ELECTRONICS ENGINEERING REVIEW
ELECTRONICS: DIODES AND APPLICATIONS
SEMICONDUCTOR PHYSICS
Intrinsic Semiconductor
Bohr’s Atomic Model – a simplified representation of
arrangement of sub-atomic particles in an atom.
- made up of semiconductor material in its purest form.
Pure Silicon crystal
2
No. of electrons in a shell: N = 2n
Energy difference between shells:
E = hf
Energy Bands
1. Conduction band – energy band where free
electrons exist
2. Valence band – energy band where outermost
electrons are located.
3. Forbidden band – located between two energy
bands where no electrons exist
Intrinsic carrier concentration: ni = Ne
Energy gap, Eg – difference in energy between two bands
(e.g. conduction and valence bands)
Current density: J = ni e ( e + h ) E
Electrical Classification of Materials
Intrinsic Carrier Concentration
Insulator – material in which electric charges do not flow
freely.
o
o
o
Valence electrons > 4
Energy gap > 5 eV
Conductivity: very low
−
Eg
2kT
Semiconductor
Electrons/cm3
Silicon
1.5 x 1010
Germanium
2.5 x 1013
Extrinsic Semiconductor
Conductor – material in which electrical charges move easily.
o
o
o
Valence electrons < 4
Energy gap ≈ 0 eV
Conductivity: very high
Semiconductor – has electrical properties between that of
conductor and insulator.
o
Valence electrons = 4
o
Energy gap ≈ 1 eV
o
Conductivity: between insulator
and conductor
- semiconductor material with impurities added through
doping.
N-type = Semiconductor + Pentavalent
Carriers
Symbol:
Construction:
P-type = Semiconductor + Trivalent
Biasing
•
Forward biasing – P connected to positive terminal of
source voltage, N to negative terminal.
Carriers
•
Electrical Parameters for Semiconductors
Reverse biasing - P connected to negative terminal of
source voltage, N to positive terminal.
Drift velocity: vd = e E
Current: I = neAe E
Conductivity:
 = qn
Current density
N-type:
Jn = e ( nn e + pn h ) E
P-type:
Jp = e np e + pp h E
(
)
Equilibrium condition (Mass Action Law) – the product of
the number of electrons in the conduction band and the
number of holes in the valence band is constant at a fixed
temperature and is independent of amount of donor and
acceptor impurity added.
ni 2 = np pp
SEMICONDUCTOR DIODE
- two-terminal device consisting of an N-type and P-type
material joined together.
Diode Currents
Shockley’s Equation for diode forward current
 kVD

ID = IS  e T − 1 




where
k=
11,600
n
For low levels of diode current, n = 1 for Ge and n = 2 for Si.
For high levels of diode current, n = 1 for both Si and Ge.
Temperature effects:
Reverse Current: IS1
T −T /10
= IS 0 2( 1 0 )
VTh1 = VTh0 + k (T1 − T0 )
Threshold voltage:
Where:
k = −2.5mV / oC for Ge
Q-factor: Q = 0.159
fRsCt
Max power dissipation: PZ max = 4IZT VZ
BASIC ZENER CIRCUITS
Fixed Vi and RL
k = −2.0mV / oC for Si
Diode Models and Characteristics
1.
Ideal Diode Model – diode acts as switch.
2.
Simplified Diode Model – threshold voltage considered
Load voltage: V = RLVi = V
L
Z
R + RL
Zener current: IZ = IR − IL
Power dissipation: PZ = I Z VZ
Fixed Vi , variable RL
3.
Linear Diode Model – internal resistance and threshold
voltage considered (for exact calculations)
RVZ
Min load resistance: R
Lmin =
Vi − VZ
Diode Resistances
•
DC Resistance: R = VD
D
ID
•
Dynamic resistance: r = 26mV
d
ID
•
VD
Ave resistance: r
ave =
ID
VZ
Max load current: I
Lmax =
RLmin
Min load current: ILmin = IR − IZM
VZ
Max load resistance: R
Lmax =
ILmin
Variable Vi , fixed RL
Power Dissipation: PDmax = IDVD
Reverse-recovery time – time needed for current to change
direction when diode is switched from forward to reverse
conditions.
trr = tr + t s
SPECIAL PURPOSE DIODES
Zener - diode used as a voltage reference.
