Carrier Transport - Electronic Devices - Electronics & Communication Engineering Previous Year Questions

1

2014 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2014 [Session 4]

Consider a silicon sample doped with N_D = 1×10¹⁵/cm³ donor atoms. Assume that the intrinsic carrier concentration n_i = 1.5×10¹⁰/cm³. If the sample is additionally doped with N_A = 1×10¹⁸/cm³ acceptor atoms, the approximate number of electrons/cm³ in the sample, at T=300 K, will be _____.

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2

2016 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2016 [Session 1]

The figure below shows the doping distribution in a p-type semiconductor in log scale.
The magnitude of the electric field (in kV/cm) in the semiconductor due to non uniform doping is ______

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3

2016 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2016 [Session 1]

Consider a silicon sample at T = 300 K, with a uniform donor density \(N_d = 5 \times 10^{16} \, cm^{-3}\), illuminated uniformly such that the optical generation rate is \(G_{opt} = 1.5 \times 10^{20} \, cm^{-3} s^{-1}\) throughout the sample. The incident radiation is turned off at \(t = 0\). Assume low-level injection to be valid and ignore surface effects. The carrier lifetimes are \(\tau_{p0} = 0.1 \, \mu s\) and \(\tau_{n0} = 0.5 \, \mu s\).
The hole concentration at \(t = 0\) and the hole concentration at \(t = 0.3 \, \mu s\), respectively, are

A

\(1.5 \times 10^{13} \, cm^{-3}\) and \(7.47 \times 10^{11} \, cm^{-3}\)

B

\(1.5 \times 10^{13} \, cm^{-3}\) and \(8.23 \times 10^{11} \, cm^{-3}\)

C

\(7.5 \times 10^{13} \, cm^{-3}\) and \(3.73 \times 10^{11} \, cm^{-3}\)

D

\(7.5 \times 10^{13} \, cm^{-3}\) and \(4.12 \times 10^{11} \, cm^{-3}\)

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4

2016 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2016 [Session 2]

Consider a region of silicon devoid of electrons and holes, with an ionized donor density of \(N_d^+ = 10^{17} \text{ cm}^{-3}\). The electric field at \(x = 0\) is \(0 \text{ V/cm}\) and the electric field at \(x = L\) is \(50 \text{ kV/cm}\) in the positive \(x\) direction. Assume that the electric field is zero in the \(y\) and \(z\) directions at all points.
Given \(q = 1.6 \times 10^{-19} \text{ coulomb}\), \(\epsilon_0 = 8.85 \times 10^{-14} \text{ F/cm}\), \(\epsilon_r = 11.7\) for silicon, the value of \(L\) in nm is ______

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5

2016 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2016 [Session 3]

The injected excess electron concentration profile in the base region of an npn BJT, biased in the active region, is linear, as shown in the figure. If the area of the emitter-base junction is 0.001 cm², μ_n = 800 cm²/(V-s) in the base region and depletion layer widths are negligible, then the collector current I_C (in mA) at room temperature is ___________.
(Given: thermal voltage V_T = 26 mV at room temperature, electronic charge q = 1.6 × 10^-19 C)

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6

2017 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2017

Given, mass of electron m = 9.11 × 10−31 kg, charge of electron e = −1.6 × 10−19 C, and permittivity ω0 = (1/36π) × 10−9 F/m

An electron (q1) is moving in free space with velocity 105 m/s towards a stationary electron (q2) far away. The closest distance that this moving electron gets to the stationary electron before the repulsive force diverts its path is ______ × 10−8 m.

A

5:10

B

7:13

C

6:11

D

9:13

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7

2021 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2021

A bar of silicon is doped with boron concentration \(10^{16} \text{ cm}^{-3}\) and assumed to be fully ionized. It is exposed to light such that electron-hole pairs are generated throughout the volume of the bar at the rate of \(10^{20} \text{ cm}^{-3} \text{s}^{-1}\). If the recombination lifetime is \(100 \text{ \mu s}\), intrinsic carrier concentration of silicon is \(10^{10} \text{ cm}^{-3}\) and assuming 100% ionization of boron, then the approximate product of steady-state electron and hole concentrations due to this light exposure is

A

\(10^{20} \text{ cm}^{-6} \)

B

\(2 \times 10^{20} \text{ cm}^{-6} \)

C

\(10^{32} \text{ cm}^{-6} \)

D

\(2 \times 10^{32} \text{ cm}^{-6} \)

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8

2022 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2022

Consider a long rectangular bar of direct bandgap \(p\)-type semiconductor. The equilibrium hole density is \(10^{17} \text{ cm}^{-3}\) and the intrinsic carrier concentration is \(10^{10} \text{ cm}^{-3}\). Electron and hole diffusion lengths are \(2 \mu\text{m}\) and \(1 \mu\text{m}\), respectively.
The left side of the bar (\(x = 0\)) is uniformly illuminated with a laser having photon energy greater than the bandgap of the semiconductor. Excess electron-hole pairs are generated ONLY at \(x = 0\) because of the laser. The steady state electron density at \(x = 0\) is \(10^{14} \text{ cm}^{-3}\) due to laser illumination. Under these conditions and ignoring electric field, the closest approximation (among the given options) of 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

\(10^3 \text{ cm}^{-3}\)

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9

2022 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2022

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.

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10

2022 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2022

A \(p\)-type semiconductor with zero electric field is under illumination (low level injection) in steady state condition. Excess minority carrier density is zero at \(x = \pm 2l_n\), where \(l_n = 10^{-4}\) cm is the diffusion length of electrons. Assume electronic charge, \(q = -1.6 \times 10^{-19}\) C. The profiles of photo-generation rate of carriers and the recombination rate of excess minority carriers (\(R\)) are shown. Under these conditions, the magnitude of the current density due to the photo-generated electrons at \(x = +2l_n\) is _____ mA/cm\(^2\) (rounded off to two decimal places).

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11

2023 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2023

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

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12

2024 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2024

The free electron concentration profile \( n(x) \) in a doped semiconductor at equilibrium is shown in the figure, where the points A, B, and C mark three different positions. 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.

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13

2025 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2025

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

A

1.65

B

1.25

C

0.85

D

1.95

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14

2025 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics & Communication Engineering (EC) 2025

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

A

26

B

98

C

38

D

10

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15

2026 · Electronics & Communication Engineering · Electronic Devices · Carrier Transport

Electronics and Communication Engineering (EC) 2026

Consider that the concentration of electrons in a semiconductor bar varies linearly from \( 2 \times 10^{17} \text{ cm}^{-3} \) at \( x = 1 \text{ } \mu\text{m} \) to \( 1 \times 10^{16} \text{ cm}^{-3} \) at \( x = 4 \text{ } \mu\text{m} \) 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 \( 1400 \text{ cm}^2\text{V}^{-1}\text{s}^{-1} \), thermal voltage is \( 25 \text{ mV} \) and electronic charge is \( 1.6 \times 10^{-19} \text{ Coulomb}. \)]
The density of electron diffusion current (in \( \text{A/mm}^2 \)) is ____.
(rounded off to two decimal places)

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