Additional Questions - Chapter 9 Semiconductor Electronics 12th Science Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

Updated On May 15, 2024

By SaraNextGen

Additional Questions
Multiple Choice Questions
Question 1.
The probability of electrons to be found in the conduction band of an intrinsic semiconductor at a finite temperature
(a) increase exponentially with increasing band gap
(b) decrease exponentially with increasing band gap
(c) decreases with increasing temperature
(d) is independent of the temperature and the band gap.
Answer:
(b) decrease exponentially with increasing band gap
Hint:
At a finite temperature, the probability of jumping an electron from valence band to conduction band decreases exponentially with the increasing band gap $\left(\mathrm{E}_{\mathrm{g}}\right)$
$
\mathrm{n}=\mathrm{n}_0 \mathrm{e}^{-\mathrm{E}_{\mathrm{g}} / \mathrm{k}_{\mathrm{B}} \mathrm{T}}
$
Question 2.
The electrical conductivity of a semiconductor increase when electromagnetic radiation of wavelength shorter than $2480 \mathrm{~nm}$ is incident on it. The band gap (in $\mathrm{eV}$ ) for the semiconductor is-
(a) 0.9
(b) 0.7
(c) 0.5
(d) 1.1 .
Answer:
(c) 0.5
Hint:
$
\mathrm{E}_g=\frac{h c}{\lambda_{\max }}=\frac{1237.5 \mathrm{eVnm}}{2480 \mathrm{~nm}}=0.5 \mathrm{eV}
$
Question 3.
Which of the following statements is not true?
(a) The resistance of intrinsic semiconductor decreases with increase of temperature.
(b) Doping pure Si with trivalent impurities gives p-type semiconductors.
(c) The majority carriers in n-type semiconductors and holes
(d) Ap-n junction can act as a semiconductor diode.
Answer:

(c) The majority carriers in $\mathrm{n}$-type semiconductors and holes
Hint:
The majority charge carriers in $\mathrm{n}$-type semiconductors are electrons not holes. Only option (c) is not true.
Question 4.
Holes are charge carrier in
(a) intrinsic semiconductors
(b) ionic solids
(c) p-type semiconductor
(d) metals.
Answer:
(a) intrinsic semiconductors
Hint:
(a) In intrinsic semiconductor, $\mathrm{n}_{\mathrm{h}}=\mathrm{n}_{\mathrm{e}}$
(b) In P-type semiconductor, $\mathrm{n}_{\mathrm{h}} \gg \mathrm{n}_{\mathrm{e}}$
Question 5.
A transistor is used in the common emitter mode as an amplifier. Then
(a) the base - emitter junction is forward - biased
(b) the base - emitter junction is reverse - biased
(c) the input signal is connected in series with the voltage applied to bias the base-emitter junction
(d) the input signal is connected in series with the voltage applied to bias the base - collector junction.
Answer:
(c) the input signal is connected in series with the voltage applied to bias the base-emitter junction Hint:
In CE - transistor amplifier, the base - emitter junction is forward bias and the input signal is connected in series with the base - emitter battery.
Question 6.
The energy band gap is maximum in-
(a) metals
(b) superconductors
(c) insulators
(d) semiconductors.

Answer:
(c) insulators
Hint:
The band gap is maximum in insulators.
Question 7.
At absolute zero, Si acts as-
(a) non - metal
(b) metal
(c) insulator
(d) none of these.
Answer:
(c) insulator
Hint:
At absolute zero, Si acts as an insulator due to the absence of free electrons in the conduction band.
Question 8.
Apiece of copper and another of germanium are cooled from room temperature to $77 \mathrm{~K}$. The resistance of
(a) each of these decreases
(b) copper strip increases and that of germanium decreases
(c) copper strip decreases and that of germanium increases
(d) each of these increases.
Answer:
(c) copper strip decreases and that of germanium increases
Hint:
With the decrease of temperature, the resistance of copper (a metallic conductor) decreased while that of germanium (a semiconductor) increases.
Question 9.
In the middle of the depletion layer of reverse biased $p-n$ junction, the-
(a) electric field is zero
(b) potential is zero
(c) potential is maximum
(d) electric field is maximum.
Answer:
(a) electric field is zero
Hint:
When a $\mathrm{p}-\mathrm{n}$ junction is reverse biased, the width of the depletion layer becomes large and so the electric field $(E=v / d)$ becomes very small, nearly zero.
Question 10.
In a full wave rectifier circuit operating from $50 \mathrm{~Hz}$ mains, frequency, the fundamental frequency in the ripple would be-
(a) $50 \mathrm{~Hz}$

