Chapter 24: Semiconductor Devices and Electronic Circuits (Set-1)
In an unbiased p–n junction, what forms near the junction due to diffusion?
A Conduction band
B Depletion region
C Drift current
D External field
Electrons and holes diffuse across the junction and recombine, leaving fixed ions behind. This creates a carrier-free depletion region with an internal electric field that opposes further diffusion.
What mainly opposes further diffusion of carriers in a p–n junction?
A Junction temperature
B Series resistance
C Internal electric field
D External battery
As the depletion region forms, fixed ions create an internal electric field. This field produces a drift current opposite to diffusion, leading to equilibrium when drift and diffusion balance.
Barrier potential in a p–n junction exists because of
A Ohmic contacts
B Fixed ion charges
C Metal coating
D Reverse current
The uncovered donor and acceptor ions in the depletion region create an electric potential barrier. This barrier prevents majority carriers from crossing the junction without external bias.
Forward bias of a diode means
A p to negative
B n to positive
C both open
D p to positive
In forward bias, p-side is connected to the positive terminal and n-side to the negative. This reduces barrier potential, narrows depletion width, and allows significant current flow.
Reverse bias of a diode causes depletion region to
A Disappear fully
B Become neutral
C Become wider
D Conduct strongly
Reverse bias increases the potential barrier, pulling majority carriers away from the junction. This widens the depletion region and only a small minority-carrier current flows.
The small current in reverse bias is mainly due to
A Minority carriers
B Majority carriers
C Metal electrons
D Lattice ions
In reverse bias, majority carriers cannot cross the widened barrier. A small reverse saturation current flows due to thermally generated minority carriers drifting across the junction.
Reverse saturation current mainly increases with
A Lower temperature
B Higher temperature
C Lower doping
D Higher barrier
As temperature rises, more electron–hole pairs are thermally generated. This increases the minority carriers available, so the reverse saturation current increases strongly with temperature.
Knee voltage of a diode is the voltage where
A Reverse current stops
B Breakdown starts
C Forward current rises fast
D Junction becomes open
Knee (cut-in) voltage is the point on the forward V–I curve where conduction becomes significant. Beyond it, a small increase in voltage produces a large rise in current.
Dynamic resistance of a diode refers to
A V/I at origin
B Fixed resistor value
C Reverse resistance only
D dV/dI at point
Dynamic (small-signal) resistance is the slope of the V–I curve at an operating point. It tells how the diode voltage changes for a small change in current.
Diode equation mainly relates current to
A Magnetic field
B Junction voltage
C Light intensity
D Inductor value
The diode equation shows how diode current depends exponentially on junction voltage. It explains why forward current increases rapidly after the cut-in region and why reverse current is small.
In Zener breakdown, the diode is operated in
A Reverse bias region
B Forward active
C Saturation region
D Open circuit
A Zener diode is designed to operate in reverse bias at breakdown. In this region, it maintains nearly constant voltage across itself, useful for voltage regulation.
Zener voltage is the voltage across diode in
A Forward conduction
B Zero bias
C Reverse breakdown
D Thermal runaway
Zener voltage is the nearly constant voltage that appears across the Zener diode when it is reverse-biased into breakdown. It serves as a stable reference voltage.
A series resistor in Zener regulator is used to
A Increase breakdown
B Limit Zener current
C Reduce load voltage
D Stop regulation
The series resistor drops excess supply voltage and limits current through the Zener diode. It protects the diode and helps keep Zener current within safe operating limits.
Line regulation in a Zener regulator means change in output due to
A Load change
B Temperature only
C Frequency change
D Supply change
Line regulation measures how well the regulator maintains output voltage when input (supply) voltage changes. A good regulator keeps output nearly constant despite input variations.
Load regulation in a Zener regulator means change in output due to
A Supply change
B Diode doping
C Load current change
D Capacitor aging
Load regulation describes the change in output voltage when load current varies. A stable regulator keeps output voltage almost constant as the load draws more or less current.
Zener diode power rating limits maximum
A Junction area
B Dissipated power
C Doping level
D Reverse leakage
Power rating specifies the maximum power the Zener can safely dissipate without overheating. Since power is Vz × Iz, it limits the allowable Zener current at breakdown.
