Arihant AIEEE Physics

D.B. Singh

Chapter 34

Semi-conductor Devices - all with Video Answers

Educators

Chapter Questions

Problem 1

A $n$ -type semi-conductor has impurity level $20 \mathrm{meV}$ below the conduction band. In a thermal collision, transferrable energy is $k T$. The value of $T$ for which electrons start to jump in conduction band is :
(a) $232 \mathrm{~K}$
(b) $348 \mathrm{~K}$
(c) $400 \mathrm{~K}$
(d) none of these

Ajay Singhal

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Numerade Educator

Problem 2

The band gap for a pure semi-conductor is $2.1 \mathrm{eV}$. The maximum wavelength of a photon which is able to create a hole- electron pair is :
(a) $600 \mathrm{~nm}$
(b) $589 \mathrm{~nm}$
(c) $400 \mathrm{~nm}$
(d) none of these

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Problem 3

Assume that the number of hole-electron pair in an intrinsic semi-conductor is proportional to $e^{-\Delta E / 2 k T}$. Here, $\Delta E=$ energy gap and $\quad k=8.62 \times 10^{-5} \mathrm{eV} / \mathrm{kelvin}$
The energy gap for silicon is $1.1 \mathrm{eV}$. The ratio of electronhole pairs at $300 \mathrm{~K}$ and $400 \mathrm{~K}$ is :
(a) $e^{-5.31}$
(b) $e^{-5}$
(c) $e$
(d) none of these

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Problem 4

Electronic current is in
(a) conduction band
(b) valence band
(c) either of the two
(d) none of these

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Problem 5

Semi-conductors are formed if the bonds are
(a) van der Waals
(b) ionic
(c) metallic
(d) covalent

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Problem 6

Semi-conductor devices are
(a) temperature dependent
(b) current dependent
(c) voltage dependent
(d) none of the above

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Problem 7

Which one of the following is not the advantage of semi-conductor device in the electron tubes?
(a) Unlimited life
(b) Greater efficiency
(c) No-warming up time for switching
(d) Low consumption of power for cathode heating

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Problem 8

If $\mu_{e}$ and $\mu_{h}$ are electron and hole mobility, $E$ be the applied electric field, the current density $J$ for intrinsic semi-conductor is equal to
(a) $n_{1} e\left(\mu_{e}+\mu_{h}\right) E$
(b) $n_{1} e\left(\mu_{n}-\mu_{h}\right) E$
(c) $\frac{n_{1} e\left(\mu_{e}+\mu_{h}\right)}{E}$
(d) $\frac{E}{n_{1} e\left(\mu_{e}+\mu_{h}\right)}$

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Problem 9

If the resistivity of copper is $1.7 \times 10^{-0} \Omega \mathrm{cm}$, then the mobility of electrons in copper, if each atom of copper contributes one free electron for conduction, is [The atomic weight of copper is $63.54$ and its density is $8.96 \mathrm{~g} / \mathrm{cc}]:$
(a) $23.36 \mathrm{~cm}^{2} / \mathrm{Vs}$
(b) $503.03 \mathrm{~cm}^{2} / \mathrm{Vs}$
(c) $43.25 \mathrm{~cm}^{2} / \mathrm{Vs}$
(d) $88.0 \mathrm{~cm}^{2} / \mathrm{Vs}$

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Problem 10

A pure silicon crystal of length $l(0.1 \mathrm{~m})$ and area $A\left(10^{-4} \mathrm{~m}^{2}\right)$ has the mobility of electron $\left(\mu_{e}\right)$ and holes
$\left(\mu_{h}\right)$ as $0.135 \mathrm{~m}^{2} / \mathrm{Vs}$ and $0.048 \mathrm{~m}^{2} / \mathrm{Vs}$, respectively. If the
voltage applied across it is $2 \mathrm{~V}$ and the intrinsic charge concentration is $n_{l}=1.5 \times 10^{6} \mathrm{~m}^{-2}$, then the total current flowing through the crystal is :
(a) $8.78 \times 10^{-17} \mathrm{~A}$
(b) $6.25 \times 10^{-17} \mathrm{~A}$
(c) $7.98 \times 10^{-17} \mathrm{~A}$
(d) $2.456 \times 10^{-17} \mathrm{~A}$

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Problem 11

In a semi-conductor diode, the barrier potential offers opposition to only
(a) majority caurier in both regions
(b) minoriiy carrier in both regions
(c) free electrons in the $n$ -region
(d) holes in the $p$ -region

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Problem 12

Which one of the following diagrams correctly represents the energy levels in the $p$ -type semi-conductor?