(b) $25 \mathrm{~Hz}$
(c) $100 \mathrm{~Hz}$
(d) $70.7 \mathrm{~Hz}$
Answer:
(c) $100 \mathrm{~Hz}$
Hint:
The frequency of the ripple in the output of a fullwave rectifier is twice the frequency of the a.c. input. Hence, it is $100 \mathrm{~Hz}$.
Question 11.
The part of a transistor, which is heavily doped to produce a large number of majority carriers is called?
(a) emitter
(b) base
(c) collector
(d) any one out of emitter, base and collector.
Answer:
(a) emitter
Hint:
Emitter of a transistor is heavily doped so as to act as source of majority charge carriers.
Question 12.
When $\mathrm{n}-\mathrm{p}-\mathrm{n}$ transistor is used as an amplifier, then-
(a) electrons move from base to collector
(b) holes move from emitter to base
(c) electrons move from collector to base
(d) holes move from base to emitter.
Answer:
(a) electrons move from base to collector
Hint:
When $\mathrm{n}-\mathrm{p}-\mathrm{n}$ transistor is in operation, the majority charge carriers, i.e., electrons move from emitter to base and then to collector.
Question 13.
In a common - base amplifier, the phase difference between the input signal voltage and the output voltage (across collector and base) is
(a) 0
(b) $\pi / 4$
(c) $\pi / 2$
(d) $\pi$.
Answer:

(a) 0

In a common - base amplifier, the input and output voltages are in the same phase.
Question 14
In a common - base mode of a transistor, the collector current is $5.488 \mathrm{~mA}$ for an emitter current of 5.60
A. The value of the base current amplification factor $(\beta)$ will be-
(a) 49
(b) 50
(c) 51
(d) 48 .
Answer:
(a) 49
Hint:
$
\beta=\frac{I_C}{I_B}=\frac{5.488}{0.112}=49
$
Question 15.
In the circuit below, $\mathrm{A}$ and $\mathrm{B}$ represent two inputs and $\mathrm{C}$ represents the output. The circuit represents

(a) OR gate
(b) NOR gate
(c) AND gate
(d) NAND gate.
Answer:
(a) OR gate
Hint:
The given circuit represents an OR gate, when either A or B or both inputs are high, the output $\mathrm{C}$ is high.
Question 16.
In a fee lattice structure, what is the effective number of atoms?
(a) 4
(b) 3
(c) 2
(d) 1 .
Answer:
(a) 4
Hint:
$
\mathrm{N}=\frac{N_C}{8}+\frac{N_F}{2}=\frac{8}{8}+\frac{6}{2}=4
$
Question 17.
Monoclinic crystal lattice has dimensions-
(a) $\alpha=\beta=\gamma$
(b) $\alpha=\beta=90^{\circ}, \gamma \neq 90^{\circ}$
(e) $\alpha \neq \beta \neq \gamma$
(d) none of these.
Answer:
(b) $\alpha=\beta=90^{\circ}, \gamma \neq 90^{\circ}$
Hint:
For a monoclinic crystal, $\alpha \neq \beta \neq \mathrm{c}$ and $\alpha=\beta=90^{\circ} \neq \gamma$.
Question 18 .
In insulators?
(a) valence band is partially filled
(b) conduction band is partially filled with electrons
(c) conduction band is filled with electrons and valence band is empty
(d) conduction band is empty and valence band is completely filled with electrons.
Answer:
(d) conduction band is empty and valence band is completely filled with electrons.
Hint:
In insulators, the conduction band is empty and valence band is completely filled with electrons.

Question 19.
The valence band and conduction band of a solid overlap at low temperature, the solid may be-
(a) a metal
(b) a semiconductor
(c) an insulator
(d) none of these.
Answer:
(a) a metal
Hint:
In metals, the valence band and conduction band may overlap at low temperature.
Question 20.
If germanium is dopped with arsenic, that will result in-
(a) n-type semiconductor
(b) p-type semiconductor
(c) intrinsic semiconductor
(d) none of these.
Answer:
(a) n-type semiconductor
Hint:
Arsenic is pentavalent. Its doping with germanium results in $\mathrm{n}$-type semiconductor.
Question 21.
n-type semiconductor is
(a) positive charged
(b) negatively charged
(c) neutral
(d) positive or negative depending upon doping material.
Answer:
(c) neutral
Hint:
Semiconductors maintain their electrical neutrality even after doping.
Question 22.
To a germanium ciystal equal number of aluminium and indium atoms are added. Then
(a) it remains an intrinsic semiconductor
(b) it becomes an n-type semiconductor
(c) it becomes a p-type semiconductor
(d) it becomes an insulator.
Answer:
(c) it becomes a p-type semiconductor
Hint:
Both A1 and In are trivalent atoms. Their doping in germanium results in a p-type semiconductor.

Question 23.
The dominant contribution to current comes from holes in case of-
(a) metals
(b) intrinsic semiconductors
(c) p-type extrinsic semiconductors
(d) n-type extrinsic semiconductors.
Answer:
(c) p-type extrinsic semiconductors
Hint:
Holes are the majority charge carriers in p-type extrinsic semiconductors.
Question 24 .
For a heavily doped n-type semiconductor, fermi - level lies-
(a) a little below the conduction band
(b) a little above the valence band
(c) a little inside the valence band
(d) at the centre of the band gap.
Answer:
(a) a little below the conduction band
Hint:
For a heavily doped n-type semiconductor, the fermi level lies slightly below the bottom of the conduction band.
Question 25.
The coordination number for a bcc crystal is-
(a) 4
(b) 8
(c) 12
(d) 6 .
Answer:
(b) 8
Hint:
The eight comer atoms of the unit cell are close neighbourers of the atom at the body centre.
Question 26.
If the forward voltage in a diode is increased the width of the depletion region-
(a) increases
(b) decreases
(c) fluctuates
(d) no change.

