charge in the form of electric field. The
capacitors store electric charge by using two electrically conducting plates

 When voltage is applied to the capacitor, charge carriers starts flowing

through the conducting wire. When these charge carriers reach the
electrodes of the capacitor, they experience a strong opposition from the
dielectric or insulating material.

 As a result, a charge carriers cannot move between the plates. However,

they exert electric field between the plates. The charge carriers which are
trapped near the dielectric material will stores electric charge. The ability
of the material to store electric charge is called capacitance.

 In a basic capacitor, the capacitance is directly proportional to the size

of electrodes or plates and inversely proportional to the distance
between two plates.
 Just like the capacitors, a reverse biased p-n junction diode also
stores electric charge at the depletion region. The depletion region
is made of immobile positive and negative ions.
 In a reverse biased p-n junction diode, the p-type and n-type regions
have low resistance. Hence, p-type and n-type regions act like the
electrodes or conducting plates of the capacitor.
 The depletion region of the p-n junction diode has high resistance.
Hence, the depletion region acts like the dielectric or insulating
material. Thus, p-n junction diode can be considered as a parallel
plate capacitor.
 In depletion region, the electric charges (positive and negative ions)
do not move from one place to another place. However, they exert
electric field or electric force.
 Therefore, charge is stored at the depletion region in the form of
electric field.
 The ability of a material to store electric charge is called
capacitance. Thus, there exists a capacitance at the depletion
region.
 The capacitance at the depletion region changes with the change in
applied reverse bias voltage.
 The capacitance at the depletion region changes with the change in
applied voltage. When reverse bias voltage applied to the p-n
junction diode is increased, a large number of holes (majority
carriers) from p-side and electrons (majority carriers) from n-side
are moved away from the p-n junction.
 As a result, the width of depletion region increases whereas the size
of p-type and n-type regions (plates) decreases.
 The capacitance means the ability to store electric charge. The p-n
junction diode with narrow depletion width and large p-type and n-
type regions will store large amount of electric charge whereas the
p-n junction diode with wide depletion width and small p- type and
n-type regions will store only a small amount of electric charge.
 Therefore, the capacitance of the reverse bias p-n junction diode
decreases when voltage increases.
 In a forward biased diode, the transition capacitance exists.
However, the transition capacitance is very small compared to
the diffusion capacitance. Hence, transition capacitance is
neglected in forward biased diode.
  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.

3. Derive the PN diode current equation.

 Temperature effect of p-n junction:
The current in a diode is given by the diode current equation

I = Io( e V/ηVT –1)

Where, I - diode current

Io - reverse saturation current
V- diode voltage
η- Semiconductor constant =1 for Ge, 2 for Si.
T- Voltage equivalent of temperature= T/11,600 (Temperature T is in Kelvin)

Note: If the temperature is given in 0C then it can be converted to Kelvin by the

help
of following relation, 0C+273 = K

1. Effect of Temperature on Forward Voltage

 The forward voltage or threshold voltage of a PN junction diode is the

minimum voltage required to overcome the potential barrier of the junction
and allow significant current flow. This threshold voltage typically ranges
between 0.6V and 0.7V for silicon diodes at room temperature (25°C).
As the temperature increases, the following effects on the forward voltage are
observed:

 Reduction in Forward Voltage: For every degree Celsius rise in

temperature, the forward voltage decreases by approximately 2 mV for
silicon diodes. This is due to the fact that the intrinsic carrier concentration
in the semiconductor material increases with temperature, reducing the
potential barrier of the PN junction.

 Increased Conductivity: With a decrease in forward voltage, the diode

becomes more conductive at higher temperatures, allowing more current to
flow for a given forward voltage. This can improve performance in some
cases but can also lead to overheating if not properly managed.

2. Effect of Temperature on Reverse Saturation Current

The reverse saturation current (I_S) is the small leakage current that flows
through the diode when it is reverse-biased. This current is typically very small
at room temperature but can increase significantly with rising temperatures.

 (Exponential Growth in Reverse Saturation Current: The reverse

saturation current increases exponentially with temperature, approximately
doubling for every 10°C rise. This is because higher temperatures generate
more electron-hole pairs in the semiconductor, which contributes to the
reverse leakage current.