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Problem 13

A potential barrier of $0.4 \mathrm{~V}$ exists across a $p-n$ junction. A constant electric field of magnitude $10^{6} \mathrm{~V} / \mathrm{m}$ exists in the depletion region. The width of depletion region is :
(a) $4 \times 10^{-7} \mathrm{~m}$
(b) $0.1 \mathrm{~mm}$
(c) $5 \times 10^{-7} \mathrm{~m}$
(d) none of these

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Problem 14

In the given circuit all diodes are ideal. The current through battery (shown in figure) is :
(a) $2 \mathrm{~A}$
(b) $1 \mathrm{~A}$
(c) $3 \mathrm{~A}$
(d) none of these

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Numerade Educator

Problem 15

In the given circuit, diode $D$ is ideal. The potential difference across $4 \Omega$ resistance is :
(a) $10 \mathrm{~V}$
(b) $5 \mathrm{~V}$
(c) $4 \mathrm{~V}$
(d) none of these

Ajay Singhal

Numerade Educator

Problem 16

In previous problem, assume that diode is not ideal. The drift current for diode is $40 \mu \mathrm{A}$. The potential difference across the $4 \Omega$ resistance is :
(a) $10 \mathrm{~V}$
(b) $40 \times 10^{-6} \mathrm{~V}$
(c) $160 \times 10^{-6} \mathrm{~V}$
(d) none of these

Ajay Singhal

Numerade Educator

Problem 17

In the given circuit, diodes are ideal. The equivalent resistance between points $A$ and $B$ is :
(a) $20 \Omega$
(b) $10 \Omega$
(c) infinity
(d) $40 \Omega$

Ajay Singhal

Numerade Educator

Problem 18

The circuit is shown in the figure. To obtain maximum current, what value of resistor must be connected in series with the diode, if the diode used in the circuit has a constant voltage drop, of $0.5 \mathrm{~V}$ at all currents and has a maximum power rating of 100 milliwatt?
(a) $1.5 \Omega$
(b) $5 \Omega$
(c) $6.67 \Omega$
(d) $200 \Omega$

Ajay Singhal

Numerade Educator

Problem 19

What is the plate current in a diode valve under the space charge limited operation, when the plate potential is $60 \mathrm{~V} ?$ In a diode valve, the plate current is $320 \mathrm{~mA}$, when the plate potential is 240 volts :
(a) $30 \mathrm{~mA}$
(b) $20 \mathrm{~mA}$
(c) $40 \mathrm{~mA}$
(d) $10 \mathrm{~mA}$

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Numerade Educator

Problem 20

A tungsten emitter works at $2500 \mathrm{~K}$. To increase the emission current densitiy by $20 \%$, how much change in the work function is required ? (Given: $\log 2=0.3$, $\log 3=0.477)$
(a) $0.016 \mathrm{eV}$
(b) $0.039 \mathrm{eV}$
(c) $2.54 \mathrm{eV}$
(d) $0.254 \mathrm{eV}$

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Numerade Educator

Problem 21

What is the work function of tungsten at $1500 \mathrm{~K}$ temperature, when a diode valve with a tungsten filament works at $1500 \mathrm{~K}$ ? Assume the work function of tungsten at $0 \mathrm{~K}$ is $4.52 \mathrm{eV}$ :
(a) $4.71 \mathrm{eV}$
(b) $0.39 \mathrm{eV}$
(c) $8.86 \mathrm{eV}$
(d) $1.25 \mathrm{eV}$

Ajay Singhal

Numerade Educator

Problem 22

Choose the correct option for the forward biased characteristics of a $p-n$ junction :

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Problem 23

In a $p-n$ junction diode, holes diffuse from the $p$ -region to the $n$ -region because:
(a) the free electrons in the $n$ -region attract them
(b) they are swept across the junction by the potential difference
(c) there is greater concentration of holes in the $p$ -region as compared to $n$ -region
(d) none of the above

Ajay Singhal

Numerade Educator

Problem 24

When we apply reverse bias to a junction diode, it:
(a) lowers the potential barrier
(b) raises the potential barrier
(c) greatly increases the minority carrier current
(d) greatly increases the majority carrier current

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Numerade Educator

Problem 25

In case of $p-n$ junction diode at high value of reverse bias, current rises sharply. The value of reverse bias is called :
(a) cut off voltage
(b) inverse voltage
(c) zener voltage
(d) critical voltage

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Numerade Educator

Problem 26

A $p-n$ diode is reverse biased. The resistance measured by an ohmmeter connected across it will be :
(a) zero
(b) low
(c) high
(d) infinite