Answer:
(b) decreases
Hint:
If the forward voltage in a diode is increased, the width of depletion region decreases.
Question 27.
In order to rectify an alternating current one uses a-
(a) thermocouple
(b) diode
(c) triode
(d) transistor.
Answer:
(b) diode
Hint:
A diode is used to rectify an alternating current.
Question 28.
Why is there sudden increase in current in zener diode?
(a) due to rupture of bonds
(b) resistance of depletion layer becomes less
(c) due to high doping
(d) none of the above.
Answer:
(a) due to rupture of bonds
Hint:
The sudden increase in current in a zener diode is due to the rupture of many covalent bonds.
Question 29.
In a transistor
(a) there is $1 \mathrm{p}-\mathrm{n}$ junction
(b) there are $2 \mathrm{p}-\mathrm{n}$ junction
(c) there are $3 \mathrm{p}-\mathrm{n}$ junction
(d) none of the above.
Answer:
(b) there are $2 \mathrm{p}-\mathrm{n}$ junction
Hint:
A transistor is like a combination of two p-n junctions placed back to back.

Question 30.
Let $i_e, i_c$ and $i_b$ represent emitter current, collector current and the base current of a transistor, then
(a) $i_c>i_e$
(b) $i_b>i_c$
(c) $i_b<i_c$
(d) $i_e>i_c$
Answer:
(d) $i_e>i_c$
Hint:
As $i_e=i_c+i_b$
$\therefore \mathrm{i}_{\mathrm{i}}>\mathrm{c}$
Question 31.
In common - emitter amplifier the ratio $\frac{I_C}{I_E}$ is 0.98 . The current gain will be
(a) 4.9
(b) 7.8
(c) 49
(d) 78 .
Answer:
(c) 49
Hint:
$
\frac{\alpha}{1-\alpha}=\frac{0.98}{1-0.98}=\frac{0.98}{0.02}=49
$
Question 32.
The gate for which output is high, if atleast one input is low?
(a) NAND
(b) NOR
(c) AND
(d) OR.
Answer:
(a) NAND
Hint:
The output of a NAND gate is high, if atleast one input is low.

Question 33.
An oscillator is nothing but an amplifier with-
(a) positive feedback
(b) large gain
(c) no feedback
(d) negative feedback.
Answer:
(a) positive feedback
Hint:
When a transistor is used as an amplifier with positive feedback, it works as an oscillator.
Question 34.
Crystalline solid are?
(a) anisotropic
(b) isotropic
(c) amporphus
(d) none of these.
Answer:
(a) anisotropic
Hint:
Crystalline solids are anisotropic as they show different physical properties along different directions.
Question 35.
Which of the following is an amorphous solid?
(a) Glass
(b) Diamond
(c) Salt
(d) Sugar.
Answer:
(a) Glass
Question 36.
Energy required to break one band in DNA is
(a) $\approx 1 \mathrm{eV}$
(b) $\approx 0.1 \mathrm{eV}$
(c) $\approx 0.01 \mathrm{eV}$
(d) $\approx 2.1 \mathrm{eV}$.
Answer:
(a) $\approx 1 \mathrm{eV}$
Hint:
The bond strength in DNA is nearly $1 \mathrm{eV}$.

Question 37.
An intrinsic semiconductor, at the absolute zero temperature, behaves like a/an?
(a) insulator
(b) superconductor
(c) n-type semiconductor
(d) p-type semiconductor.
Answer:
(a) insulator
Hint:
At the absolute zero temperature, an intrinsic semiconductor behaves like an insulator.
Question 38 .
In a semiconducting material the mobilities of electrons and holes are $\mu_{\mathrm{e}}$ and $\mu_{\mathrm{h}}$ respectively. Which of the following is true?
(a) $\mu_{\mathrm{e}}>\mu_{\mathrm{h}}$
(b) $\mu_{\mathrm{e}}<\mu_{\mathrm{h}}$
(c) $\mu_{\mathrm{e}}=\mu_{\mathrm{h}}$
(d) $\mu_{\mathrm{e}}<0 ; \mu_{\mathrm{h}}>0$.
Answer:
(a) $\mu_{\mathrm{e}}>\mu_{\mathrm{h}}$
Hint:
The mobility of an electron in the conduction is more than the mobility of a hole in the valence band.
Question 39.
In a pure semiconductor crystal, if current flows due to breakage of crystal bonds, then the semiconductor is called
(a) acceptor
(b) donor
(c) intrinsic semiconductor
(d) extrinsic semiconductor.
Answer:
(c) intrinsic semiconductor
Hint:
Pure semiconductors are called intrinsic semiconductors.

