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Semiconductor Physics and Carrier Transport(17)Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices(10)

Semiconductor Physics and Carrier Transport

17 Questions

GATE→2026→Electronic Devices→Semiconductor Physics and Carrier Transport

Q1

MCQ[1 Mark]

Consider carrier transport in a Zener diode in the breakdown region. Which is the dominant transport mechanism for current flow in this case?

(A)

Drift

(B)

Diffusion

(C)

Tunneling

(D)

Ballistic transport

Explanation

GATE→2026→Electronic Devices→Semiconductor Physics and Carrier Transport

Q2

NAT[2 Marks]

Consider that the concentration of electrons in a semiconductor bar varies linearly from $2\times10^{17}cm^{-3}$ at $x=1~\mu m$ to $1\times10^{16}cm^{-3}$ at $x=4~\mu m$ along the x-direction. Assume that the concentration of electrons is not varying along other directions (that is along y- and z-directions). [Given: the mobility of electron is $1400~cm^{2}V^{-1}s^{-1}$, thermal voltage is 25 mV and electronic charge is $1.6\times10^{-19}$ Coulomb.] The density of electron diffusion current $(in~A/mm^{2})$ is ______ (rounded off to two decimal places)

Explanation

GATE→2025→Electronic Devices→Semiconductor Physics and Carrier Transport

Q3

MSQ[1 Mark]

Which of the following can be used as an n-type dopant for silicon? Select the correct option(s).

(A)

Arsenic

(B)

Boron

(C)

Gallium

(D)

Phosphorous

Explanation

GATE→2025→Electronic Devices→Semiconductor Physics and Carrier Transport

Q4

MCQ[2 Marks]

The intrinsic carrier concentration of a semiconductor is $2.5 \times 10^{16} / m^3$ at 300 K. If the electron and hole mobilities are $0.15~m^2/Vs$ and $0.05~m^2/Vs$, respectively, then the intrinsic resistivity of the semiconductor (in $k\Omega \cdot m$) at 300 K is (Charge of an electron $e = 1.6 \times 10^{-19} C$.)

(A)

1.65

(B)

1.25

(C)

0.85

(D)

1.95

Explanation

GATE→2025→Electronic Devices→Semiconductor Physics and Carrier Transport

Q5

MCQ[2 Marks]

The electron mobility $\mu_n$ in a non-degenerate germanium semiconductor at 300 K is $0.38~m^2/Vs$. The electron diffusivity $D_n$ at 300 K (in $cm^2/s$, rounded off to the nearest integer) is (Consider the Boltzmann constant $k_B = 1.38 \times 10^{-23} J/K$ and the charge of an electron $e = 1.6 \times 10^{-19} C$.)

(A)

26

(B)

98

(C)

38

(D)

10

Explanation

GATE→2024→Electronic Devices→Semiconductor Physics and Carrier Transport

Q6

MCQ[1 Mark]

For non-degenerately doped n-type silicon, which one of the following plots represents the temperature (T) dependence of free electron concentration (n)?

(A)

(A)

(B)

(B)

(C)

(C)

(D)

(D)

Explanation

GATE→2024→Electronic Devices→Semiconductor Physics and Carrier Transport

Q7

MSQ[1 Mark]

The free electron concentration profile $n(x)$ in a doped semiconductor at equilibrium is shown in the figure. Which of the following statements is/are true?

(A)

For x between B and C, the electron diffusion current is directed from C to B.

(B)

For x between B and A, the electron drift current is directed from B to A.

(C)

For x between B and C, the electric field is directed from B to C.

(D)

For x between B and A, the electric field is directed from A to B.

Explanation

GATE→2024→Electronic Devices→Semiconductor Physics and Carrier Transport

Q8

NAT[2 Marks]

A non-degenerate n-type semiconductor has 5% neutral dopant atoms. Its Fermi level is located at 0.25 eV below the conduction band ($E_C$) and the donor energy level ($E_D$) has a degeneracy of 2. Assuming the thermal voltage to be 20 mV, the difference between $E_C$ and $E_D$ (in eV, rounded off to two decimal places) is

Explanation

GATE→2023→Electronic Devices→Semiconductor Physics and Carrier Transport

Q9

MCQ[1 Mark]

In a semiconductor, if the Fermi energy level lies in the conduction band, then the semiconductor is known as

(A)

degenerate n-type.

(B)

degenerate p-type.

(C)

non-degenerate n-type.

(D)

non-degenerate p-type.

