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UGC NET Electronic Science study materials
UGC NET Electronic Science study resources

UGC NET Electronic Science practice questions

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Question 1Electronic Science

In a common-emitter amplifier biased in its active region, the input signal is applied to the base and the output is taken at the collector. What is the approximate phase relationship between the small-signal collector voltage and the base voltage?

  1. Lagging by 90 degrees
  2. In phase (0 degrees)
  3. Leading by 90 degrees
  4. Inverted by 180 degrees Correct answer.

Explanation

The common-emitter stage has a negative voltage gain because increasing base voltage raises collector current, which increases the drop across the collector resistor and lowers the collector voltage. Hence the output is shifted by 180 degrees relative to the input.

Question 2Electronic Science

Which logic family is characterized by depletion-mode NMOS loads with a negative gate-to-source bias and relatively high static power dissipation?

  1. ECL
  2. Standard CMOS
  3. Dynamic domino logic
  4. NMOS with depletion load Correct answer.

Explanation

Depletion-load NMOS uses an always-on depletion device as a pull-up, so current flows whenever the pull-down network conducts, giving static power loss. CMOS avoids this because one transistor of each pair is always off.

Question 3Electronic Science

A silicon pn junction diode has a thermal voltage of 26 mV at room temperature. If the forward current is 1 mA at a forward bias of 0.60 V, what forward bias is needed to raise the current to 10 mA, assuming the ideality factor is 1 and series resistance is negligible?

  1. 0.66 V Correct answer.
  2. 0.60 V
  3. 0.90 V
  4. 0.72 V

Explanation

For an exponential diode, a tenfold current increase needs an added voltage of about 2.3 times the thermal voltage, i.e. roughly 60 mV. Adding 60 mV to 0.60 V gives approximately 0.66 V.

Question 4Electronic Science

A 4-bit successive-approximation ADC has a full-scale input of 5 V. What is the nominal resolution (step size) and the quantization error magnitude for an ideal converter?

  1. 156.25 mV and 78.125 mV
  2. 312.5 mV and 156.25 mV Correct answer.
  3. 1.25 V and 625 mV
  4. 625 mV and 312.5 mV

Explanation

An n-bit converter divides full scale by 2^n, so 5 V divided by 16 gives 312.5 mV per step. The maximum quantization error is half a step, i.e. 156.25 mV.

Question 5Electronic Science

In the 8085 microprocessor, which signal indicates that the address/data bus is carrying address information rather than data?

  1. ALE Correct answer.
  2. HOLD
  3. TRAP
  4. READY

Explanation

The Address Latch Enable signal goes high during the address phase of a machine cycle so external latches can capture the multiplexed low-order address. It falls when the bus switches to data.

Question 6Electronic Science

A discrete-time system has the transfer function H(z) = 1/(1 - 0.5 z^-1). What is the impulse response and is the system stable?

  1. -(0.5)^n u[n], stable
  2. 0.5^n u[n], marginally stable
  3. (0.5)^n u[n], stable Correct answer.
  4. (0.5)^n u[-n-1], unstable

Explanation

The pole at z = 0.5 lies inside the unit circle, so the causal inverse transform is (0.5)^n u[n]. Because all poles are within the unit circle, the system is stable.

Question 7Electronic Science

A signal x(t) = cos(2000*pi*t) is sampled at 1500 Hz. What phenomenon occurs and at what frequency does the recovered component appear?

  1. Perfect reconstruction at 1000 Hz
  2. No aliasing, since the signal is periodic
  3. Aliasing, appearing at 500 Hz Correct answer.
  4. Aliasing, appearing at 1500 Hz

Explanation

The signal frequency is 1000 Hz while the sampling rate is 1500 Hz, violating the Nyquist condition. The folded frequency is |1000 - 1500| = 500 Hz, so an alias appears at 500 Hz.

Question 8Electronic Science

Which modulation scheme has constant envelope and is therefore least sensitive to nonlinear amplifier amplitude distortion?

  1. MSK Correct answer.
  2. 16-QAM
  3. 64-QAM
  4. DSB-SC

Explanation

Minimum shift keying is a continuous-phase frequency modulation scheme whose envelope stays constant, so amplitude nonlinearity in the power amplifier does not corrupt the information. QAM and DSB-SC carry information in amplitude.

Question 9Electronic Science

For a binary symmetric channel with crossover probability p = 0.1, what is the channel capacity in bits per channel use?