Question 40.
Which of the following, when added as an impurity into the silicon, produces n-type semiconductor?
(a) phosphorous
(b) aluminium
(c) magnesium
(d) both (b) and (c).
Answer:
(a) phosphorous
Hint:
As phosphorous is pentavalent, it produces n-type semiconductor when added to silicon.
Question 41.
In n-type semiconductors, majority charge carriers are
(a) holes
(b) protons
(c) neutrons
(d) electrons.
Answer:
(d) electrons
Question 42.
In p-type semiconductor,
(a) major current carrier are electrons
(b) major carrier are mobile negative ions
(c) major carrier are mobile holes
(d) the number of mobile holes exceeds the number of acceptor atoms.
Answer:
(c) major carrier are mobile holes
Hint:
In p-type semiconductors, holes are the majority charge carriers.

Question 43.
The potential barrier in the depletion layer is due to-
(a) ions
(b) holes
(c) electrons
(d) forbidden band.
Answer:
(a) ions
Hint:
The potential barrier in the depletion layer is due to the presence of immobile ions.
Question 44.
When a p-n diode is reverse biased, then
(a) no current flows
(b) the depletion region is increased
(c) the depletion region is reduced
(d) the height of the potential barrier is reduced.
Answer:
(b) the depletion region is increased
Hint:
When a p-n junction is reverse biased, its depletion region is widened.
Question 45.
If a $\mathrm{p}-\mathrm{n}$ diode is reverse biased, then the resistance measured by an ohm meter, will be
(a) zero
(b) low
(c) high
(d) infinite.
Answer:
(c) high
Hint:
When a p-n diode is reverse biased, it offers a high resistance.
Question 46.
Diode is used as an/a?
(a) oscillator
(b) amplifier
(c) rectifier
(d) modulator.

Answer:
(c) rectifier
Question 47.
In the half wave rectifier circuit operating from $50 \mathrm{~Hz}$ main frequency, the fundamental frequency in the ripple would be-
(a) $25 \mathrm{~Hz}$
(b) $50 \mathrm{~Hz}$
(c) $70.7 \mathrm{~Hz}$
(d) $100 \mathrm{~Hz}$
Answer:
(b) $50 \mathrm{~Hz}$
Hint:
In a half wave rectifier, fundamental frequency in the ripple $=$ Input frequency $=50 \mathrm{~Hz}$.
Question 48.
Zener diode acts as a/an?
(a) oscillator
(b) regulator
(c) rectifier
(d) filter.
Answer:
(b) regulator
Question 49.
A transistor is a/an?
(a) chip
(b) insulator
(c) semiconductor
(d) metal.
Answer:
(c) semiconductor
Question 50.
The minimum potential difference between the base and emitter required to switch a silicon transistor $\mathrm{ON}$ is approximately.
(a) IV
(b) $3 \mathrm{~V}$
(c) $5 \mathrm{~V}$
(d) $4.2 \mathrm{~V}$
Answer:
(a) IV
Hint:
For switching on a silicon transistor, $\left(\mathrm{V}_{\mathrm{BE}}\right)_{\min } \approx 1 \mathrm{~V}$.

Question 51.
When n-p-n transistor is used as an amplifier, then
(a) holes moves from emitter
(b) electrons move from base to collector
(c) holes move from base to emitter
(d) electrons move from collector to base.
Answer:
(b) electrons move from base to collector
Question 52.
The current gain for a transistor working as common base amplifier is 0.96 . If the emitter current is 7.2 $\mathrm{mA}$, then the base current is-
(a) $0.29 \mathrm{~mA}$
(b) $0.35 \mathrm{~mA}$
(c) $0.39 \mathrm{~mA}$
(d) $0.43 \mathrm{~mA}$
Answer:
(a) $0.29 \mathrm{~mA}$
Hint:
$
\begin{aligned}
& \alpha=\frac{I_C}{I_E} \text { (or) } 0.96=\frac{I_C}{7.2 m A} \\
& \mathrm{I}_{\mathrm{C}}=0.96 \times 7.2=6.91 \mathrm{~mA} \\
& \mathrm{I}_{\mathrm{B}}=\mathrm{I}_{\mathrm{C}}-\mathrm{I}_{\mathrm{E}}=7.2-6.91=0.29 \mathrm{~mA}
\end{aligned}
$
Question 53.
Consider an n-p-n transistor amplifier in common - emitter configuration. The current gain of the transistor is 100 . If the collector current changes by $1 \mathrm{~mA}$, what will be the change in emitter current?
(a) $1.1 \mathrm{~mA}$
(b) $1.01 \mathrm{~mA}$
(c) $0.01 \mathrm{~mA}$
(d) $10 \mathrm{~mA}$.
Answer:
(b) $1.01 \mathrm{~mA}$
Hint:
$
\begin{aligned}
& \beta=\frac{\Delta I_C}{\Delta I_E} \therefore \Delta \mathrm{I}_{\mathrm{B}}=\frac{\Delta I_C}{\beta}=\frac{1 m A}{\beta}=0.01 \mathrm{~mA} \\
& \Delta \mathrm{I}_{\mathrm{E}}=\Delta \mathrm{I}_{\mathrm{B}}+\Delta \mathrm{I}_{\mathrm{C}}=0.01+1=1.01 \mathrm{~mA}
\end{aligned}
$