Explanation

GATE→2023→Electronic Devices→Semiconductor Physics and Carrier Transport

Q10

MCQ[1 Mark]

For an intrinsic semiconductor at temperature $T = 0$ K, which of the following statement is true?

(A)

All energy states in the valence band are filled with electrons and all energy states in the conduction band are empty of electrons.

(B)

All energy states in the valence band are empty of electrons and all energy states in the conduction band are filled with electrons.

(C)

All energy states in the valence and conduction band are filled with holes.

(D)

All energy states in the valence and conduction band are filled with electrons.

Explanation

GATE→2023→Electronic Devices→Semiconductor Physics and Carrier Transport

Q11

NAT[2 Marks]

In an extrinsic semiconductor, the hole concentration is given to be $1.5 n_i$ where $n_i$ is the intrinsic carrier concentration of $1 \times 10^{10} \text{ cm}^{-3}$. The ratio of electron to hole mobility for equal hole and electron drift current is given as _______ (rounded off to two decimal places).

Explanation

GATE→2023→Electronic Devices→Semiconductor Physics and Carrier Transport

Q12

NAT[2 Marks]

In a semiconductor device, the Fermi-energy level is 0.35 eV above the valence band energy. The effective density of states in the valence band at $T = 300$ K is $1 \times 10^{19} \text{ cm}^{-3}$. The thermal equilibrium hole concentration in silicon at 400 K is _______ $\times 10^{13} \text{ cm}^{-3}$ (rounded off to two decimal places). Given $kT$ at 300 K is 0.026 eV.

Explanation

GATE→2023→Electronic Devices→Semiconductor Physics and Carrier Transport

Q13

NAT[2 Marks]

A non-degenerate n-type semiconductor has 5% neutral dopant atoms. Its Fermi level is located at 0.25 eV below the conduction band ($E_C$) and the donor energy level ($E_D$) has a degeneracy of 2. Assuming the thermal voltage to be 20 mV, the difference between $E_C$ and $E_D$ (in eV, rounded off to two decimal places) is

Explanation

GATE→2022→Electronic Devices→Semiconductor Physics and Carrier Transport

Q14

MCQ[1 Mark]

Consider a long rectangular bar of direct bandgap p-type semiconductor... The steady state electron density at $x=2~\mu\text{m}$ is

(A)

$0.37 \times 10^{14}~\text{cm}^{-3}$

(B)

$0.63 \times 10^{13}~\text{cm}^{-3}$

(C)

$3.7 \times 10^{14}~\text{cm}^{-3}$

(D)

$10^3~\text{cm}^{-3}$

Explanation

GATE→2022→Electronic Devices→Semiconductor Physics and Carrier Transport

Q15

MCQ[1 Mark]

In a non-degenerate bulk semiconductor with electron density $n = 10^{16}~\text{cm}^{-3}$, the value of $E_C - E_{Fn} = 200$ meV... For $n = 0.5 \times 10^{16}~\text{cm}^{-3}$, the closest approximation of the value of $(E_C - E_{Fn})$, among the given options, is

(A)

226 meV

(B)

174 meV

(C)

218 meV

(D)

182 meV

Explanation

GATE→2022→Electronic Devices→Semiconductor Physics and Carrier Transport

Q16

MSQ[2 Marks]

Select the CORRECT statement(s) regarding semiconductor devices.

(A)

Electrons and holes are of equal density in an intrinsic semiconductor at equilibrium.

(B)

Collector region is generally more heavily doped than Base region in a BJT.

(C)

Total current is spatially constant in a two terminal electronic device in dark under steady state condition.

(D)

Mobility of electrons always increases with temperature in Silicon beyond 300 K.

Explanation

GATE→2022→Electronic Devices→Semiconductor Physics and Carrier Transport

Q17

NAT[2 Marks]

A p-type semiconductor with zero electric field is under illumination... The magnitude of the current density due to the photo-generated electrons at $x = +2l_n$ is _______ $\text{mA/cm}^2$ (rounded off to two decimal places).

Explanation

Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

10 Questions

GATE→2026→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q18

MSQ[1 Mark]

Consider a p-n junction diode when it is forward biased with 2 V. Which of the following is/are the correct magnitude(s) of the energy difference between quasi Fermi-levels, $E_{fn}$ in the n-side and $E_{fp}$ in the p-side?