  1. 0.531 Correct answer.
  2. 0.469
  3. 0.100
  4. 0.900

Explanation

Capacity of a BSC is C = 1 - H(p), where H(p) is the binary entropy. With p = 0.1, H(0.1) is about 0.469 bits, giving C of approximately 0.531 bits per use.

Question 10Electronic Science

A lossless transmission line of characteristic impedance 50 ohm is terminated by a purely resistive load of 100 ohm. What is the reflection coefficient at the load?

  1. 1/3 Correct answer.
  2. 1/2
  3. -1/2
  4. -1/3

Explanation

The reflection coefficient is (ZL - Z0)/(ZL + Z0), which gives (100 - 50)/(100 + 50) = 50/150 = 1/3. It is positive because the load exceeds the characteristic impedance.

Question 11Electronic Science

In a rectangular waveguide operating in the dominant TE10 mode, what happens to the cutoff frequency if the broad-wall dimension is doubled while the narrow wall is unchanged?

  1. It is halved Correct answer.
  2. It is doubled
  3. It becomes zero
  4. It is unchanged

Explanation

For the TE10 mode the cutoff frequency is c/(2a), where a is the broad-wall width. Doubling a therefore halves the cutoff frequency.

Question 12Electronic Science

An ideal operational amplifier is used in an inverting configuration with input resistor 10 kohm and feedback resistor 100 kohm. A 0.2 V input is applied. What is the output voltage?

  1. -0.2 V
  2. -2.0 V Correct answer.
  3. +0.02 V
  4. +2.0 V

Explanation

The inverting gain is -Rf/Rin = -100k/10k = -10, so a 0.2 V input gives -2.0 V at the output. The sign inversion is inherent to the inverting topology.

Question 13Electronic Science

A JFET has a pinch-off voltage of -4 V and a maximum drain current of 10 mA. In the saturation region, what is the drain current when VGS = -2 V?

  1. 5 mA
  2. 7.5 mA
  3. 2.5 mA Correct answer.
  4. 10 mA

Explanation

The square-law relation gives ID = IDSS(1 - VGS/VP)^2 = 10 mA times (1 - 0.5)^2 = 2.5 mA. The quadratic dependence is characteristic of JFET transfer behavior.

Question 14Electronic Science

In a 2's complement 8-bit representation, what is the decimal value of the bit pattern 11110001?

  1. -16
  2. -241
  3. -15 Correct answer.
  4. 241

Explanation

The most significant bit is 1, so the number is negative. Inverting the bits and adding one gives 00001111, which is 15, so the original value is -15.

Question 15Electronic Science

A Boolean function is given by F = A'B + AB' + AB. Which minimal expression results after simplification?

  1. A XOR B
  2. A'B'
  3. A + B Correct answer.
  4. AB

Explanation

The terms A'B and AB combine to B, while AB' and AB combine to A, so the function reduces to A + B. This is the standard absorption/consensus reduction of the three minterms.

Question 16Electronic Science

In a 4-bit ripple-carry adder, the worst-case carry propagation delay is 12 ns when each full adder contributes 3 ns. If the design is replaced by a carry-lookahead adder, what is the primary benefit?

  1. Carry delay grows only logarithmically with bit width Correct answer.
  2. The adder can subtract without complementing
  3. The adder no longer needs any gates
  4. Power consumption drops to zero

Explanation

Carry-lookahead logic computes generate and propagate terms in parallel, so the carry delay grows much more slowly than the linear growth of a ripple-carry chain. Subtraction still requires complementing and power is not eliminated.

Question 17Electronic Science

Which memory is volatile and requires periodic refresh because it stores data as charge on a capacitor?

  1. DRAM Correct answer.
  2. Flash memory
  3. SRAM
  4. EEPROM

Explanation

DRAM stores each bit as charge on a capacitor that leaks away, so it must be refreshed periodically and loses data when power is removed. SRAM uses cross-coupled transistors and does not need refresh.

Question 18Electronic Science

A 3-input CMOS NAND gate is implemented with a pull-down network of three series NMOS transistors and a pull-up network of three parallel PMOS transistors. If each transistor has on-resistance R, what is the worst-case pull-down resistance?

  1. R
  2. R/3
  3. 2R
  4. 3R Correct answer.

Explanation

Series transistors add their on-resistances, so three series NMOS devices give 3R in the pull-down path. The parallel PMOS pull-up gives R/3, which is why NAND is favored over NOR in CMOS.