Question 54.
An amplifier has voltage gain $=1000$. The voltage gain (in $\mathrm{dB}$ ) is-
(a) $30 \mathrm{~dB}$
(b) $60 \mathrm{~dB}$
(c) $3 \mathrm{~dB}$
(d) $20 \mathrm{~dB}$.
Answer:
(b) $60 \mathrm{~dB}$
Hint:
Voltage gain in $\mathrm{dB}=20 \log _{10} \mathrm{~A}_{\mathrm{V}}=20 \log _{10}(1000)=20 \times 3=60 \mathrm{~dB}$.
Question 55.
Boolean algebra is essentially based on-
(a) logic
(b) truth
(c) numbers
(d) symbol.
Answer:
(a) logic
Question 56.
The number (0) zero is required for-
(a) transistor
(b) abacus
(c) computer
(d) calculator.
Answer:
(c) computer
Hint:
A computer work on binary digits 0 and 1.
Question 57.
Which of the following logic gates in a universal gate?
(a) OR
(b) NOT
(c) AND
(d) NAND.
Answer:
(d) NAND.
Hint:

NAND gate is a universal gate because its repeated use can give all basic gates like OR, AND and NOT gates.
Question 58.
Which of the following is the weakest kind of the bonding in solids?
(a) Ionic
(b) Metallic
(c) Van der waals
(d) Covalent.
Answer:
(c) Van der waals
Question 59.
The cations and anions are arranged in alternate form in-
(a) metallic crystal
(b) ionic crystal semi - conductor
(c) covalent crystal
(d) crystal.
Answer:
(b) ionic crystal semi - conductor
Question 60.
Diamond is very hard because-
(a) it is covalent solid
(b) it has large cohesive energy
(c) high melting point
(d) insoluble in all solvents.
Answer:
(b) it has large cohesive energy
Question 61.
Number of atoms per unit cell in bec lattice is-
(a) 9
(b) 4
(c) 2
(d) 1 .
Answer:
(c) 2
Hint:
$
\mathrm{N}=\mathrm{N}_{\mathrm{B}}+\frac{N_C}{8}=1+\frac{8}{8}=2
$

Question 62.
At absolute zero, Si acts as?
(a) non metal
(b) metal
(c) 2
(d) 1 .
Answer:
(c) 2
Hint:
At absolute zero, $\mathrm{Si}$ acts as an insulator because it has no free electrons in the conduction band.
Question 63.
Which of the following, when added as an impurity into the silicon produces n-type semi-conductor?
(a) $\mathrm{B}$
(b) $\mathrm{AL}$
(c) $\mathrm{P}$
(d) $\mathrm{Mg}$.
Answer:
(c) $\mathrm{P}$
Hint:
Only $P$ is a pentavalent impurity atom, its doping with germanium produces a p-types semi - conductor.
Question 64.
To obtain a p-type germanium semiconductor, it must be doped with-
(a) indium
(b) phosphorus
(c) arsenic
(d) antimony.
Answer:
(a) indium
Hint:
Only In is a trivalent impurity atom, its doping with germanium produces a p-type semi-conductor.
Question 65.
When arsenic is added as an impurity to silicon, the resulting material is-
(a) $\mathrm{n}$-type conductor
(b) n-type semiconductor
(c) $\mathrm{P}$ - type semiconductor
(d) none of these.
Answer:

(b) $\mathrm{n}$-type semiconductor
Hint:
When pentavalent arsenic is doped to silicon, it forms $\mathrm{n}$-type semi - conductor.
Question 66.
In a p-type semiconductor, the majority carriers of current are-
(a) protons
(b) electrons
(c) holes
(d) neutrons.
Answer:
(c) holes
Question 67.
The depletion layer in the $\mathrm{p}-\mathrm{n}$ junction region is caused by-
(a) drift of holes
(b) diffusion of charge carriers
(c) migration of impurity ions
(d) drift of electrons.
Answer:
(b) diffusion of charge carriers
Hint:
The depletion layer in the $\mathrm{p}-\mathrm{n}$ junction region is caused by diffusion of charge carriers.
Question 68.
In the depletion region of an unbiased $\mathrm{p}-\mathrm{n}$ junction diode, there are-
(a) holes
(b) mobile ions
(c) electrons
(d) immobile ions.
Answer:
(d) immobile ions.
Hint:
The depletion layer consists of immobile ions.
Question 69.
In forward bias, the width of potential barrier in a $\mathrm{p}-\mathrm{n}$ junction adiode.
(a) remain constant
(b) decreases
(c) increases
(d) first (a) then (b).
Answer:
(b) decreases