 Impact on Device Reliability: As the reverse saturation current increases,

the diode becomes more prone to leakage, which can degrade its efficiency,
especially in rectification applications. Prolonged operation under high
temperatures can also lead to device failure due to excessive leakage
current.)
 4. Write short notes on Clipping Circuit and Clamping Circuit
Clipping and Clamping circuits
 A wave shaping circuit which controls the shape of the output waveform
by removing or clipping a portion of the applied wave.
• Half wave rectifier is the simplest example. (It clips negative half cycle).

• Also referred as voltage limiters/ amplitude selectors/ slicers.

Positive Clipper

 A positive clipper is that which removes the positive half-cycles of the

input voltage.
Fig.1 shows the circuit of a positive clipper using a diode.

Circuit Action

 During the positive half cycle of the input voltage, the diode is forward
biased and conducts heavily. Therefore, the voltage across the diode, which
behaves as a short, is zero. And hence, voltage across the load RL is zero.

 During the negative half cycle of the input voltage, the diode is reverse
biased and behaves as an open.

In this condition, the circuit behaves as a voltage divider with an output given by
:
Generally, RL is much greater than R. So,

Fig.2(ii)

Input-Output Waveform

Fig.3 shows the input-output waveforms of a positive clipper. As shown, the

output voltage has all the positive half cycles removed or clipped off.

Negative Clipper
If it is desired to remove the negative half cycle of the input, the only thing to
be done is to reverse the polarities of the diode in the circuit shown in fig 1.
Such a clipper is known as negative clipper.

Applications:

- In radio receivers for communication circuits.

- In radars, digital computers and other electronic systems.

- Generation for different waveforms such as trapezoidal, or square waves.

- Helps in processing the picture signals in television transmitters.

- In television receivers for separating the synchronising signals from composite

picture signals.

Biased Clipper

Sometimes, it is desired to remove a small portion of positive or negative half

cycle of the signal voltage. For this purpose, biased clipper is used.

Fig.4 shows the circuit diagram of a biased clipper.

Fig.4
It consists of a diode D with a battery of Vdc volts. With the polarities of the
battery shown in Fig.4, a portion of each positive half cycle will be clipped off.
However, the negative half cycles, will appear as such across the load, Such, a
clipper is called biased positive clipper.

Circuit Action
During the positive half-cycle, so long as the input voltage is greater than +V dc,
the diode will conduct heavily.

Fig.5(i)
When the input voltage is greater than +V, the diode is forward biased and
behaves as a short and the output voltage equals to +Vdc.

The output will stay at +Vdc, so long as the input voltage is greater than +V dc.

During the period the input voltage is less than +Vdc, the diode is reverse biased
and behaves as an open. Therefore, most of the input voltage appears across the
output.

Fig.5(ii)
In this way the biased positive clipper removes input voltage, above +V dc.

During the negative half cycle of the input voltage, the diode remains reverse
biased. Therefore, almost entire negative half cycle appears across the load. i.e.

VO = Vin . Fig.5(iii)

Input-Output Waveform
Fig.6 shows the input-output waveforms of a positive biased clipper. As shown,
the biased positive clipper removes input voltage above +V dc.
 Fig.6
Biased Negative Clipper
If it is desired to clip a portion of negative half cycle of input voltage, the only
thing to be done is to reverse the polarities of diode or battery. Such a circuit is
known as biased negative clipper.

Clamping Circuit
A circuit that places either the positive or negative peak of a signal at a desired
d.c. level is known as a clamping circuit.

Fig.10
A clamping circuit essentially adds a d.c. component to the signal.
The input signal is a sine wave having peak to peak value of 10 V.
The clamper adds the d.c. component and pushes the signal upward so that
the negative peaks fall on the zero level.
It may be noted that the shape of the original signal has not changed; only
there is a vertical shift in the signal. Such a clamper is known as positive
clamper.

The negative clamper dos the reverse i.e. it pushes the signal downwards so that
the positive peaks fall on the zero level.

The following points may be noted carefully:

1. The clamping circuit does not change the peak-to-peak or r.m.s. value of
the waveform.

2. A clamping circuit changes the peak value and average value of a

waveform.

In the above circuit, it is easy to see that input waveform has a peak value of 5 V
and average value over a cycle is zero. The clamped output varies between 10 V
and 0 V. Therefore, the peak value of camped output is 10 V. And

Positive Clamper
Fig.11 shows the circuit diagram of a positive clamper.