Symbol:
Temperature coefficient:
TC =
VZ
VZ (T1 − T0 )
Min input voltage:
Vi min =
( R + RL )VZ
RL
Max series current: IR max = IZM + IL
Max input voltage: Vi max = IR max R + VZ
Point contact diode
Very high capacitance
Schottky diode
Metal-conductor rectifier
Tunnel diode
Negative resistance
Gunn diode
Wideband diode
PIN diode
Offers impedance at microwave
LED
Emits light
Photodiode
Current varies with light
Thyrector
Bidirectional trigger device
Light Emitting Diode (LED)
- diode which emits light when forward biased.
Light wavelength:  = c
f
Energy: Eg =
hc

LED Color Construction:
Negative resistance – current decreases as voltage increases
across the device (opposite of Ohm’s Law).
Gunn effect – production of rapid fluctuations of current
when the voltage applied to a semiconductor device exceeds
a critical value with the result that microwave power is
generated.
DIODE APPLICATIONS
Rectification – process of converting AC to pulsating DC.
Important quantities:
Varactor
Ripple factor: r = Vr (rms)
Vdc
- diode with variable capacitance.
Ripple voltage: Vr (rms) = Vrms2 − Vdc 2
Capacitance:
CT =
where
Half-wave Rectifier
k
(VT + VR )n
1
for alloy junction
2
1
n = for diffused junction
3
n=
In terms of np bias capacitance: C =
T
Temperature Coefficient: TC
=
C (0)
(1 + VR / VT )
n
C
C0 (T1 − T0 )
Special Purpose Diodes
Diode
Characteristics
Varactor
Variable capacitance
Average value:
Ideal: Vdc = 0.318Vm
Practical: Vdc = 0.318 (Vm − Vth )
Peak Inverse Voltage: PIV = Vm
Output frequency: fout = fin
Full-wave Rectifier
Clipper
- circuit which clips a portion of the input signal.
Positive Clipper
Bridge type
Negative Clipper
Average value:
Ideal: Vdc = 0.636Vm
Practical: Vdc = 0.636 (Vm − 2Vth )
Clamper
- shifts the input signal to a different dc signal.
Peak Inverse Voltage: PIV = Vm
Output frequency: fout = 2 fin
Center-tapped
Note: The peak-to-peak values for the input and output
signals are equal.
Voltage Multiplier
- a circuit which gives an output that is an integral multiple of
the input.
Voltage Doubler
Voltage Tripler
Average value:
Ideal: Vdc = 0.636Vm
Practical: Vdc = 0.636 (Vm − Vth )
Peak Inverse Voltage: PIV = 2Vm
Output frequency: fout = 2 fin
Review Questions
1.
2.
3.
A diode (P-N junction) has two
terminals. The P-terminal is known as
A. Anode*
B. Cathode
C. Either anode or cathode
D. None of these
9.
A working diode must have
A. High resistance when forward or
reverse biased
B. Low resistance when forward
biased, while high resistance when
reverse bias*
C. High resistance when forward
biased, while low resistance when
reverse bias
D. Low resistance when forward or
reverse biased
10. When a junction diode is Forward
Biased the thickness of the depletion
region is
A. Larger
C. Smaller*
B. Medium
D. None of these
Electrons in p-type material of a semiconductor are called as
A. either minority carriers or majority
carriers
B. minority carriers*
C. majority carriers
D. valance carriers
4.
A diode (p-n junction) when reverse
biased act as
A. On Switch
C. Zener diode
B. Capacitor
D. Off Switch*
5.