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Numerade Educator

Problem 27

The depletion layer in a $p-n$ junction diode consists of layer of :
(a) positively charged donors on the $p$ -side and negatively charged acceptors on the $n$ -side
(b) negatively charged donors on the $p$ -side and positively charged acceptors on the $n$ -side
(c) positively charged donors on the $n$ -side and negatively charged acceptors on the $p$ -side
(d) negatively charged donors on the $n$ -side and positively charged acceptors on the $p$ -side

Ajay Singhal

Numerade Educator

Problem 28

A transistor operating in a common base configuration has forward current gain factor, $\alpha=0.99 .$ If the emitter current changes by $1 \mathrm{~mA}$, then the changes in the base current will be :
(a) $100 \mathrm{~mA}$
(b) $0.01 \mathrm{~mA}$
(c) $0.99 \mathrm{~mA}$
(d) $99 \mathrm{~mA}$

Ajay Singhal

Numerade Educator

Problem 29

If both the collector and emitter junctions of a transistor are forward biased, the transistor is sald to operate in the :
(a) active region
(b) saturation region
(c) cut off region
(d) none of these

Ajay Singhal

Numerade Educator

Problem 30

In the active region operation of a transistor:
(a) the collector-emitter junction is forward biased
(b) the collector-emitter junction is reverse biased
(c) the collector-base junction is forward biased and the emitter junction is reverse biased
(d) the collector junction is reverse biased and emitter-base junction is forward biased

Ajay Singhal

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Problem 31

Choose the correct option for $n-p-n$ transistor :

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Problem 32

If the ratio of change in current in emitter and corresponding change in current in collector is $1.013$, then the value of $\alpha$ is :
(a) $0.987$
(b) $0.100$
(c) $0.900$
(d) none of these

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Numerade Educator

Problem 33

In previous problem, the value of $\beta$ is :
(a) $75.92$
(b) $76.92$
(c) $78.32$
(d) $0.987$

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Numerade Educator

Problem 34

A signal of $20 \mathrm{mV}$ is applied to common emitter transistor amplifier circuit. Due to this, the change in base current and the change in collector current are $20 \mu \mathrm{A}$ and $2 \mathrm{~mA}$. The load resistance is $10 \mathrm{k} \Omega$. The voltage gain is:
(a) $20 \mathrm{~V}$
(b) $10 \mathrm{~V}$
(c) $50 \mathrm{~V}$
(d) none of these

Ajay Singhal

Numerade Educator

Problem 35

In previous problem, transconductance is
(a) $0.1 \Omega^{-1}$
(b) $0.2 \Omega^{-1}$
(c) $10 \Omega^{-1}$
(d) none of these

Ajay Singhal

Numerade Educator

Problem 36

In previous problem, input resistance is
(a) $10 \Omega$
(b) $0.1 \mathrm{k} \Omega$
(c) $2 \mathrm{k} \Omega$
(d) none of these

Ajay Singhal

Numerade Educator

Problem 37

What is the current gain for a transistor used as common emitter amplifier, if the current gain of the common base $n-p-n$ transistor is $0.96 ?$
(a) 16
(b) 24
(c) 20
(d) 32

Ajay Singhal

Numerade Educator

Problem 38

The input resistance of a common emitter transistor amplifier, if the output resistance is $500 \mathrm{k} \Omega$, the current gain $\alpha=0.98$ and the power gain is $6.0625 \times 10^{6}$, is :
(a) $198 \Omega$
(b) $300 \Omega$
(c) $100 \Omega$
(d) $400 \Omega$

Ajay Singhal

Numerade Educator

Problem 39

The plate resistance, if for $5 \mathrm{k} \Omega$ of load the value of voltage gain of an amplifier be $1 / 3$ of the amplification factor, is :
(a) $12 \mathrm{k} \Omega$
(b) $10 \mathrm{k} \Omega$
(c) $15 \mathrm{k} \Omega$
(d) $5 \mathrm{k} \Omega$

Ajay Singhal

Numerade Educator

Problem 40

In the operation of $n-p-n$ transistor compared to that of a triode, the $p$ base acts as :
(a) emitter
(b) cathode
(c) grid
(d) plate

Ajay Singhal

Numerade Educator

Problem 41

A transistor, when connected in common emitter mode, has a :
(a) high input resistance and a low output resistance
(b) medium input resistance and high output resistance
(c) very low input resistance and a low output resistance
(d) high input resistance and a high output resistance

Ajay Singhal

Numerade Educator