Fig.11
The input signal is assumed to be a square wave with time period T.
The clamped output is obtained across RL.
The circuit design incorporates two main features. Firstly, the value of C and
RL are so selected that time constant τ = C RL is very large.
This means that voltage across the capacitor will not discharge significantly
during the interval the diode is non-conducting.

Secondly, RL C time constant is deliberately made much greater than the time
period T of the incoming signal.

Operation
During the negative half cycle of the input signal, the diode is forward biased.
Therefore, the diode behaves as a short as shown in fig.12.

Fig.12
The charging time constant (= CRf, where Rf = forward resistance of the diode) is
very small so that the capacitor will charge to V volts very quickly.

It is easy to see that during this interval, the output voltage is directly across the
short circuit. Therefore, Vout = 0. When the input switches to positive half cycle,
the diode is reverse biased and behaves as an open as shown in fig 13.

Fig.13
Since the discharging time constant (= CRL) is much greater than the time period
of the input signal, the capacitor remains almost fully charged to V volts during
the off time of the diode.

Now applying Kirchhoff’s voltage law to the input loop:

The resulting waveform is shown in fig.14.

Fig.14
It is clear from the fig.14 that it is a positively clamped output. That is the input
signal is pushed upward by V volts so that negative peaks fall on the zero level.

Negative Clamper

Fig.15 shows the circuit of a negative clamper.

Fig.15
The clamped output is taken across RL. The only change from the positive clamper
is that the connections of diode are reversed.

Operation
During the positive half cycle of the input signal, the diode is forward biased.
Therefore, the diode behaves as a short as shown in fig.16.

Fig.16
The charging time constant ( = CRf) is very small so that the capacitor will charge
to V volts very quickly. It can be seen that during this interval, the output voltage
is directly across the short circuit. Therefore, Vout = 0.

When the input switches to negative half cycle, the diode is reverse biased and
behaves as an open as shown in fig.17.

Fig.17
Since the discharging time constant (= CRL) is much greater than time period of
the input signal, the capacitor almost remains fully charged to V volts during the
off time of the diode.

Now applying Kirchhoff’s law to the input loop:

The resulting waveform is shown in fig.18. It can be noted that total swing of the
output signal is equal to the total swing of the input signal.

Fig.18

Rectifiers
5. What is halfwave rectifier? Explain the working principle with
neat sketch?
 The Half wave rectifier is a circuit, which converts an ac voltage
to dc voltage. The primary of the transformer is connected to ac
supply. This induces an ac voltage across the secondary of the
transformer.
There are two types of rectifiers:

(a) Half Wave (HW) rectifier

(b) Full Wave (FW) rectifier

Half -wave Rectifier: Construction:

 It consists of a single diode in series with a load resistor. The input to half
wave rectifier is supplied from the 50 Hz a.c supply. The circuit diagram
for halfwave rectifier is shown in fig.
 Positive half cycle:
 During the positive half cycle of the input signal the anode of
the diode becomes positive with respect to the cathode and
hence the diode D conducts.
 For an ideal diode, the forward voltage drop is zero. So, the
whole-input voltage will appear across load resistance RL.

Negative half cycle:

 During negative half cycle of the input signal, the anode of the
diode becomes negative with respective to the cathode and
hence the diode D does not contact.
 For an ideal diode the impedance by the diode is infinity. So,
the whole input voltage appears across the diode D. hence the
voltage drop across R, is zero.
Working of Half Wave Rectifier:
The working of the half-wave rectifier is as follows:
1. Half wave rectifier works as a step-down transformer, the left side is
the primary side and the right side is the secondary side.
2. AC voltage is connected to the primary side and the secondary side
is equipped with the diode and the load.
3. A high AC voltage is applied to the primary side of the transformer.
 The secondary low voltage that is acquired is then applied to
the diode.
4. The diode works in two ways; it is forward biased during the
positive half cycle and reverse biased during the negative half cycle.
5. When the diode is forward biased, it acts as a closed switch while in
reverse biased it acts as an open switch. As a result, no current can
flow to the load and the output voltage is equal to zero.