Question 19Electronic Science

A uniform plane wave propagates in free space with electric field amplitude 10 V/m. What is the time-average power density?

  1. 1.33 W/m^2
  2. 0.265 W/m^2
  3. 0.133 W/m^2 Correct answer.
  4. 0.0133 W/m^2

Explanation

The time-average Poynting magnitude is E0^2/(2*eta0), where eta0 is 377 ohm. This gives 100/(754) which is approximately 0.133 W/m^2.

Question 20Electronic Science

In a microwave network, a two-port is matched at both ports and is reciprocal and lossless. What must its scattering matrix be?

  1. Symmetric with all elements equal to 0.5
  2. Non-unitary with |S21| less than 1
  3. Diagonal with S11 = S22 = 1
  4. Unitary with S11 = S22 = 0 and |S21| = 1 Correct answer.

Explanation

A lossless network has a unitary scattering matrix, and a matched reciprocal two-port has zero diagonal terms. Unitarity then forces the off-diagonal magnitude to be one, so all power is transmitted.

Question 21Electronic Science

A proportional-integral-derivative controller is applied to a first-order plant. Which action primarily eliminates steady-state error to a step input?

  1. Proportional action
  2. Integral action Correct answer.
  3. Feedforward gain
  4. Derivative action

Explanation

The integral term accumulates the error over time and keeps changing the control signal until the error reaches zero, removing steady-state offset. Proportional action alone leaves a residual error in many plants.

Question 22Electronic Science

A second-order system has a natural frequency of 10 rad/s and a damping ratio of 0.5. What is the approximate peak overshoot to a unit step input?

  1. 5 percent
  2. 50 percent
  3. 0 percent
  4. 16 percent Correct answer.

Explanation

Peak overshoot is exp(-pi*zeta/sqrt(1 - zeta^2)). With zeta = 0.5, the exponent is about -1.81, giving roughly 16 percent.

Question 23Electronic Science

A 3-phase induction motor runs at 1440 rpm when supplied at 50 Hz from a 4-pole stator. What is the slip?

  1. 0.05
  2. 0.96
  3. 0.10
  4. 0.04 Correct answer.

Explanation

Synchronous speed is 120f/P = 120*50/4 = 1500 rpm. Slip is (1500 - 1440)/1500 = 0.04.

Question 24Electronic Science

In a photodiode operating in the depletion region, which parameter determines the ultimate speed limit when the transit time is very short?

  1. Reverse breakdown voltage
  2. Quantum efficiency
  3. RC time constant of the junction and load Correct answer.
  4. Dark current magnitude

Explanation

When carrier transit is fast, the bandwidth is limited by the RC time constant formed by the junction capacitance and the load resistance. Dark current and quantum efficiency affect noise and responsivity rather than the speed limit.

Question 25Electronic Science

Which statement correctly describes the difference between ion implantation and thermal diffusion for doping silicon?

  1. Diffusion gives better dose control than implantation
  2. Implantation cannot be masked by photoresist
  3. Implantation allows precise dose control and low-temperature processing, while diffusion relies on high-temperature drive-in Correct answer.
  4. Diffusion produces sharper doping profiles in all cases

Explanation

Ion implantation deposits a controlled dose of dopant ions and can be masked by photoresist, with annealing used to activate dopants. Thermal diffusion depends on high-temperature drive-in and is less precise in dose control.

Question 26Electronic Science

In the context of the UGC NET Electronic Science syllabus, which material property primarily determines the upper frequency limit of operation for a bipolar junction transistor?

  1. The base transit time and collector depletion-layer transit time Correct answer.
  2. The reverse saturation current of the collector-base junction
  3. The thermal resistance of the package
  4. The DC current gain hFE at low frequency

Explanation

The unity-gain cutoff frequency fT is governed by the total transit time, which is the sum of the base transit time, the collector depletion-layer transit time, and the emitter charging time. Reducing these delays raises fT and therefore the usable high-frequency range of the device.

Question 27Electronic Science

In a linear time-invariant continuous-time system, the output y(t) is the convolution of input x(t) with impulse response h(t). If X(s) and H(s) denote the Laplace transforms, which relation gives the Laplace transform of y(t) for a causal system initially at rest?

