Chapter 24: Semiconductor Devices and Electronic Circuits (Set-2)
In a p–n junction at equilibrium, diffusion current is balanced by which current?
A Drift current
B Convection current
C Eddy current
D Displacement current
In an unbiased junction, diffusion of carriers creates an internal field. That field produces drift current in opposite direction. At equilibrium, drift current exactly balances diffusion current, so net current is zero.
The depletion region contains mainly
A Free electrons
B Free holes
C Immobile ions
D Neutral atoms
When carriers recombine near the junction, mobile electrons and holes leave the region. The remaining donor and acceptor ions are fixed in the lattice, so depletion region has immobile charges and very few carriers.
Forward bias primarily reduces
A Doping density
B Depletion width
C Junction area
D Barrier charge
Forward bias lowers the barrier potential and pushes majority carriers toward the junction. This decreases the depletion region width, allowing easier carrier flow and a rapid increase in forward current.
Reverse bias primarily increases
A Minority injection
B Forward voltage
C Depletion width
D Junction heating
Reverse bias pulls majority carriers away from the junction, increasing barrier potential. The depletion layer becomes wider and the diode blocks current except for a small minority-carrier reverse saturation current.
Reverse saturation current is mainly set by
A Minority carriers
B Majority carriers
C Series resistor
D Junction area only
Reverse saturation current comes from thermally generated minority carriers drifting across the junction under reverse bias. It is small but strongly temperature dependent, unlike majority-carrier forward conduction.
For silicon diode, cut-in voltage is roughly
A 0.1 V
B 0.3 V
C 0.7 V
D 2.0 V
A silicon diode typically starts conducting significantly around 0.7 V in forward bias. Below this, current is small; beyond this, current rises steeply due to the exponential diode behavior.
Piecewise linear diode model mainly includes
A Only diode equation
B Only capacitance effect
C Only reverse leakage
D Threshold plus resistance
The piecewise linear model approximates a diode as an ideal diode with a fixed threshold voltage and a small series resistance. It simplifies circuit analysis while staying reasonably accurate for many designs.
An ideal diode in forward bias behaves like
A Short switch
B Open switch
C Current source
D Voltage source
In the ideal diode model, forward bias makes the diode conduct with zero voltage drop, like a closed switch. Reverse bias makes it block completely, like an open switch.
Diode junction capacitance is significant mainly at
A Low frequency
B DC only
C High frequency
D Zero bias only
Junction capacitance affects how fast a diode voltage can change. At high frequencies or fast switching, this capacitance stores and releases charge, influencing response time and waveform shape.
Zener diode is used mainly as
A Rectifier element
B Current amplifier
C Light detector
D Voltage reference
In reverse breakdown, a Zener diode maintains almost constant voltage over a range of currents. This stable voltage makes it useful as a reference and for simple voltage regulation circuits.
In Zener regulation, output is taken across
A Series resistor
B Zener diode
C Transformer winding
D Input source
The load is connected in parallel with the Zener diode. When the Zener is in breakdown, the voltage across it remains nearly constant, so the load receives a regulated voltage.
If input voltage rises, Zener current usually
A Decreases
B Becomes zero
C Increases
D Reverses direction
With higher supply voltage, extra voltage appears across the series resistor. This causes additional current to flow, which mainly increases Zener current, helping keep the load voltage nearly constant.
Zener dynamic resistance ideally should be
A Very low
B Very high
C Infinite
D Negative
Low dynamic resistance means the Zener voltage changes very little when current changes. That improves regulation against line and load variations, keeping output voltage more stable.
Zener diode must always be used with
A Parallel capacitor
B Inductor choke
C Series resistor
D Heat sensor
Without a series resistor, the Zener could draw excessive current in breakdown and overheat. The resistor limits current and sets safe operating conditions for both Zener and load.
Tunnel diode is made by
A Light doping
B Heavy doping
C No doping
D Metal junction
Tunnel diodes are extremely heavily doped, making the depletion region very thin. This allows quantum tunneling of carriers, producing a special V–I curve with a negative resistance region.
Negative resistance means current decreases when
A Voltage decreases
B Temperature decreases
C Light increases
D Voltage increases
In the negative resistance region of a tunnel diode, increasing forward voltage reduces tunneling probability, so current drops. This unusual property is useful for oscillators and fast switching circuits.