Analysis of Half wave rectifier:

Let Vi be the input voltage to the rectifier
𝑉𝑖 = 𝑉𝑚𝑠𝑖𝑛𝜔𝑡
Where,
𝑉𝑚 = Maximum value of the input voltage
Let I be the current flowing though the circuit when the diode is
conducting.
I= {𝐼𝑚𝑠𝑖𝑛𝜔𝑡 𝐹𝑜𝑟 0 ≤ 𝜔𝑡 ≤ 𝜋
0 f𝑜𝑟 𝜋 ≤ 𝜔𝑡 ≤ 2 𝜋 }

Where, 𝐼𝑚 = 𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡

Im= Vm/ Rf +Rl

Where, 𝑅𝐹-Forward dynamic resistance of diode.

𝑅𝐿-Load resistance

(a) Average or DC value of output current (Idc):

 From Fig., it is seen that the output current is not steady but
contains fluctuations even though it is DC current. The average
value of this fluctuating current is called DC current (Idc). It can
be calculated as follows.
Average value = (Area under the curve / Period)
Disadvantages of HWR:
 Low output because one half cycle only delivers output
 A.C. component more in the output
 Requires heavy filter circuits to smooth out the output Peak inverse
Voltage.
Application of Half Wave Rectifier

There are some applications of Half Wave Rectifier −

 They are used for rectification

 They are used for demodulation

 They are used for signal peak applications

 In FWR, current flows through the load during both half cycles
of the input a.c. supply. Like the half wave circuit, a full wave
rectifier circuit produces an output voltage or current which is
purely DC orhas some specified DC component.
 Full wave rectifiers have advantages over their half wave
rectifier.
 The average (DC) output voltage is higher than for half
wave, the output ofthe full wave rectifier has much less
ripple than that of the half wave rectifier producing a
smoother output waveform.

Full Wave Rectifier:

6. Explain the operation of full wave rectifier with centre tap
transformer. Also derive the following for this rectifier?
 A full wave rectifier is an electronic circuit which converts AC
voltage into a pulsating DC voltage using both half cycles of the
applied AC voltage.
 A full wave rectifier is a circuit which allows a unidirectional
current to flow through the load during the entire input cycle as
shown in fig.
 The result of full wave rectification is a d.c. output voltage that
pulsates every half-cycle of the input. On the other hand, a half
 wave rectifier allows the current to flow through the load during
positive half-cycle only.

Positive half cycle:

 The circuit uses two diodes which are connected to secondary
winding of the transformer. The input signal is applied to the
primary winding of the transformer.
 During the positive input half cycle, the polarities of the
secondary voltage are shown in fig. These forward biases the
diode D1, and reverse biases the diode D2. As a result of this,
the diode D1, conducts some current whereas the diode D2, is
off.
 The current through load RL is as indicated in through D1, and
the voltage Drop across RL shown in fig. The load current flow
is equal to the input voltage.
Negative half cycle:

 During the negative input half cycle, the polarities of the

secondary voltage are interchanged. The reverse-bias the diode
D1, and forward Biases the diode D2.
 As a result of this, the diode D1 is OFF and the diode D2
conducts some current. The current through the load R, is an
indicated in the fig.
 The load current flows through D2 and the voltage drop across
RL will be equal to the input voltage. The maximum efficiency
of a fall-wave rectifier is 8l,2℅Vo and ripple factor is 0.48.

Input and output waveforms:

 Analysis of Full Wave Rectifier:
Let Vi be the input voltage to the rectifier, 𝑉𝑖 = 𝑉𝑚𝑠𝑖𝑛𝜔𝑡
Where, 𝑉𝑚 = Maximum value of the input voltage.
Let I be the current flowing though the circuit when the diode is
conducting.
{𝐼𝑚𝑠𝑖𝑛𝜔𝑡 𝐹𝑜𝑟 0 ≤ 𝜔𝑡 ≤𝜋
𝑖=

0 𝐹𝑜𝑟 𝜋 ≤ 𝜔𝑡 ≤ 2𝜋}
Where, 𝐼𝑚 = 𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑣𝑎𝑙𝑢𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡;

Im = 𝑉𝑚

𝑅𝐹 +𝑅𝐿

Where, 𝑅𝐹-Forward dynamic resistance of diode; 𝑅𝐿-Load

resistance.
 Peak inverse voltage is the maximum possible voltage across a diode
when it is not conducting.
Advantages:
1. The D.c load voltage and current are more than halfwave.
2. No D.c current thro transformer windings hence no possibility of
saturation.
3. TUF is better.
4. Efficiency is higher.
5. Ripple factor less.
Disadvantages:
1. PIV rating of diode is higher

2. Higher PIV diodes are larger in size ad costlier.

3. Cost of transformer is high.

 Compare different types of rectifiers?
Type HW FW
No of diodes used 1 2
Need of transformer Not necessary Necessary
Ripple factor 1.21 0.48
Efficiency 40.6% 81.2%
PIV Vm 2Vm
TUF 0.287 0.812
From factor 1.57 1.11
Peak factor 2 √2
Ripple frequency f 2f

Display devices:

7. Discuss the working principle, characteristics and application of

LED in detail. (OR)

Explain the principle and operation of light emitting diode
(LED) with necessary expressions for current densities and
efficiency of light generation.