Question 70.
Reverse bias applied to a junction diode-
(a) lowers the potential barrier
(b) raises the potential barrier
(c) increases the majority carrier current
(d) increases the minority carrier current.
Answer:
(b) raises the potential barrier
Question 71.
Barrier potential of a $\mathrm{p}-\mathrm{n}$ junction diode does not depend on-
(a) diode design
(b) temperature
(c) forward bias
(d) doping density.
Answer:
(a) diode design
Hint:
Barrier potential depends upon temperature, doping density and forward biasing.
Question 72.
The peak voltage in the output of a half wave diode rectifier fed with a sinusoidal signal without filter is $10 \mathrm{~V}$. The d.c component of the output voltage is-
(a) $\frac{10}{\sqrt{2 V}}$
(b) $\frac{10}{\pi V}$
(c) $10 \mathrm{v}$
(d) $\frac{20}{\pi V}$.
Answer:
(b) $\frac{10}{\pi V}$
Hint:
$
\mathrm{V}_{\mathrm{dc}}=\mathrm{V}_{\mathrm{m}}=\frac{V_0}{\pi}=\frac{10}{\pi} \mathrm{V}
$
Question 73.
A p-n junction diode can be used as-
(a) condenser
(b) regulator
(c) amplifier
(d) rectifier.

Answer:
(d) rectifier.
Question 74.
Zener diode is used for-
(a) amplification
(b) rectification
(c) stabilisation
(d) producing oscillations in an oscillator.
Answer:
(d) producing oscillations in an oscillator.
Hint:
Zener diode can be used for stabilisation of voltage.
Question 75.
When n-p-n transistor is used as an amplifier, then
(a) electrons move from collector to base
(b) holes move from base to emitter
(c) electrons move from base to collector
(d) electrons move from emitter to base
Answer:
(c) electrons move from base to collector
Hint:
When n-p-n transistor is used an amplifier, the majority carrier electrons move from base to collector.
Question 76.
The correct relationship between the two current gains a and $P$ in a transistor is-
(a) $\alpha=\frac{\beta}{1+\beta}$
(b) $\alpha=\frac{1+\beta}{\beta}$
(c) $\beta=\frac{\alpha}{1+\alpha}$
(d) $\beta=\frac{\alpha}{\alpha-1}$.
Answer:
(a) $\alpha=\frac{\beta}{1+\beta}$
Hint:
$
\beta=\frac{\alpha}{1+\alpha} \text { (or) } \beta-\beta \alpha=\alpha ; \alpha=\frac{\beta}{1+\beta} \text {. }
$

Question 77.
The correct relation for $\mathrm{a}, \mathrm{P}$ for a transistor is-
(a) $\beta=\frac{1-\alpha}{\alpha}$
(b) $\beta=\frac{\alpha}{\alpha}$
(c) $\alpha=\frac{\beta-1}{\beta}$
(d) $\alpha \beta=1$.
Answer:
(b) $\beta=\frac{\alpha}{1-\alpha}$
Question 78.
For a common base circuit if $\frac{I_C}{I_E}=0.98$, then current gain for common emitter circuit will be-
(a) 49
(b) 98
(c) 4.9
(d) 25.5
Answer:
(a) 49
Hint:
Here $\frac{I_C}{I_E}=\alpha=0.98 ; \beta=\frac{\alpha}{1-\alpha}=\frac{0.98}{1-0.98}=49$.
Question 79.
Radio waves of constant amplitude can be generated with-
(a) FET
(b) filter
(c) rectifier
(d) oscillator.
Answer:
(d) oscillator.
Question 80.
An oscillator is an amplifier with-
(a) a large gain
(b) negative feedback
(c) positive feedback
(d) no feedback.
Answer:
(c) positive feedback

Question 81.
The output of OR gate is 1
(a) if both inputs are zero
(b) if either or both inputs are 1
(c) only if both inputs are 1
(d) if either input is zero.
Answer:
(b) if either or both inputs are 1
Question 82.
The device that can act as a complete electronic circuit is-
(a) junction diode
(b) integrated circuit
(c) junction transistor
(d) zener diode.
Answer:
(b) integrated circuit
Question 83.
At absolute zero temperature, a semiconductor acts as a/an.
(a) dielectric
(b) conductor
(c) insulator
(d) none of these.
Answer:
(c) insulator
Question 84.
At which temperature, a pure semiconductor behave slightly as a conductor?
(a) low temperature
(b) room temperature
(c) high temperature
(d) both (a) and (b).
Answer:
(b) room temperature

Question 85.
In germanium crystal, the forbidden energy gap in joule is.
(a) $1.6 \times 10^{-19}$
(b) zero
(c) $1.12 \times 10^{-19}$
(d) $1.76 \times 10^{-19}$
Answer:
(c) $1.12 \times 10^{-19}$
Hint:
For a germanium crystal,
$
\mathrm{E}_{\mathrm{g}}=0.7 \mathrm{eV}=0.7 \times 1.6 \times 10^{-19} \mathrm{~J}=1.12 \times 10^{-19} \mathrm{~J} \text {. }
$
Question 86.
In a p-type semiconductor, germanium is doped with.
(a) gallium
(b) boron
(c) aluminium
(d) all of these
Answer:
(d) all of these
Hint:
$\mathrm{Ga}, \mathrm{B}$ and $\mathrm{A} 1$ are all trivalent atoms, they produce p-type semiconductor.
Question 87.
The major carrier of current in a p-type semiconductor will be.
(a) neutrons
(b) protons
(c) electrons
(d) holes.
Answer:
(d) holes.
Hint:
Holes are the major carriers of current in a p-type semiconductor.
Question 88.
Rectification is the process of conversion of
(a) a.c into d.c
(b) low a.c into high a.c
(c) d.c into a.c
(d) low d.c into high d.c
Answer:
(a) a.c into d.c

Question 89.
Which type of gate is represented by the given figure?