Depletion region of a p-n junction is
formed
A. During Reverse bias
B. During Forward bias
C. During Manufacturing*
D. During Heating
6.
7.
8.
A zener diode voltage regulator has
load requirement of 12 V and 2 Amp.
The zener diode's
minimum current requirement is 0.2
A. The minimum voltage at input is 24
V. What is maximum efficiency of
circuit?
A. 34.3 %
C. 45.5 %*
B. 52.8 %
D. 66.3 %
A 3 - phase diode bridge rectifier is
fed from a 400 V rms, 50 Hz, 3 phase
AC source. If the load is purely
resistive, then peak instantaneous
output voltage is equal to
A. 400√2 V*
C. 400 V
B. 400√(2/3) V
D. 400 / √3 V
A LED produces light when
A. forward biased* C. reverse biased
B. unbiased
D. none of these
A Schottky diode is a
A. majority carrier device*
B. minority carrier device
C. fast recovery diode
D. both a majority and a minority
carrier diode
11. If temperature increases then
conductivity of a semiconductor is
A. Decreases
C. Increases*
B. Constant
D. None of these
12. A semiconductor has a
A. Negative Temperature coefficient
of resistance*
B. Positive Temperature coefficient of
resistance
C. Constant Temperature coefficient
of resistance
D. None of the above
13. The leakage current in a pn junction is
in order of
A. A
C. kA
B. µA*
D. None of these
14. N-type extrinsic semiconductor is
obtained by adding
A. trivalent impurity
B. pentavalent impurity*
C. tetravalent impurity
D. all of the above
15. The amount of time between the
creation and disappearance of a free
electrons is called
A. recombination
B. bound electrons
C. drift velocity
D. life time of the carriers*
16. Single phase, 230 V, 1 KW heater is
connected across single phase 230 V,
50 Hz supply through a diode. The
power delivered to the heater
element
A. 100 W
C. 1000 W
B. 500 W*
D. None of above
17. For a diode, reverse recovery time is
defined as the time between the
instant diode current becomes zero
and the instant reverse recovery
current decays to
A. 0
B. 10% of reverse peak current
C. 25% of reverse peak current*
D. 15% of reverse peak current
18. When the pn junction is forward
biased the sequence of events that
take place are
A. diffusion, drift and recombination
B. injection, diffusion and
recombination*
C. diffusion, injection and drift
D. none of above
19. The depletion region of a pn junction
is one, that is depleted of
A. atoms
B. mobiles charges*
C. immobile charges
D. velocity of the carriers
20. The junction capacitance of linearly
graded junction varies with the
applied reverse bias, VR as
A. (VR)- 1
C. (VR)- ½
B. (VR)- 1/3*
D. (VR)1/2
21. The diffusion capacitance of a
forward biased P+N
junction diode with a steady electric
current I depends on
A. width of the depleted region
B. mean life time of the holes
C. mean life time of the electrons*
D. junction area
22. Avalanche breakdown in
a diode occurs when
A. potential barrier is reduced to zero
B. forward electric current exceeds
certain value
C. reverse bias exceeds a certain
value*
D. none of the above
23. Which are the mobile charge carriers
present in semiconductor diode?
A. Holes
C. Neutrons
B. Electrons
D. Both A & B*
24. If the series resistance increases in an
unloaded zener regulator, the zener
current
A. decreases*
B. stays the same
C. increases
D. equals the voltage divided by the
resistance
25. The load voltage is approximately
constant when a zener diode is
A. forward biased
B. reverse biased
C. operating in breakdown region*
D. unbiased
26. When the source voltage increases in
a zener regulator, which of
these electric current remain
approximately constant?
A. Series current C. Zener current
B. Load current* D. None of these
27. A silicon material has an intrinsic
concentration ni=1010 per cubic
centimeter at room temperature. If it
is doped with 1015 antimony atoms
per cubic centimeter, what is now the
approximate electron concentration
at the conduction band?