 A light-emitting diode (LED) is a semiconductor light source

LEDs are used as indicator lamps in many devices and are
increasingly used for other lighting.
 When a light-emitting diode is forward-biased (switched on),
electrons are able to recombine with electron holes within the
device, releasing energy in the form of photons. This effect is
called electroluminescence and the color of the light
(corresponding to the energy of the photon) is determined by the
energy gap of the semiconductor.
 LEDs are often small in area (less than 1 mm2), and integrated
optical components may be used to shape its radiation pattern.
 LEDs present many advantages over incandescent light sources
 including lower energy consumption, longer lifetime, improved
robustness, smaller size, and faster switching.
 LEDs powerful enough for room lighting are relatively
expensive and require more precise current and heat
management than compact fluorescent lamp sources of
comparable output.
 Light Emitting Diodes are made from exotic semiconductor
compounds such as Gallium Arsenide (GaAs), Gallium
Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP),
Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all
mixed together at different ratios to produce a different
wavelength of colour.

 Different LED compounds emit light in specific regions of the

visible light spectrum and therefore produce different intensity
levels.
 Gallium Arsenide Phosphide (GaAsP) - red to infra-red, orange
 Gallium Phosphide (GaP) - red, yellow and green
 Gallium Nitride (GaN) - green, emerald green
 Gallium Indium Nitride (GaInN) - near ultraviolet, bluish-green and
blue
 Light-emitting diodes are used in applications as different as aviation
lightning, automotive lighting, advertising, general lighting, and traffic
signals.
 LEDs have allowed new text, video displays, live video, and sensors to be
developed, while their high switching rates are also useful in advanced
communications technology.
 Infrared LEDs are also used in the remote control units of many
commercial products including televisions, DVD players, and other
domestic appliances.
Construction of LED Lamp

For the construction of LED lamps, following semiconductor materials are used

 Gallium Phosphide (Green / Red)

 Gallium Arsenide Phosphide (Yellow / Red)
 Gallium Nitride (White)

Fig. Equivalent circuit

  In order to construct an LED lamp, a layer of P-type semiconductor
material is placed above the N-type semiconductor layer.
 A metal film is used on the P-type layer to provide anode connection to
the device. Similarly, a gold-film is formed on the N-type layer to provide
cathode connection. The gold-film also provides reflection of light from
the bottom surface of the device. This increases the efficiency of the LED
lamp. The above figure shows the basic construction of an LED lamp.

Working of LED Lamp

 When a DC power supply is applied to an LED lamp in forward bias, the

majority charge carriers start drifting, i.e., electrons towards the positive
terminal and holes towards the negative terminal of the source.
 At the PN junction, the recombination of electrons and holes takes place,
due to the recombination of these charge carriers, the energy is released
either in the form of heat or light.
 The semiconductor materials such as gallium phosphide (GaP), gallium
Arsenide Phosphide (GaAsP), gallium nitride (GaN), etc. emit light on the
recombination of electrons and holes at the PN junction.
 The electrons in these semiconductor materials lose their energy by the
emission of light photons. If the semiconductor material is translucent, the
emitted light at the PN junction will be transferred outside. In this way, an
LED lamp emits light.

Advantages of LED Lamps

The advantages of using LED lamps are as follows −

 The operating cost is very less.

 They are compact in size.
 LED lamps provide easy control of light.
  Remote switching and control can be implemented in the LED lamps.
 LED lamps have longer life, up to 100000 hours.
 LED lamps are energy efficient and low current consuming.
 These lamps are mechanically robust.
 LED lamps provide excellent color rendering.
 These lamps are environment friendly, as not having any toxic content.

Disadvantages of LED Lamps

Listed below are some of the disadvantages of using LED lamps −

 The initial cost of the LED lamps is very high.