Avalanche breakdown occurs mainly due to
A Impact ionization
B Tunneling only
C Recombination
D Forward diffusion
In avalanche breakdown, carriers accelerated by a strong electric field collide with atoms, creating more carriers. This multiplication causes a sudden rise in reverse current.
Zener breakdown is prominent in junctions that are
A Lightly doped
B Undoped
C Heavily doped
D Metallic
Heavy doping produces a very thin depletion region and strong electric field at lower reverse voltage. This enables tunneling of carriers, causing Zener breakdown.
Tunnel diode shows negative resistance because of
A Heating effect
B Quantum tunneling
C Rectification only
D Photoelectricity
In a tunnel diode, heavy doping makes a very thin barrier. Electrons can tunnel through, and in a certain voltage range current decreases as voltage increases, giving negative resistance.
Peak current in tunnel diode occurs at
A Highest forward voltage
B Reverse saturation
C Open circuit point
D Negative resistance start
The tunnel diode current rises to a peak at low forward voltage due to tunneling. As voltage increases further, tunneling reduces and current drops into the negative resistance region.
Valley current in tunnel diode is the minimum current after
A Thermal region
B Reverse region
C Peak region
D Zero bias
After the peak, tunnel current decreases and reaches a minimum called valley current. Beyond this, normal diode conduction dominates and current again increases with voltage.
Tunnel diode is best suited for
A Low-speed rectifiers
B High-speed switching
C DC motors
D Power transformers
Because tunneling is very fast and the device has a unique negative resistance region, tunnel diodes can switch quickly and are used in high-frequency and microwave applications.
LED light emission occurs due to
A Electroluminescence
B Thermionic emission
C Photoelectric effect
D Magnetic induction
In an LED, forward bias causes electrons and holes to recombine at the junction. The recombination releases energy as photons, producing light—this process is electroluminescence.
LEDs commonly use materials with
A Indirect band gap
B No band gap
C Direct band gap
D Metallic bonding
Direct band-gap semiconductors efficiently convert electron–hole recombination energy into photons. Indirect band-gap materials lose much energy as heat, so they are poor light emitters.
LED color mainly depends on
A Wire thickness
B Band gap energy
C Junction area
D Heat sink size
Photon energy equals the semiconductor band gap. A larger band gap produces higher-energy photons (shorter wavelength), changing the emitted color from red toward blue/violet.
A current-limiting resistor is used with LED to
A Increase brightness always
B Reverse the polarity
C Raise band gap
D Prevent excess current
LEDs have low forward resistance once conducting, so current can rise sharply. A series resistor limits current to a safe value and protects the LED from overheating damage.
LCD works mainly by controlling
A Magnetic flux
B Heat conduction
C Light polarization
D Sound intensity
Liquid crystals can rotate the plane of polarized light when an electric field changes their alignment. With polarizers, this controls light transmission to form visible segments or pixels.
LCD needs a light source because it is a
A Light-modulating device
B Light-emitting device
C Current generator
D Heat radiator
LCD does not produce light itself. It changes how much light passes through by controlling polarization, so it usually needs ambient light or a backlight to be visible.
Solar cell works on the principle of
A Electrolysis
B Photovoltaic effect
C Seebeck effect
D Hall effect
In a solar cell, light creates electron–hole pairs in a p–n junction. The built-in field separates charges, producing a voltage and current without external bias.
Open-circuit voltage of a solar cell is measured when
A Current is maximum
B Load is zero ohm
C Output terminals open
D Light is absent
Open-circuit voltage is the voltage across the solar cell when no current is drawn, meaning the circuit is open. It depends on illumination and the junction properties.
Short-circuit current of a solar cell is measured when
A Load is infinite
B Light is off
C Reverse bias applied
D Terminals are shorted
Short-circuit current is the current produced when the solar cell terminals are directly connected with negligible resistance. It mainly depends on light intensity and cell area.
Maximum power point of a solar cell is where
A Product V×I is max
B Voltage is zero
C Current is zero
D Resistance is infinite
Solar cell power output equals V×I. The maximum power point is the operating point on the I–V curve where this product is highest, giving best usable power.
Fill factor of a solar cell indicates
A Cell thickness
B Doping type
C Squareness of I–V
D Wire resistance
Fill factor compares the maximum power rectangle to Voc×Isc. A higher fill factor means the I–V curve is more “square,” indicating better power extraction and efficiency.