A. 105 electrons
C. 1010 electrons
B. 1015 electrons* D. 1020 electrons
B. decrease
C. decrease exponentially
D. increase*
34. As the operating temperature of a
reverse-biased diode is increased, its
leakage or reverse saturation current
will
A. Increase
B. increase exponentially*
C. decrease
D. decrease exponentially
35. For a silicon diode, calculate the
current at room temperature if the
forward voltage VF = 0.3 V and the
reverse saturation current IS = 100
nA.
A. 32.8 μA*
C. 10.8 μA
B. 32.8 mA
D. 10.8 mA
28. An external voltage applied to a
junction reduces its barrier and aid
current to flow through the junction
A. reverse bias
C. external bias
B. junction bias
D. forward bias*
36. Calculate the new threshold voltage
of a germanium diode when it
operates at 100 0C.
A. 0.113 V*
C. 0.188 V
B. 0.215 V
D. 0.513 V
29. A device containing an anode and a
cathode or a pn junction of a
semiconductor as the principal
elements and provides unidirectional
conduction.
A. diode*
C. diac
B. triode
D. triac
37. A silicon diode has a reverse
saturation current of 50 nA at room
temperature. If the operating
temperature is raised by 50°C, what is
now the reverse saturation current?
A. 105.56 nA
C. 287.73 nA
B. 827.89 nA
D. 1.66 µA*
30. What do you call the very small
amount of current that will flow in
the diode when it is reverse biased?
A. saturation current
B. reverse saturation current*
C. cut-off current D. holding current
38. In every increase of 10°C in the
operating temperature of a diode will
cause its reverse saturation current to
A. decrease
C. double*
B. triple
D. quadruple
31. The minimum voltage required
before a diode can totally conduct in
a forward direction.
A. triggering voltage
B. breakdown voltage
C. saturation voltage
D. threshold voltage*
32. What will happen to the threshold
voltage of the diode when it operates
at higher temperatures?
A. increases
B. increases exponentially
C. decreases*
D. decreases exponentially
33. The forward current in a conducting
diode will _____ as the operating
temperature increases.
A. not be affected
39. When a diode is reverse biased the
depletion region widens, since it is in
between positively charge holes and
negatively charge electrons, it will
have an effect of a capacitor, this
capacitance is called what?
A. diffusion capacitance
B. storage capacitance
C. stray capacitance
D. transition capacitance*
40. A diode that is especially designed to
operate as a voltage-variable
capacitor. It utilizes the junction
capacitance of a semiconductor
diode.
A. varactor
C. varicap
B. varistor
D. A and C *
41. The capacitance of a varactor will
_______ when the forward bias
voltage is increased.
A. increase*
B. decrease
C. exponentially decrease
D. not change
42. In operating a diode at high-speed
switching circuits, one of the most
important parameters to be
considered is
A. ac resistance
B. diode capacitance
C. noise figure
D. reverse recovery time*
43. The maximum power the diode can
handle.
A. maximum derating power
B. maximum consumption power
C. breakdown power
D. maximum dissipation power*
44. What will happen to the power
handling capability of the diode if it is
to be operated at a higher
temperature?
A. decreases*
B. increases
C. increases exponentially
D. will not be affected
45. A certain diode has a maximum
power dissipation of 500 mW at room
temperature and a linear power
derating factor of 5.0 mW/°C. How
much power the diode can handle if
operated at 50°C.
A. 625 mW
C. 505 mW
B. 495 mW
D. 375 mW*
46. Refers to a special type of diode
which is capable of both amplification
and oscillation.
A. Junction diode
B. Tunnel diode*
C. Point contact diode
D. Zener diode
47. Diode whose negative resistance
depends on a specific form of
quantum-mechanical bond structure
of the material
A. Gunn diode*
B. tunnel diode
C. TRAPATT diode
D. backward diode
48. What semiconductor diode that have
a fine wire (called a cat-whisker)
whose point is in permanent contact
with the surface of a wafer of
semiconductor material such as
silicon, germanium or gallium
arsenide?