 As LED lamps are semiconductor devices. Thus, they are temperature
sensitive and temperature dependent.
 LED lamps require DC supply, therefore, rectification unit is required in
the lamp circuit.
 The high intensity light produced by an LED lamp has impact on the
insects.
Applications of LED Lamps

LED lamps are used in the following applications −

 For domestic and commercial lightings.

 Used as indicating lamps.
 Used as bi-color indicators.
 LED lamps are also used as fault indicators in control panels.
 In display boards.
 Used for decorative lighting.
 In mobile phone and wrist watches screens.
 Used as head lamps in automobiles.
 LASER DIODES

7. Explain in detail about LASER DIODE?

The term laser comes from the acronym for light amplification for stimulated
emission of radiation. The laser medium can be a gas, liquid, amorphous solid
or semiconductor

Definition

It is a specially fabricated pn junction diode. This diode emits laser light when
it is forward - biased.

Principle

 When the p-n junction diode is forward-biased (fig. (a)), the electrons from
n-region and holes from p-region cross the junction and recombine
with each other.
 During the recombination process, the light radiation (photons) is
released from direct band gap semiconductors like GaAs. This light
radiation is known as recombination radiation (fig (b)).
The photon emitted during recombination stimulates other electrons and holes to
recombine. As a result, stimulated emission takes place and laser light is
produced.

Construction

The construction of laser diode is shown in fig

i. The active medium is a p - n junction diode made from a single crystal of
gallium arsenide. This crystal is cut in the form of a platelet(colourless) having
a thickness of 0.5 mm. noise. This platelet consists of two regions n - type and p
- type.

ii. The metal electrodes are connected to both upper(p-region) and lower (n-
region) surfaces of the (p-region) semiconductor diode. The forward bias voltage
is applied through metal electrodes.
Working

i. The energy level diagram of laser diode is shown in fig 4.25.

ii. When the pn junction is forward-biased, the electrons and holes are injected
into junction region.

iii. The region around junction contains a large number of electrons in the
conduction band and holes in the valence band.

iv. Now the electrons and holes recombine with each other. During
recombination, light photons are produced.

v. When the forward - biased voltage is increased, more light photons are emitted.
These photons trigger a chain of stimulated recombination resulting in the
emission of more light photons in phase.

 These photons moving at the plane of the junction travel back and forth by
reflection between two polished surfaces of the junction. Thus, the light
photons grow in strength

After gaining enough strength, laser beam of wavelength 8400 Å is emitted from
the junction.
The wavelength of laser light is given by

The end faces of the pn junction are well polished and parallel to each other. They
act as an optical resonator through which the emitted light comes out.

V-I Characteristics of Laser diode

 The forward voltage of laser diode is generally around 1.5 V. Although the
forward voltage depends on operating temperature.

 The variance of current in the diode with the voltage can be understood
with the help of below diagram.
 Characteristics of Laser Diode

 The Ideal light output against current characteristics for

semiconductor laser is shown in Fig.

 The solid line represents the laser characteristics. It may

be observed that the device gives low light output in the
region, the threshold with corresponds to spontaneous
emission only within the structure.

 After the threshold current value the light output

increases substantially for small increases in current
through the device.
Characteristics:

i. Type: Solid state semiconductor laser.

ii. Active medium: A pn junction diode made from a single crystal of gallium
arsenide.

iii. Pumping method: Direct conversion method.

iv. Power output: a few mW.si

v. Nature of output: Continuous wave or pulsed output.

vi. Wavelength of output: 8300 Å to 8500 Å.

Advantages

i. This laser is very small in size and compact.

ii. It has high efficiency.

iii. The laser output can be easily increased by increasing the junction current.

iv. It is operated with less power than ruby and CO2 lasers.

v. It requires very little additional equipment.

vi. It emits a continuous wave output or pulsed output.

Disadvantages

i. Laser output beam has large divergence.

ii. The purity and monochromacity are poor.

iii. It has poor coherence and stability.

Applications of Laser Diode

i. Used in fibre optic communication.

ii. Used in various measuring devices such as range finders, bar-code readers.

iii. Used in printing industry both as light sources for scanning images and for
resolution printing plate manufacturing.

iv. Infrared and red laser diodes are common in CD players, CD-ROM and DVD
technology. Violet lasers are us ed in HD-DVD and Blue-ray technology.

v. High power laser diodes are used in industrial applications such as heat treating,
cladding, steam and for pumping other lasers.

vi. Used in laser medicine especially, dentist

ZENER DIODE

8. Explain the construction & working principle of Zener diode.

Explain the Break down mechanisms in semiconductor devices.