Tunnel diode is often used in
A Power rectifiers
B DC generators
C Microwave oscillators
D Heating coils
The negative resistance region allows tunnel diodes to sustain oscillations at very high frequencies. Their fast carrier transport also supports high-speed RF and microwave circuit applications.
LED is generally operated in
A Forward bias
B Reverse breakdown
C Zero bias only
D Open circuit
LEDs emit light when forward biased. Electrons and holes recombine in the junction and release energy as photons. Reverse bias is avoided because it can damage the LED.
LED brightness mainly depends on
A Reverse current
B Junction temperature only
C Forward current
D Wire length
More forward current increases recombination rate, producing more photons and higher brightness. However, current must be limited to prevent overheating and permanent damage to the LED.
LCD uses liquid crystals to control
A Heat flow
B Light transmission
C Sound waves
D Magnetic fields
Liquid crystals change alignment under an electric field. With polarizers, this controls how much light passes through each segment or pixel, forming visible characters and images.
A solar cell is basically a
A p–n junction
B Forward diode
C Inductor coil
D Capacitor bank
A solar cell is a p–n junction designed to convert light into electrical energy. Light-generated carriers are separated by the built-in field, producing a voltage and current at the terminals.
Solar cell delivers maximum current when
A Circuit open
B Light removed
C Circuit shorted
D Reverse biased
Short-circuit current is the current at zero terminal voltage, measured when terminals are shorted. It represents the maximum current the cell can produce under given illumination.
Solar cell delivers maximum voltage when
A Circuit shorted
B Circuit open
C Load minimum
D Frequency high
Open-circuit voltage is measured when no current flows, so terminals are open. It is the maximum voltage a solar cell can produce for a given light intensity and temperature.
Clamping circuit shifts waveform by adding
A AC component
B Magnetic flux
C Heat energy
D DC component
A clamper changes the DC level of a waveform using diode and capacitor action. The waveform shape remains mostly the same, but it is shifted upward or downward relative to zero.
Clipping circuit removes part of waveform above a
A Set level
B Resonant level
C Noise level
D Drift level
Clippers use diodes to limit voltage beyond a chosen reference. They cut off peaks above (or below) a level, protecting circuits and shaping signals for digital or communication uses.
A flyback diode is placed across
A Resistor load
B Battery terminals
C DC motor coil
D LED series resistor
Inductive loads generate a high reverse voltage when switched off. A flyback diode provides a safe path for current, protecting switches and transistors from voltage spikes.
A diode OR gate uses diodes to perform
A Addition
B Logic OR
C Logic AND
D Subtraction
In a diode OR gate, any high input forward-biases its diode and raises the output. If all inputs are low, output stays low, so the circuit behaves like a basic OR function.
A voltage doubler circuit is used to
A Increase DC level
B Halve AC
C Reduce ripple only
D Increase frequency
Voltage doublers use diodes and capacitors to charge on alternate half-cycles and add capacitor voltages. This produces a DC output approximately twice the peak input (minus diode drops).
Full-wave rectifier output frequency is
A Same as input
B Half input
C Twice input
D Zero frequency
Full-wave rectification uses both half-cycles, producing two pulses per input cycle. Therefore, ripple frequency becomes twice the AC supply frequency, making filtering easier than half-wave rectification.
Ripple factor is a measure of
A DC smoothness
B Transformer ratio
C Diode breakdown
D AC content in output
Ripple factor compares the RMS value of AC component to the DC component in rectifier output. Lower ripple factor means smoother DC and better performance of the power supply.
Rectifier efficiency mainly compares
A AC input voltage
B Diode temperature
C DC output power
D Transformer size
Rectifier efficiency is the ratio of DC power delivered to the load to AC power supplied to the rectifier. Full-wave rectifiers are more efficient than half-wave rectifiers.
Transformer utilization factor is higher for
A Bridge rectifier
B Half-wave rectifier
C Open circuit
D Zener regulator
Bridge rectifier uses the transformer secondary effectively during both half-cycles without needing a center tap. This improves transformer utilization compared with half-wave and center-tap full-wave rectifiers.
In CB configuration, current gain is
A β
B γ
C α
D 1/β
Common base current gain is α = Ic/Ie, slightly less than 1. Most emitter current reaches the collector, with only a small part becoming base current due to recombination.