A clipping circuit is used to
A Increase frequency
B Limit waveform peaks
C Convert AC to DC
D Store charge only
Clipping circuits use diodes to remove or limit parts of an input waveform above or below a chosen level. They protect circuits and shape signals by restricting peaks.
A clamping circuit is used to
A Amplify voltage
B Increase current
C Reduce noise only
D Shift DC level
Clamping circuits add a DC component to shift the entire waveform upward or downward without changing its shape much. Diode and capacitor action sets the new reference level.
Rectification means converting
A DC to AC
B Heat to light
C AC to DC
D Light to heat
Rectification uses diodes to allow current mainly in one direction, converting alternating voltage into a unidirectional (pulsating DC) output used in power supplies.
Half-wave rectifier uses how many diodes?
A Two diodes
B One diode
C Three diodes
D Four diodes
A half-wave rectifier uses a single diode, conducting only during one half-cycle of AC input. The other half-cycle is blocked, giving lower efficiency and higher ripple.
Full-wave rectifier with center-tap uses
A Two diodes
B One diode
C Four diodes
D No diode
A center-tapped full-wave rectifier uses two diodes, each conducting on alternate half-cycles. The center-tap provides the return path, producing rectified output in both halves.
Bridge rectifier typically uses
A One diode
B Two diodes
C Four diodes
D Six diodes
A bridge rectifier uses four diodes arranged so that current through the load flows in the same direction during both half-cycles. It does not require a center-tapped transformer.
Peak inverse voltage in half-wave rectifier equals
A Vm/2
B Vm
C 2Vm
D 4Vm
In a half-wave rectifier, during the non-conducting half-cycle the diode must withstand the full peak secondary voltage across it. Therefore PIV requirement is Vm.
PIV of each diode in a bridge rectifier is about
A 2Vm
B Vm/2
C 4Vm
D Vm
In a bridge rectifier, each reverse-biased diode typically sees a maximum reverse voltage approximately equal to the peak secondary voltage Vm, lower than the center-tap full-wave case.
A capacitor filter mainly reduces
A Peak voltage
B Ripple voltage
C Supply frequency
D Transformer ratio
A capacitor across the load charges near the peak and discharges slowly between peaks, smoothing the output. This reduces ripple and increases the average DC level.
In a BJT, collector current is controlled mainly by
A Base current
B Collector voltage
C Emitter resistor
D Heat sink
In a BJT, a small base current controls a much larger collector current due to transistor action. This current amplification is the basis of using BJTs as amplifiers.
In common emitter configuration, current gain is
A α
B 1/β
C β
D 1−α
In common emitter (CE), current gain is β = Ic/Ib. It is typically much greater than 1, making CE configuration widely used for current and voltage amplification.
Relation between α and β is
A β=1−α
B β=α/(1−α)
C α=1/(1+β)
D β=1/α
α is common base current gain and β is common emitter current gain. Since Ie = Ic + Ib, the relation β = α/(1−α) connects both gains.
CE amplifier output has phase shift of
A 0°
B 90°
C 360°
D 180°
A CE amplifier inverts the signal. When base voltage increases, collector current increases, causing a larger drop across collector resistor, reducing collector voltage—giving 180° phase inversion.
A transistor operates as an amplifier mainly in
A Cutoff region
B Saturation region
C Active region
D Breakdown region
For amplification, the transistor must be biased so it stays in the active region where Ic is proportional to Ib. Cutoff or saturation makes it behave like a switch instead.
FET has very high input impedance because
A It uses base current
B Gate current is tiny
C It is heavily doped
D It has two junctions
In a FET, the gate controls channel current using an electric field, and gate current is nearly zero (especially in MOSFET). This gives very high input impedance.
Pinch-off voltage in JFET is the Vgs where
A Channel closes at drain
B Channel fully opens
C Gate conducts forward
D Current becomes infinite
Pinch-off refers to a condition where increasing Vds causes the depletion region to constrict the channel near the drain. Current then saturates and changes little with Vds.
MOSFET conduction begins when gate voltage exceeds
A Zener voltage
B Knee voltage
C Threshold voltage
D Breakdown voltage
In an enhancement MOSFET, no conducting channel exists at zero gate voltage. When Vgs exceeds threshold voltage, an inversion channel forms, allowing significant drain current flow.