(OR) Explain the Concept of Zener Breakdown and its VI characteristics.

The Zener Diode is a PN junction semiconductor device.

It is fabricated with precise breakdown voltages, by controlling the doping level
during manufacturing. Practically, Zener Diodes are operated in reverse biased
mode.

Fig. Zener Diode

CHARACTERISITCS OF ZENER DIODE:

FORWARD CHARACTERISITCS:
In forward biased condition, the normal rectifier diode and the Zener diode
operate in similar fashion.
(Refer: PN diode forward characteristics)

Zener reverse characteristics

REVERSE CHARACTERISITCS:

 Zener diode is designed to operate in the reverse biased condition.

 In reverse biased condition, the diode carries reverse saturation
current till the reverse voltage applied is less than the reverse
breakdown voltage.

 When the reverse voltage exceeds reverse breakdown voltage,

the current through it changes severelybut the voltage across it
 remains almost constant.

 Such a breakdown region is a normal operating region for a Zener

diode.

 The normal operating regions for both diode and Zener are shown in
below Fig.

Fig. The normal operating region for a rectifier diode and Zener diode

 When the applied reverse voltage is increased then, the current through it
is very small (few µA) and it is called Reverse Leakage Current (Io)
 At certain reverse voltage, the current will increase rapidly. The
breakdown occurs and the current at this point (knee or Zener knee) is
called Zener knee current (IZK or IZmin).
 Zener knee current is the minimum Zener current which is must to carry
out the operate in Reverse Breakdown Region.
 The reverse voltage at which the breakdown occurs is called Zener
Breakdown Voltage or Zener Voltage (VZ).
 Below the knee, the reverse breakdown voltage increases slightly as
Zener current (IZ) increases but, remains almost CONSTANT.
 As the current increases, the power dissipation (P Z = VZ IZ) will be
increased and if this power dissipation is increased beyond a certain current
 value, the Zener diode may get damaged. So, there is a maximum current
that a Zener diode can carry safely is called Zener Maximum Current (IZM
or IZmax).
 In practical circuits, a current limiting resistor is used in series with Zener
diode in order to limit the current between IZmin to IZmax.
 The complete VI characteristics of Zener Diode is shown in Fig.

Fig. VI characteristics of Zener Diode

EQUIVALENT CIRCUIT OF ZENER DIODE:

 When the breakdown occurs then IZ may increase from IZmin

to IZmax but voltage across Zener remains almost constant. The
internal impedance decreases as current increases in Zener
region.
 But this impedance is very small and hence ideally Zener diode
is indicated by a battery of voltage VZ. This VZ remains
almost constant in the Zener region which is shown in Fig.
Fig. Ideal equivalent circuit of Zener diode

 In practical circuit, the Zener internal resistance is to be

considered (even though it is very small) and called as Zener
Dynamic Resistance ZZ. Due to this resistance the Zener
 region is not exactly vertical, i.e., for the small change in the
Zener current ∆IZ produces a small change in Zener voltage
∆VZ.
 The ratio of VZ to IZ is called Zener resistance ZZ.
 Hence, the practical Zener diode equivalent circuit should be
indicated with a battery of V Z along with series resistance ZZ
as shown in Fig.
∆𝑉𝑧
Dynamic resistance, 𝑅𝑧 = ∆𝐼𝑧

= = 1⁄∆∆𝑉𝐼𝑧𝑧
1
=𝑅𝑧 = 𝑆𝑙𝑜𝑝𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑒𝑣𝑒𝑟𝑠𝑒 𝑐ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐𝑠 𝑖𝑛 𝑟𝑒𝑣𝑒𝑟𝑠𝑒 𝑟𝑒𝑔𝑖𝑜𝑛

BREAKDOWN MECHANISM IN ZENER DIODE:

Two distinct breakdown mechanism:

 Zener Breakdown
 Avalanche Breakdown

 For devices with breakdown voltage less than 5V - Zener Breakdown

For devices with breakdown voltage between 5V and 8V - Zener Breakdown
and Avalanche Breakdown