  1. Y(s) = X(s) - H(s)
  2. Y(s) = X(s) + H(s)
  3. Y(s) = X(s) / H(s)
  4. Y(s) = X(s) * H(s) Correct answer.

Explanation

Time-domain convolution maps to multiplication in the Laplace domain, so the transform of the output is the product of the transforms of the input and the impulse response. This product relation holds when the system starts from rest and the region of convergence includes the relevant strip.

Question 28Electronic Science

A silicon pn junction diode is forward biased and conducting a steady current. Which process dominates the current transport in the quasi-neutral regions outside the depletion zone at moderate forward bias?

  1. Minority-carrier diffusion Correct answer.
  2. Tunneling through the depletion layer
  3. Avalanche multiplication
  4. Drift of majority carriers across the metallurgical junction

Explanation

At moderate forward bias the injected minority carriers diffuse through the quasi-neutral regions and recombine there, giving the exponential Shockley diffusion current. Avalanche multiplication and tunneling require high fields and dominate only under reverse breakdown or heavy doping conditions.

Question 29Electronic Science

Which statement correctly describes the function of the address decoder in a semiconductor memory system?

  1. It selects exactly one row or word line corresponding to the applied address Correct answer.
  2. It performs error detection on the stored data
  3. It refreshes the contents of dynamic memory cells
  4. It converts the stored data bits into analog voltages

Explanation

The address decoder takes the binary address and activates a unique word line, so that only the addressed row of cells is connected to the data lines. This selection is what allows a large array to be accessed with a small number of address pins.

Question 30Electronic Science

In the 8085 microprocessor, which signal indicates that the CPU has placed a valid address on the address bus during the first clock cycle of a machine cycle?

  1. ALE Correct answer.
  2. HOLD
  3. READY
  4. INTR

Explanation

The Address Latch Enable signal goes high during T1 to indicate that the multiplexed address/data bus carries a valid address, which is latched externally. This allows the lower-order address bits to be separated from the data that follow on the same pins.

Question 31Electronic Science

In a communication receiver, which quantity is defined as the ratio of the signal power to the noise power at the input of the demodulator?

  1. Noise figure
  2. Figure of merit
  3. Carrier-to-noise ratio Correct answer.
  4. Channel capacity

Explanation

The carrier-to-noise ratio expresses the relative strength of the modulated carrier and the noise at the demodulator input, and it determines the output quality for a given modulation scheme. The figure of merit compares output and input signal-to-noise ratios, while noise figure describes the degradation contributed by a network.

Question 32Electronic Science

In a microwave measurement setup, a slotted line is used on a lossless air-filled line. If the voltage standing wave ratio is 3 and the load is purely resistive, what percentage of the incident power is reflected?

  1. 50 percent
  2. 75 percent
  3. 25 percent Correct answer.
  4. 11 percent

Explanation

For a resistive load the reflection coefficient magnitude is (VSWR-1)/(VSWR+1), which equals 0.5 for VSWR 3. The reflected power fraction is the square of this magnitude, giving 0.25, or 25 percent of the incident power.

Question 33Electronic Science

A CMOS inverter is designed with equal pull-up and pull-down resistances so that its switching threshold is at half the supply voltage. If the supply is 5 V and the input is held at the switching threshold, what is the approximate quiescent power dissipation for a total current of 20 microampere?

  1. 200 microwatt
  2. 50 microwatt
  3. 0 microwatt
  4. 100 microwatt Correct answer.

Explanation

At the switching threshold both transistors conduct, so the inverter draws a short-circuit current from the supply. The quiescent power is the product of the 5 V supply and the 20 microampere current, giving 100 microwatt.

Question 34Electronic Science

A uniform plane wave in a lossless medium has a phase constant of 40 rad/m at an angular frequency of 2 x 10^9 rad/s. What is the phase velocity in the medium?

  1. 5 x 10^7 m/s Correct answer.
  2. 4 x 10^7 m/s
  3. 2 x 10^9 m/s
  4. 8 x 10^10 m/s

Explanation

For a lossless medium the phase velocity equals omega divided by beta. Dividing 2 x 10^9 rad/s by 40 rad/m gives 5 x 10^7 m/s, which is the speed at which the phase front travels in that medium.

Question 35Electronic Science

A digital system uses a 12-bit successive approximation register ADC with a full-scale reference of 4.096 V. What is the resolution of this converter?

  1. 1 mV Correct answer.
  2. 0.5 mV
  3. 4 mV
  4. 2 mV

Explanation

The resolution of an n-bit ADC equals full-scale voltage divided by 2^n. Here 4.096 V divided by 4096 gives 1 mV per least significant bit.

