➢ Review question
1.What is a p-n junction diode? How does a barrier field appear across a p-n
junction?
When one side of single-crystal semiconductor is doped with acceptors and the
other side is doped with doners, a p-n junction is produced
The positive and the negative uncovered charges generate electric field across
the junction directed form n-side to p-side
2.For unbiased p-n junction, sketch the variation of the space charge, electric
field, electrostatic potential, and electron energy as a function of distance
across the junction.
3.What is an abrupt and a linearly graded p-n junction?
In a step-graded or abrupt p-n junction, the acceptor and the donor
concentration are constants up to the junction
In a linearly-graded junction, the impurity concentration varies almost linearly
with distance from the junction
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4.(a)Discuss the operation of p-n junction diode.
1) The electrons diffuse through the junction to the p-side and the
holes to n-side
2) The electrons and the holes combine with each other
3) As a result, the acceptor ions near the junction in the p-side and
the donor ions near the junction in the n-side are left
unneutralized ions (uncovered charges)
4) The positive and the negative uncovered charge generate electric
field across the junction called the barrier field
5) The barrier field opposes the diffusion of electrons and holes
through the junction
6) Equilibrium is established when the electric field is sufficient to
prevent further diffusion of electrons and holes
(b)Mention some application of semiconductor diodes.
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Rectifiers
Clipper circuits
Clamping circuits
Reverse current protection circuits
5.Draw the energy-band diagram of an open-circuited p-n junction
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Explain the following terms:
Uncovered charges
When The electrons and the holes combine with each other, the acceptor ions
near the junction in the p-side and the donor ions near the junction in the n-side
are left unneutralized ions. The unneutralized ions are termed uncovered
charges
Barrier field
The positive and the negative uncovered charge generate electric field across
the junction directed from n-side to p-side. The direction of this field is called
barrier field
Depletion region
When the barrier field prevent further diffusion of
electrons and holes, at this situation there no movement of charge carriers
across the junction. Since the neighbourhood of the junction is depleted of
mobile charge, it is referred to as the depletion region
6.(a) "The barrier potential across a p-n junction diode cannot be measured
by placing a voltmeter across the diode terminals". Explain
7.When is a p-n junction said to be (i)forward-biased,(ii) reversed-biased? If
the biasing voltage is the same, will the same current flow under both
forward and reverse bias conditions? If not, why?
A p-n junction is said to be forward-biased when the positive pole of a
battery is connected to the p-side and the negative pole to the n-side of the
junction
A p-n junction is said to be reverse-biased when the positive pole of a
battery is connected to the n-side and the negative pole to the p-side of the
junction
No, the current will not the same
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In the forward-biased: the barrier energy is reduced and allowing a large
current to flow across the junction
In the reverse-biased: the barrier energy is enhanced and the small current
can flow across the junction
8.(a)Why dose most of the battery voltage applied across the diode
terminals appear across the depletion region of a p-n diode?
If a positive voltage applied across the junction , it can supply free electrons
and holes with extra energy they require to cross the junction. As the width
of depletion layer is decreased.
By applying a negative voltage results in the free charges being pulled away
from the junction. As the width of depletion layer is increased.
(b)How are the width of the space-charge region and the barrier height
affected when a p-n junction is (i)forward-biased, (ii)reverse-biased?
In forward-biased: the width of space charge and the barrier energy is
diminished
In reverse-biased: the width of space charge and the barrier energy increase
9. What is the origin of the reverse saturation current in a p-n junction? Dose
the reverse saturation current change with the applied reverse bias and the
diode temperature? Explain
The number of minority carriers moving down the potential barrier is left
unaffected since this number depends on the temperature only, as a result a
small current called reverse saturation current(Is) flows.
The reverse saturation current is independent of the reverse bias,
but increase with the rise of temperature
10.(a)Draw the circuit diagrams of a forward-biased and a reverse-biased pn junction. Write the expression for the volt-ampere characteristics of the
diode. Draw the characteristics curve and explain its natural.
Forward-biased
reverse-biased
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the expression for the volt-ampere characteristics of the diode:
the characteristics curve:
▪ When the forward bias v is less than a value v γ ,the current is very
small. As v exceeds v γ The current increases sharply
▪ When the diode is reverse bias, there is a small reverse current which
independent of the applied bias and equals the reverse saturation current.
At reverse voltage corresponding to the point p, the reverse current
increases abruptly almost parallel to the current axis. The portion PQ is
called the break down region.
(b)Drive the current-voltage relationship for p-n junction.
11.Define the static resistance and dynamic resistance of p-n diode. Are they
equal? If not, why?
Do these resistance depend on temperature and bias voltage?