A. point-contact diode*
B. diac
C. PiN diode
D. thyrector
49. The appearance of RF current
oscillations in a dc-biased slab of ntype gallium arsenide in a 3.3 kV
electric field
A. Gunn effect*
C. Hall effect
B. Zener effect
D. avalanche
50. What will happen to the magnitude
of the load-line slope when the load
resistance is decreased?
A. it will also decrease
B. it will increase*
C. it will increase exponentially
D. is not affected by the load
51. A germanium diode is connected to a
load resistance of 1.5 kΩ and is
supplied with 12-V such that the
diode will be forward biased. What is
the voltage across the diode?
A. approximately 12 V
B. approximately 0.7 V
C. approximately 0.3 V*
D. can’t be solved
52. What is the drop across the diode
when it is connected in series to a
resistor of 1.8 kΩ and a supply
voltage of 50 V? The supply voltage
causes the diode to be reversebiased.
A. 50 V*
C. 0.7 V
B. 0.3 V
D. can’t be solved
53. Two germanium diodes are
connected in series and have a load
resistance of 10 kΩ and a forward
supply voltage of 5 V. Calculate the
voltage across the load resistor.
A. 4.7 V
C. 4.4 V*
B. 0.6 V
D. 0.3 V
54. A silicon diode is in parallel with a
germanium diode and is connected to
a load resistor having a value of 20 kΩ
and a forward supply voltage of 10 V.
What is the approximate voltage
across the silicon diode?
A. 10 V
C. 1.0 V
B. 0.7 V
D. 0.3 V*
55. At room temperature, in a perfect
silicon crystal, the equilibrium
concentration of thermally generated
electrons in the conduction band is
about
A. 1.5 x 105 per cubic cm
B. 1.5 x 1010 per cubic cm*
C. 1.5 x 1015 per cubic cm
D. 1.5 x 1020 per cubic cm
56. What is the output voltage across a
load resistor if it is paralleled with a
forward biased silicon diode? The
resistor network is supplied with 10
V.
A. 0.7 V*
C. 9.3 V
B. 10 V
D. lack of data
57. How many capacitors are used in a
diode-capacitor half- wave voltage
doubler?
A. 1
C. 2*
B. 3
D. 4
58. In a diode-capacitor voltage
quadrupler, what is the voltage across
the third stage capacitor?
A. Vmax
C. 2 Vmax*
B. 3 Vmax
D. 4 Vmax
59. A light emitting diode (LED) is to be
used in a circuit with a supply voltage
of 5 V. What should be the value of
the resistor needed by the LED to
operate normally?
A. 25 Ω
C. 250 Ω*
B. 25 kΩ
D. 250 kΩ
60. In a semiconductor device, a p-n
junction formed by alloying a suitable
material such as indium with the
semiconductor.
A. alloy junction*
B. diffused junction
C. depletion junction
D. storage junction
61. Which of the following is not a valid
form of a diode equivalent circuit?
A. Piecewise Linear Model
B. Ideal Diode Model
C. Simplified Model
D. Differential Model*
62. Which model of the diode equivalent
circuit is represented by the given
diagram?
A. Piecewise Linear Model*
B. Ideal Diode Model
C. Simplified Model
D. Differential Model
63. AC resistance of a diode was found to
be r1 and r2, when measured with two
different values of diode current i.e.
10 mA and 25 mA respectively, for
the same diode voltage. Which of the
following options hold true?
A. r1 = r2
B. r1 > r2*
C. r1 < r2
D. Can’t be determined
64. The reverse saturation current for a
Germanium diode at a temperature
of 293 K is found to be 2 μA. What is
the reverse saturation current Is at a
temperature of 313 K?