Emitter follower is also called
A Common base
B Common emitter
C Common drain
D Common collector
In an emitter follower, output is taken from the emitter and collector is common to input and output circuits. It provides high input impedance, low output impedance, and voltage gain near unity.
CE configuration is widely used because it gives
A No current gain
B High voltage gain
C Zero phase shift
D No amplification
CE amplifiers provide significant voltage and power gain along with moderate input and output impedance. They are easy to bias and widely used in multistage amplifiers.
Q-point of an amplifier is set by
A AC signal only
B Heat sink only
C DC biasing
D Output load only
The Q-point is the steady DC operating point (Ic and Vce) when no signal is applied. Proper biasing keeps the transistor in active region for faithful, low-distortion amplification.
Saturation region in BJT means both junctions are
A Forward biased
B Reverse biased
C Unbiased
D Broken down
In saturation, both base–emitter and base–collector junctions are forward biased. The transistor acts like a closed switch with low Vce, and it cannot amplify linearly in this state.
Cutoff region in BJT means base–emitter junction is
A Forward biased
B Shorted
C Reverse biased
D Broken down
In cutoff, base–emitter junction is not forward biased, so base current is nearly zero. Collector current becomes very small and the transistor behaves like an open switch.
A JFET is a
A Bipolar device
B Unipolar device
C Light device
D Thermal device
In a JFET, current is carried mainly by one type of charge carrier (electrons in n-channel or holes in p-channel). Therefore it is called a unipolar device.
JFET gate is normally kept
A Forward biased
B Shorted to drain
C Floating always
D Reverse biased
Reverse bias on the gate junction keeps gate current extremely small, giving high input impedance. The reverse bias controls depletion width, thereby controlling the channel conductivity and drain current.
FET transconductance relates change in
A Vds with Id
B Ig with Vgs
C Id with Vgs
D Vgs with Vds
Transconductance gm is defined as the rate of change of drain current with gate-source voltage, gm = dId/dVgs. It shows how effectively the gate voltage controls output current.
Depletion-mode MOSFET can conduct at
A Zero gate voltage
B Only high Vgs
C Only reverse Vgs
D Only at breakdown
A depletion MOSFET has a pre-formed channel, so it conducts even when Vgs = 0. Applying suitable gate voltage can deplete or enhance the channel to control drain current.
Enhancement-mode MOSFET needs Vgs
A Below threshold
B Above threshold
C Exactly zero
D Negative always
Enhancement MOSFET has no channel at zero gate voltage. When Vgs exceeds the threshold, an inversion layer forms, creating a conducting channel and allowing drain current to flow.
In MOSFET, gate oxide mainly provides
A Low resistance
B High current gain
C Electrical insulation
D Light emission
The thin oxide layer electrically isolates the gate from the channel. This results in extremely small gate current and very high input impedance, which is a key advantage of MOSFET devices.
Common source FET amplifier generally gives
A Voltage gain
B Unity gain
C No phase change
D Current blocking
Common source amplifier provides voltage gain because variations in gate voltage cause larger variations in drain current and drain voltage. It typically produces an inverted output signal.
Common drain amplifier is also called
A Emitter follower
B Common base
C Common gate
D Source follower
In common drain, output is taken from the source, so it “follows” the gate signal with nearly unity voltage gain. It offers high input impedance and low output impedance for buffering.
RC coupling in amplifiers is mainly used for
A Power transmission
B Microwave circuits
C Audio frequency stages
D High-voltage rectifiers
RC-coupled amplifiers are simple, inexpensive, and provide good voltage gain over the audio frequency range. Coupling capacitors block DC while passing AC between amplifier stages.
Bandwidth of an amplifier is defined by
A fH − fL
B Midband gain only
C fH + fL
D 2fL − fH
Bandwidth is the frequency range over which amplifier gain stays within acceptable limits, usually between lower and upper cutoff frequencies (fL and fH). It equals fH minus fL.
Negative feedback generally makes amplifier gain
A Increase greatly
B Become infinite
C Decrease slightly
D Become unstable
Negative feedback feeds a fraction of output back in opposite phase. It reduces overall gain but improves stability, reduces distortion and noise, and often increases bandwidth and input impedance.
Negative feedback usually changes output impedance to
A Increase greatly
B Decrease
C Become infinite
D Become negative
Negative feedback tends to reduce output impedance, making the amplifier better at driving loads with less voltage drop. This improves regulation and reduces the effect of load changes on output.