The static resistance (rdc):
The ratio between the voltage V across the junction and the current I flowing
through the junction
The dynamic resistance(rac):
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The inverse of the slope of the volt-ampere characteristic of the p-n diode
No, they are not equal, because rdc=dc voltage/ dc current ,
rac=change in voltage/change in current
Yes, they depend on temperature and bias voltage
12.(a)Define the cutin voltage of a p-n junction. What are its typical values for
Ge and Si diodes?
The voltage at which the forward diode current starts increasing rapidly( for
Ge=0.2 v , for Si=0.6 v )
(b)what do you mean by the transition capacitance of p-n junction? Dose it
depend on the depletion region width and the applied reverse voltage?
When p-n junction is reverse biased the depletion region act as an insulator and
the p-type and n-type region have low resistance and acts as the plates. Thus
this p-n junction can be considered as a parallel plate capacitor. This junction
capacitance is called as transition capacitance.
Yes, it dose, the depletion region increases with the increase in reverse bias
voltage the resulting transition capacitance decreases.
(c)what is the diffusion capacitance?
Under forward bias, holes are injected into the n-side where they become
minority carriers, and electrons are injected into the p-side where they become
minority carriers. The injected minority concentration decreases as we move
away from the junction. This injected charge on either side of the junction gives
rise to a capacitance, termed the diffusion or storage capacitance
(d)Explain the use of p-n diodes as varistors.
Used as safety devices to prevent excess transient voltage in the circuit
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(e) A p-n diode has threshold voltage and a forward resistance Rf .Draw its
piecewise linear volt-ampere characteristics and the equivalent circuit.
(f)Draw the volt-ampere characteristics of an ideal diode
(e) ‘A p-n junction is a nonlinear unilateral element.’_ Explain
A p-n junction is a nonlinear unilateral element as behaves differently under
forward and reverse bias conditions
13.What is a breakdown diode? Discuss the origin of breakdown of a junction.
Breakdown diodes are p-n junction diodes operated in the breakdown region
In reverse biased p-n junction. If the reverse voltage applied on the diode is
increased, the width of depletion region increases. This depletion region blocks
the majority carrier current. However, it allows the minority carrier current. The
width of the depletion region increases with the increase in voltage only up to
certain value. If the reverses voltage is increased beyond that value, the junction
breaks down and allow a large reverse current.
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14.(a)What is the difference between avalanche breakdown and Zener
breakdown of a p-n junction?
Avalanche Breakdown
Zener Breakdown
Occurs due to collision of electrons
moving at high speed
Occurs in a junction having a wide
depletion region
Occurs when the high electric field is
created across the junction
Occurs in a junction having a narrow
depletion region
The avalanche breakdown
voltage(>6 v) increases with a rise of
temperature
The temperature coefficient of the
avalanche breakdown is positive
The Zener breakdown voltage(<6 v)
decreases with a rise of temperature
The temperature coefficient of the
Zener breakdown is negative
(b) What is the difference between an ordinary semiconductor diode and a
Zener diode?
Ordinary diode
Zener diode
Conducts the current in forward bias
only and get damaged when used in
reverse bias conditions
Used as rectifier
Conducts in forward and reverse bias
Used as voltage regulator
15.(a) ‘Zener breakdown voltage has a negative temperature coefficient
whereas the avalanche breakdown voltage has a positive temperature
coefficient’. Explain
Zener breakdown:
With a rise of temperature, the energy of the valance electron increases. So that
a lower applied voltage can pull theses electrons out of their covalent bonds, so
the Zener breakdown voltage(<6 v) decreases with increasing temperature
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avalanche breakdown:
with a rise of temperature, the crystal ions vibrate with a greater amplitude,
increasing the possibility of collisions of the carriers and the crystal ions. So that
The avalanche breakdown voltage(>6 v) increases with a rise of temperature
(b) ‘the dynamic resistance of an ideal Zener diode is zero but the dc resistance
is not so’. Why?
Dynamic resistance (rz)=change in vz /change in Iz
In ideal Zener diode: change in vz is zero ,where the Zener characteristics is
parallel to the current axis . So that rz=0
Static resistance(rdc)=vz/Iz , So that rdc is not zero
(c)Give the equivalent circuit of (i)an ideal Zener diode and (ii)a nonideal Zener
diode.
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16.Explain with a circuit diagram the use of a Zener diode as a reference diode.
The voltage vz across the load resistance RL remains fairly constant although the
supply voltage V and the load current IL can vary.
17.Write short notes on:
(i)current components in a p-n junction
(ii)Zener diode and its use
Zener diode is a silicon semiconductor device that permits current to flow in
either a forward or reverse direction, designed to conduct in the reverse
direction when a certain specified voltage is reached.
Zener diode is used for voltage regulation ,as reference element
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