For devices with breakdown voltage above 8V - Avalanche Breakdown

ZENER BREAKDOWN:
 Zener breakdown occurs at Reverse biased condition because of heavy
doping;
 Practically, Zener breakdown is observed in the Zener diodes with
breakdown voltage less than 6V.
 In Zener breakdown, the value of the breakdown voltage
decreases as PN junction temperature increases, i.e. Negative
Temperature Coefficient (NTC)
 For applied reverse biased voltage of less than 6V causes a high
magnitude electric field (3 X 105 V/cm) across the depletion region, at
the PN junction.
 This electric field applies a large force on the valence electron of the
atom, tending it to separate them from their respective nuclei.
 Electron-hole pairs are generated in large numbers and there will be a
sudden increase in current. (To limit this current, a current limiting
resistor is used in order to protect the Zener diode from being
destroyed because of excessive heating at the junction)
 AVALANCHE BREAKDOWN:
 Avalanche Breakdown occurs at Reverse biased condition due to ionization
of electron and hole pairs Practically, Avalanche breakdown is observed
in the Zener diodes with breakdown voltage greater than 6V.
 In avalanche breakdown, the value of the breakdown voltage increases as
PN junction temperature increases, i.e. Positive Temperature Coefficient
(PTC)
 For applied reverse biased voltage of greater than 6V causes increased
acceleration of minority charge particles. Thus, collision between
accelerated charge particles with high velocity and kinetic energy with
adjacent atom is involved in breaking the covalent bonds of the crystal
structure. This process is called Carrier Multiplication.
 At this stage, junction is said to be in breakdown and current starts
increasing rapidly. To limit this current below IZmax, a current limiting
resistor is necessary.
 Fig. Breakdown
Mechanism in Zener
Diode

8. (a) Explain the working of a Zener diode as a regulator?

Zener Diodes are widely used as Shunt Voltage Regulators to

regulate the voltage across small loads. Zener Diodes have a sharp
reverse breakdown voltage and breakdown voltage will be constant
for a wide range of currents. Thus, we will connect the Zener diode
parallel to the load such that the applied voltage will reverse bias it.
Thus, if the reverse bias voltage across the zener diode exceeds the
knee voltage, the voltage across the load will be constant.
 Fig. VI characteristics of Zener Diode

The Zener Diode is used to regulate the Load Voltage. Here, the Zener is used in
reverse biased condition
Under reverse biased condition, the current through the zener diode is very small
of the order of few µA, up to certain limit.
When enough reverse bias voltage is applied, electrical breakdown occurs and
large current flows through the zener diode. The voltage at which the
breakdown occurs is called
Zener Voltage (VZ).
Under this condition, whatever may be the current, the voltage across the
Zener is constant and equal toVZ}
Since, voltage across the Zener Diode is CONSTANT & equal to VZ, it is
connected across the load.
⸫ The Load Voltage (Vo) is equal to Zener Voltage (VZ).
i.e. The Zener Diode acts as an ideal voltage source which maintains a
constant load voltage, independent of the current.
REGULATION WITH VARYING INPUT VOLTAGE (Line Regulation)
Zener Regulator under
varying input voltage
condition is shown in Fig.

Vo = VZ is constant
 𝑉0 𝑉𝑍
𝐼𝐿 = 𝑅 = 𝑅𝐿
= 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝐿

And 𝐼 = 𝐼𝑍 + 𝐼𝐿

As long 𝑰𝒁 is between 𝑰𝒁𝒎𝒊𝒏 and 𝑰𝒁𝒎 , the VZ i.e. output voltage

Vo is constant. Thus, the changes in the input voltage is get
compensated and output is maintained constant.
The maximum power dissipation for the zener diode is fixed, 𝑷𝑫 = 𝑽𝒁
𝑰𝒁𝒎𝒂𝒙

REGULATION WITH VARYING LOAD (Load Regulation)

Zener Regulator under varying load condition (R L is variable) and

constant input voltage (Vin is constant) is shown in Fig.

Fig. Varying load

condition

Vo = VZ is constant and Vin is Constant, then for constant R,

the current (I) is constant

𝑉 −𝑉
𝐼 = 𝑖𝑛 𝑍 =
𝑅
Constant
As long 𝑰𝒁 is between 𝑰𝒁𝒎𝒊𝒏 and 𝑰𝒁𝒎 , the VZ i.e. output
voltage Vo is constant. Thus, the changes in the load is get
compensated and output is maintained constant.