(A)

2 eV

(B)

1 eV

(C)

2 V

(D)

1 V

Explanation

GATE→2026→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q19

MSQ[1 Mark]

Figure shows the output characteristics of two different Bipolar Junction Transistors (BJT), BJT 1 with magnitude of Early voltage $|V_{A1}|$, and BJT 2 with magnitude of Early voltage $|V_{A2}|$. Which of the following options is/are correct regarding the Early voltages?

(A)

$|V_{A1}|>|V_{A2}|$ dak

(B)

$|V_{A1}|$ is infinitely large

(C)

$|V_{A1}|<|V_{A2}|$

(D)

$|V_{A2}|$ is finite

Explanation

GATE→2026→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q20

MSQ[2 Marks]

Consider an LED based on a direct bandgap semiconductor material with energy bandgap 1.3 eV. Given: Plank's constant, $h=6.63\times10^{-34}$ Js and speed of light in free space is $3\times10^{8}ms^{-1}$ In which of the following wavelength ranges the LED will NOT emit?

(A)

$1410\pm20~nm$

(B)

$1090\pm20~nm$

(C)

$950\pm20~nm$

(D)

$510\pm20~nm$

Explanation

GATE→2025→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q21

NAT[2 Marks]

An ideal p-n junction germanium diode has a reverse saturation current of $10~\mu A$ at 300 K. The voltage (in Volts, rounded off to two decimal places) to be applied across the junction to get a forward bias current of 100 mA at 300 K is ______ (Consider the Boltzmann constant $k_B = 1.38 \times 10^{-23} J/K$ and the charge of an electron $e = 1.6 \times 10^{-19} C$.)

Explanation

GATE→2024→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q22

MSQ[2 Marks]

Which of the following statements is/are true for a BJT with respect to its DC current gain $\beta$?

(A)

Under high-level injection condition in forward active mode, $\beta$ will decrease with increase in the magnitude of collector current.

(B)

Under low-level injection condition in forward active mode, where the current at the emitter-base junction is dominated by recombination-generation process, $\beta$ will decrease with increase in the magnitude of collector current.

(C)

$\beta$ will be lower when the BJT is in saturation region compared to when it is in active region.

(D)

A higher value of $\beta$ will lead to a lower value of the collector-to-emitter breakdown voltage.

Explanation

GATE→2024→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q23

NAT[2 Marks]

Consider a MOS capacitor made with p-type silicon. It has an oxide thickness of 100 nm, a fixed positive oxide charge of $10^{-8}\text{ C/cm}^2$ at the oxide-silicon interface, and a metal work function of 4.6 eV. Assume that the relative permittivity of the oxide is 4 and absolute permittivity of free space is $8.85 \times 10^{-14}\text{ F/cm}$. If the flatband voltage is 0 V, the work function of the p-type silicon (in eV, rounded off to two decimal places) is

Explanation

GATE→2024→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q24

NAT[2 Marks]

An NMOS transistor operating in the linear region has $I_{DS} = 5~\mu\text{A}$ at $V_{DS} = 0.1$ V. Keeping $V_{GS}$ constant, the $V_{DS}$ is increased to 1.5 V. Given that $\mu_n C_{ox} \frac{W}{L} = 50~\mu\text{A/V}^2$, the transconductance at the new operating point is

Explanation

GATE→2024→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q25

NAT[2 Marks]

The photocurrent of a PN junction diode solar cell is 1 mA. The voltage corresponding to its maximum power point is 0.3 V. If the thermal voltage is 30 mV, the reverse saturation current of the diode (in nA, rounded off to two decimal places) is

Explanation

GATE→2023→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q26

MCQ[1 Mark]

For a MOS capacitor, $V_{fb}$ and $V_t$ are the flat-band voltage and the threshold voltage, respectively. The variation of the depletion width ($W_{dep}$) for varying gate voltage ($V_g$) is best represented by

(A)

Option A

(B)

Option B

(C)

Option C

(D)

Option D

Explanation

GATE→2022→Electronic Devices→Diodes, BJTs, MOS Capacitors, MOSFETs, and Optoelectronic Devices

Q27

NAT[1 Mark]

An ideal MOS capacitor (p-type semiconductor) is shown in the figure. The MOS capacitor is under strong inversion with $V_G = 2$ V. The corresponding inversion charge density $(Q_{IN})$ is $2.2~\mu\text{C/cm}^2$. Assume oxide capacitance per unit area as $C_{ox} = 1.7~\mu\text{F/cm}^2$. For $V_G = 4$ V, the value of $Q_{IN}$ is _______ $\mu\text{C/cm}^2$ (rounded off to one decimal place).

Explanation