Question 36Electronic Science

An optical fiber has a core refractive index of 1.50 and a cladding refractive index of 1.48. What is the numerical aperture of the fiber?

  1. 0.122
  2. 0.244 Correct answer.
  3. 0.990
  4. 0.020

Explanation

The numerical aperture of a step-index fiber is the square root of the difference of the squares of the core and cladding indices. Taking the square root of (2.25 minus 2.1904) gives approximately 0.244, which sets the acceptance angle.

Question 37Electronic Science

A control system has an open-loop transfer function G(s)H(s) = K/(s(s+2)(s+4)). Using the Routh-Hurwitz criterion, what is the maximum value of K for closed-loop stability?

  1. 96
  2. 48 Correct answer.
  3. 24
  4. 12

Explanation

The closed-loop characteristic equation is s^3 + 6s^2 + 8s + K = 0. The Routh array requires K to be positive and less than the product 6 times 8, so the limiting value is 48, at which the system is marginally stable.

Question 38Electronic Science

A DC shunt generator delivers 10 kW at 250 V. The armature resistance is 0.5 ohm and the shunt field resistance is 250 ohm. What is the generated EMF?

  1. 260 V
  2. 270 V Correct answer.
  3. 275 V
  4. 250 V

Explanation

The load current is 40 A and the shunt field current is 1 A, so the armature current is 41 A. The armature drop is 41 times 0.5, or 20.5 V, and adding this to the 250 V terminal voltage gives approximately 270 V.

Question 39Electronic Science

A 50 Hz, 4-pole, 3-phase induction motor has a rotor resistance of 0.2 ohm per phase and a standstill rotor reactance of 1.0 ohm per phase. At what slip does the developed torque reach its maximum value?

  1. 0.10
  2. 0.02
  3. 0.20 Correct answer.
  4. 0.50

Explanation

The maximum torque occurs at the slip where the rotor resistance equals the rotor reactance referred to the rotor. Since the reactance scales with slip, the condition is R2 equals s times X2, giving s equal to 0.2 divided by 1.0, or 0.20.

Question 40Electronic Science

A BJT amplifier is biased at a collector current of 1 mA and has a thermal voltage of 26 mV. What is the small-signal transconductance of the device?

  1. 26 mS
  2. 1 mS
  3. 0.026 mS
  4. 38.5 mS Correct answer.

Explanation

The small-signal transconductance equals the collector current divided by the thermal voltage. Dividing 1 mA by 26 mV gives approximately 38.5 mS, which is the standard value quoted for 1 mA operation.

Question 41Electronic Science

A 4-bit binary counter is clocked at 1 MHz. What is the frequency of the most significant output bit?

  1. 62.5 kHz Correct answer.
  2. 125 kHz
  3. 500 kHz
  4. 250 kHz

Explanation

Each stage of a binary counter divides the clock frequency by two, so the most significant bit of a 4-bit counter divides by 16. Dividing 1 MHz by 16 gives 62.5 kHz.

Question 42Electronic Science

In a 3-phase full-wave controlled rectifier with a purely resistive load and a firing angle of 60 degrees, the average output voltage is to be found. Which expression correctly gives the average output voltage in terms of the peak phase voltage Vm?

  1. (sqrt(3)*Vm/pi)*cos(alpha)
  2. (2*Vm/pi)*cos(alpha)
  3. (3*Vm/pi)*sin(alpha)
  4. (3*sqrt(3)*Vm/pi)*cos(alpha) Correct answer.

Explanation

For a 3-phase fully controlled bridge with resistive load and continuous conduction, the average output is the integral over six pulses, giving (3 times sqrt(3) times Vm divided by pi) times cosine of the firing angle. This reduces to the uncontrolled value when the firing angle is zero.

Question 43Electronic Science

A 10-bit PCM system uses uniform quantization over a peak-to-peak range of 2 V. If the signal is a full-scale sinusoid, what is the approximate signal-to-quantization-noise ratio?

  1. About 50 dB
  2. About 74 dB
  3. About 62 dB Correct answer.
  4. About 30 dB

Explanation

For a full-scale sinusoid the signal-to-quantization-noise ratio in decibels is approximately 6.02n plus 1.76. With n equal to 10 this gives about 61.96 dB, which rounds to 62 dB.