(a) NAND
(b) NOT
(c) AND
(d) OR.
Answer:
(b) NOT
Hint:
When both the inputs of a NAND gate are joined, it functions as a NOT gate.
Question 90.
Which of the following gates can be served as a building block for any digital circuit?
(a) $O R$
(b) NOT
(c) AND
(d) NAND.
Answer:
(d) NAND.
Hint:
A NAND gate is a universal gate.
Short Answer Questions
Question 1.
What are passive and active components.
Answer:
Passive components:
Components that cannot generate power in a circuit.
Active components:
components that can generate power in a circuit.

Question 2.
What are energy band?
Answer:
The band of very large number of closely spaced energy levels in a very small energy range is known as energy band.
Question 3.
What are valence band, conduction band and forbidden energy gap.
Answer:
The energy band formed due to the valence orbitals is called valence band and that formed due to the unoccupied orbitals is called the conduction band. The energy gap between the valence band and the conduction band is called forbidden energy gap.
Question 4.
What are intrinsic semiconductor?
Answer:
A semiconductor in its pure form without impurity is called an intrinsic semiconductor. In intrinsic semiconductors, the number of electrons in the conduction band is equal to the number of holes in the valence band.
Question 5.
What are extrinsic semiconductor?
Answer:
An extrinsic semiconductor is a semiconductor doped by a specific impurity which is able to deeply modify its electrical properties, making it suitable for electronic applications (diodes, transistors etc.) or optoelectronic applications (light emitters and detectors).
Question 6.
What are holes?
Answer:
The vacancy or absense of an electron in the bond of a covalently bonded crystal is called a hole.
Question 7.
What is meant by biasing and bias voltage?
Answer:
Biasing means providing external energy to charge carriers to overcome the barrier potential and make them move in a particular direction. The external voltage applied to the $\mathrm{p}-\mathrm{n}$ junction is called bias voltage.

Question 8.
What are called forward bias and reverse bias?
Answer:
- If the positive terminal of the external voltage source is connected to the p-side and the negative terminal to the $\mathrm{n}$-side, it is called forward bias.
- If the positive terminal of the battery is connected to the $\mathrm{n}$-side and the negative potential to the $\mathrm{p}$ side, the junction is said to be reverse biased.
Question 9.
Define knee voltage or threshold voltage or cut-in voltage.
Answer:
At room temperature, a potential difference equal to the barrier potential is required before a reasonable forward current starts flowing across the diode. This voltage is known as threshold voltage or cut-in voltage or knee voltage $\left(\mathrm{V}_{\text {th }}\right)$.
Question 10.
What is a Rectification?
Answer:
Rectification is the process of converting alternating current into direct current is called rectification.
Question 11.
Define - Efficiency of rectifier.
Answer:
Efficiency $(\eta)$ is the ratio of the output dc power to the ac input power supplied to the circuit.
Question 12.
What is meant by zener effect?
Answer:
The electric field is strong enough to break or rupture the covalent bonds in the lattice and thereby generating electron-hole pairs. This effect is called Zener effect.
Question 13.
What is a zener diode?
Answer:
A junction diode specially designed to operate only in the reverse breakdown region continuously (without getting damaged) is called a zener diode.

Question 14.
Write down the applications of zener diode.
Answer:
The zener diode can be used as:
1. Voltage regulators
2. Peak clippers
3. Calibrating voltages
4. Provide fixed reference voltage in a network for biasing
5. Meter protection against damage from accidental application of excessive voltage.
Question 15.
What is peak inverse voltage (PIV)?
Answer:
The maximum reverse bias that can be applied before entering into the Zener region is called the Peak inverse voltage.
Question 16.
What is Optoelectronic devices?
Answer:
Optoelectronics deals with devices which convert electrical energy into light and light into electrical energy through semiconductors.
Question 17.
Write down the applications of LED's?
Answer:
- Indicator lamps on the front panel of the scientific and laboratory equipments.
- Seven-segment displays.
- Traffic signals, exit signs, emergency vehicle lighting etc.
- Industrial process control, position encoders, bar graph readers.
Question 18 .
Write down the applications of photodiodes?
Answer:
- Alarm system
- Count items on a conveyer belt
- Photoconductors
- Compact disc players, smoke detectors
- Medical applications such as detectors for computed tomography etc.
Question 19.
Write down the applications of solar cell.
Answer:

- Solar cells are widely used in calculators, watches, toys, portable power supplies, etc.
- Solar cells are used in satellites and space applications
- Solar panels are used to generate electricity.
Question 20.
What is a solar cell.
Answer:
A solar cell, also known as photovoltaic cell, converts light energy directly into electricity or electric potential difference by photovoltaic effect.
Question 21.
Write down the applications of Oscillators.
Answer:
Applications of oscillators:
- to generate a periodic sinusoidal or non sinusoidal wave forms.
- to generate RF carriers.
- to generate audio tones
- to generate clock signal in digital circuits.
- as sweep circuits in TV sets and CRO.
Question 22.
Write down concept of Barkhausen conditions for sustained oscillations.
Answer:
Barkhausen conditions for sustained oscillations
The following condition called Barkhausen conditions should be satisfied for sustained oscillations in the oscillator.
- The loop phase shift must be $0^{\circ}$ or integral multiples of $2 \pi$.
- The loop gain must be unity. $|\mathrm{A} \beta|=1$ Here; $A \rightarrow$ Voltage gain of the amplifier, $\mathrm{b} \rightarrow$ feedback ratio; (fraction of the output that is fed back to the input)
Long Answer Questions
Question 1.
Explain the classification of materials.
Answer:
Insulators:
The valence band and the conduction band are separated by a large energy gap. The forbidden energy gap is approximately $6 \mathrm{eV}$ in insulators. The gap is very large that electrons from valence band cannot move into conduction band even on the application of strong external electric field or the increase in temperature. Therefore, the electrical conduction is not possible as the free electrons are almost nil and hence these materials are called insulators. Its resistivity is in the range of $10^{11}-10^{19} \Omega \mathrm{m}$.
Metals:
In metals, the valence band and conduction band overlap. Hence, electrons can move freely into the conduction band which results in a large number of free electrons in the conduction band. Therefore, conduction becomes possible even at low temperatures. The application of electric field provides sufficient energy to the electrons to drift in a particular direction to constitute a current. For metals, the resistivity value lies between $10^{-2}$ and $10^{-8} \Omega \mathrm{m}$.
Semiconductors:
In semiconductors, there exists a narrow forbidden energy gap $\left(\mathrm{E}_{\mathrm{g}}<3 \mathrm{eV}\right)$ between the valence band and the conduction band. At a finite temperature, thermal agitations in the solid can break the covalent bond between the atoms (covalent bond is formed due to the sharing of electrons to attain stable electronic configuration). This releases some electrons from valence band to conduction band. Since free electrons are small in number, the conductivity of the semiconductors is not as high as that of the conductors. The resistivity value of semiconductors is from $10^{-5}$ to $10^6 \Omega \mathrm{m}$.
Question 2.
Explain zener diode as a voltage regulator.
Answer:
Zener diode as a voltage regulator:
A Zener diode working in the breakdown region can serve as a voltage regulator. It maintains a constant output voltage even when input voltage $V_i$ or load current $I_L$ varies. Here, in this circuit the input voltage $\mathrm{V}_{\mathrm{i}}$ is regulated at a constant voltage, $\mathrm{V}_{\mathrm{z}}$ (Zener voltage) at the output represented as $\mathrm{V}_0$ using a Zener diode. The output voltage is maintained constant as long as the input voltage does not fall below $\mathrm{z}_{\mathrm{z}}$.

output voltage even when input voltage $\mathrm{V}_{\mathrm{i}}$ or load current $\mathrm{I}_{\mathrm{L}}$ varies. Here, in this circuit the input voltage $\mathrm{V}_{\mathrm{i}}$ is regulated at a constant voltage, $\mathrm{V}_{\mathrm{z}}$ (Zener voltage) at the output represented as $\mathrm{V}_0$ using a Zener diode. The output voltage is maintained constant as long as the input voltage does not fall below $\mathrm{Z}_{\mathrm{z}}$

When the potential developed across the diode is greater than $\mathrm{V}_{\mathrm{z}}$, the diode moves into the Zener breakdown region. It conducts and draws relatively large current through the series resistance $\mathrm{R}_{\mathrm{i}}$. The total current I passing through $R_i$ equals the sum of diode current $I_z$ and load current $I_L\left(I=I_z+I_L\right)$ It is to be noted that the total current is always less than the maximum Zener diode current.
Under all conditions $V_0=V_z$ Thus, output voltage is regulated.
Question 3.
Write down the concept in details of Integrated Chips (IC's) Integrated Chips
Answer:
An integrated circuit is also referred as an IC or a chip or a microchip. It consists of thousands to millions of transistors, resistors, capacitors, etc. integrated on a small flat piece of semiconductor material that is normally Silicon. Integrated circuits (ICs) are the keystone of modem electronics. With the advancement in technology and the emergence of Very Large Scale Integration (VLSI) era it is possible to fit more aind more transistors on chips of same piece.

ICs have two main advantages over ordinary circuits: cost and performance. The size, speed, and capacity of chips have progressed enormously with the advancement in technology. Computers, mobile phones, and other digital home appliances are now made possible by the small size and low cost of ICs. ICs can function as an amplifier, oscillator, timer, microprocessor and computer memory.

These extremely small ICs can perform calculations and store data using either digital or analog technology. Digital ICs use logic gates, which work only with values of ones and zeros. A low signal sent to a component on a digital IC will result in a value of 0 , while a high signal creates a value of 1 .