A. 2 μA
B. 8 μA*
C. 4 μA
D. Can’t be determined
65. The maximum load current that can
be drawn is
A. 1.4 mA*
B. 1.8 mA
C. 2.3 mA
D. 2.5 mA
66. For the circuit shown diode cutting
voltage is Vin = 0. The ripple voltage is
to be no more than vrip = 4 V. The
minimum load resistance, that can be
connected to the output is (in kilo
ohm)
A. 6.25*
B. 25
C. 12.5
D. 30
67. An ideal diode has ____ & _____
A. some forward voltage drop, some
reverse recovery time
B. high switching losses, high reverse
voltage drop
C. no forward voltage drop, negligible
reverse recovery time*
D. no reverse recovery time, high
leakage current
68. The figure below shows a circuit for
charging a 12-V battery. If Vs is a
sinusoid with 24-V peak amplitude,
the fraction of each cycle during
which the diode conducts is
A. One quarter of a cycle
B. One-third of a cycle*
C. One half of the cycle
D. Three quarters of a cycle
69. Diodes can be used in the making of
A. Rectifiers
B. LED lamps
C. Logic gates
D. All of the mentioned*
70. For the connections shown below,
the equivalent logic gate is
A. OR gate*
B. XOR gate
C. AND gate
D. NAND gate
71. For the connections shown below,
the equivalent logic gate is
73. A zener diode works on the principle
of_________
A. tunneling of charge carriers across
the junction*
B. thermionic emission
C. diffusion of charge carriers across
the junction
D. hopping of charge carriers across
the junction
74. When the voltage across the zener
diode increases_________
A. temperature remains constant and
crystal ions vibrate with large
amplitudes
B. temperature increases and crystal
ions vibrate with large amplitudes*
C. temperature remains constant and
crystal ions vibrate with smaller
amplitudes
D. temperature decreases and crystal
ions vibrate with large amplitudes
75. Avalanche breakdown in zener diode
is ______
A. electric current multiplication
takes place*
B. phenomenon of voltage
multiplication takes place
C. electrons are decelerated for a
period of time
D. sudden rise in voltage takes place
76. If the positive terminal of the battery
is connected to the anode of the
diode, then it is known as
A. Forward biased*
B. Reverse biased
C. Equilibrium
D. Schottky barrier
77. Ideally the voltage drop across a
conducting diode must be
A. ∞
B. 0*
C. higher than the forward biased
voltage
D. equal to the forward biased
voltage
A. OR gate
B. XOR gate
C. AND gate*
D. NAND gate
72. The units frequently used to measure
the forward bias and reverse bias
current of a diode are
A. µA and µA respectively
B. µA and mA respectively
C. mA and µA respectively*
D. mA and mA respectively
78. Schottky diodes are also called as
A. metal diode
B. hot carrier diode*
C. signaling diode
D. easy turn on diode
79. In a Schottky diode, the aluminum
metal acts as a __________
A. anode*
C. cathode
B. gate
D. none of these
80. Zener diodes allow a current to flow
in the reverse direction, when the
A. voltage reaches above a certain
value*
B. temperature reaches above a
certain value
C. current always flows in the reverse
direction only
D. current cannot flow in the reverse
direction
81. Which of the following equipment
can’t be used to check the condition
of a diode?
A. Digital Display Meter
B. Ohmmeter
C. Curve Tracer
D. CRO*
82. What is the expected reading
obtained on a Digital Display Meter
with diode-checking function when a
proper functioning silicon
semiconductor diode is connected
across its leads in the forward bias
configuration?
A. 0.67 V*
B. 0.3 V
C. Open Loop Indication
D. Varies with the diode
83. Even after the forward current
reduces to zero value, a practical
diode continues to conduct in the
reverse direction for a while due to
the
A. resistance of the diode
B. high junction temperature
C. stored charges in the depletion
region*
D. none of the mentioned
84. For a p-n junction diode, the peak
inverse current & the reverse
recovery time are dependent on
A. inverse voltage
B. forward Voltage
C. di/dt*
D. all of the above mentioned
85. In an AC-DC converter, a diode might
be used as a
A. voltage source
B. phase angle controller
C. freewheeling Diode*
D. filter