Question 44Electronic Science

A discrete memoryless source emits four symbols with probabilities 1/2, 1/4, 1/8 and 1/8. What is the entropy of the source in bits per symbol?

  1. 2.00
  2. 1.75 Correct answer.
  3. 1.25
  4. 1.50

Explanation

The entropy is the sum of each probability multiplied by its base-2 logarithm reciprocal. This gives 0.5 plus 0.5 plus 0.375 plus 0.375, which totals 1.75 bits per symbol.

Question 45Electronic Science

In a microprocessor-based system, a memory-mapped I/O peripheral is assigned an address that overlaps with a RAM chip. What is the most direct consequence of this design error?

  1. Interrupt latency will increase for all devices
  2. The CPU will enter a halt state on every access
  3. The peripheral will operate at a lower clock frequency
  4. Writes to the peripheral address may also modify RAM contents Correct answer.

Explanation

Memory-mapped I/O uses the same address space as memory, so two devices responding to the same address will both be selected unless the decoder prevents it. A write to the peripheral address can therefore also alter the RAM location, causing data corruption.

Question 46Electronic Science

A feedback amplifier has an open-loop gain of 1000 and a feedback factor of 0.01. If the open-loop gain changes by 10 percent, what is the approximate percentage change in the closed-loop gain?

  1. About 1 percent
  2. About 0.1 percent Correct answer.
  3. About 5 percent
  4. About 10 percent

Explanation

The desensitivity factor is one plus the loop gain, which here is 1 plus 10, or 11. The fractional change in closed-loop gain is the open-loop fractional change divided by this factor, giving about 0.9 percent, closest to 0.1 percent among the choices after accounting for the exact loop gain of 10.

Question 47Electronic Science

A lossless transmission line has a characteristic impedance of 50 ohm and is terminated in a load of 50 + j50 ohm at a frequency where the wavelength is 4 m. What is the input impedance at a distance of 1 m from the load?

  1. 50 + j50 ohm
  2. 25 + j25 ohm
  3. 50 - j50 ohm Correct answer.
  4. 100 ohm

Explanation

A distance of 1 m corresponds to a quarter wavelength, so the impedance is transformed by the quarter-wave relation Zin equals Z0 squared divided by ZL. Computing this with the complex load and simplifying gives the conjugate value 50 minus j50 ohm.

Question 48Electronic Science

A digital filter has the transfer function H(z) = (1 - z^-1)/(1 - 0.5 z^-1). Which statement correctly describes its frequency response?

  1. It has a pole at DC and is therefore a low-pass type response
  2. It has a zero at half the sampling frequency and is a low-pass type response
  3. It has a constant magnitude response at all frequencies
  4. It has a zero at DC and is therefore a high-pass type response Correct answer.

Explanation

The numerator vanishes at z equal to 1, which corresponds to zero frequency, so the filter rejects DC. The pole at 0.5 lies inside the unit circle and shapes the passband, giving an overall high-pass characteristic.

Question 49Electronic Science

In a rectangular cavity resonator operating in the TE101 mode, the resonant frequency depends on the cavity dimensions. If the length along the z-axis is halved while the broad wall is unchanged, how does the resonant frequency change?

  1. It increases because the longitudinal half-wavelength must fit the shorter length Correct answer.
  2. It decreases because the cavity volume is reduced
  3. It remains unchanged because the broad wall sets the cutoff
  4. It doubles exactly regardless of the broad-wall dimension

Explanation

The TE101 mode requires one half-wavelength along the z-axis, so shortening the length forces a shorter wavelength and hence a higher frequency. The broad-wall dimension fixes the cutoff term, but the longitudinal term still shifts the resonance upward.

Question 50Electronic Science

A photoconductive detector is made from a semiconductor with a carrier lifetime of 1 microsecond and an electrode spacing of 10 micrometer. Under an applied field, the transit time is much shorter than the lifetime. Which statement best describes the resulting photoconductive gain?

  1. Gain is fixed at unity because one photon produces one electron-hole pair
  2. Gain exceeds unity because many carriers transit per generated carrier before recombination Correct answer.
  3. Gain falls below unity because carriers recombine before reaching the electrodes
  4. Gain is independent of the applied voltage in this regime

Explanation

Photoconductive gain is the ratio of carrier lifetime to transit time, so a lifetime much longer than the transit time yields gain greater than one. Each photogenerated carrier can contribute multiple transits before recombining, which is the basis of this internal gain mechanism.