NEET UG Physics Semiconductor Electronics MCQs

* LOGIC GATES

·  A digital circuit with one or more input signals but only one output signal is known as logical gate.
·  The logic gates are the building blocks of a digital system. Each logic gate follows a certain logical relationship between input and output voltage.
·  There are three basic logic gates :
·  OR gate
·  AND gate
·  NOT gate

Truth table

·  It is a table that shows all possible input combinations and corresponding output combination for a logic gate.

OR gate

·  An OR gate has two or more inputs but only one output.
·  It is called OR gate because the output is high if any or all the inputs are high.
·  The logic symbol of OR gate is

·  The Boolean expression for OR gate is

Y = A + B

AND gate

·  An AND gate has two or more inputs but only one output.
·  It is called AND gate because output is high only when all the inputs are high.
·  The logic symbol of AND gate is

MCQ

For the answer of the following questions choose the correct alternative fromamong the given ones.

(1) C, Si and Ge have same no. of valence electrons. C is an insulator because energy required to take one electron out from
(A) Si is more
(B) C is more
(C) Ge is more
(D) C is less

(2) Ionization energy of isolated phosphorous atonis 10 eV. Ionization energy of same atom in Si is nearlyeV (Relative Permitivity of silicon = 12)
(A) 0.1
(B) 0.2
(C) 0.3
(D) 0.4

(3) By adding impurity in intrinsic semiconductor P type semiconductor is made. charge of these P type semiconductor is
(A) trivalent, neutral
(B) pentaralent, neutral
(C) pentavalent, positive
(D) trivalent, negative

(4) Strong overlaping of different atomic orbitals makes
(A) different energy level
(B) energy band
(C) Conductor
(D) Insulators

(5) We can not make p-n junction diode by making P type semi-condutor join with N - type semi-conductor, because
(A) Inter-atomic spacing becomes less than 1A^O
(B) P - type will repel N - type
(C) There will be discontinuity for the flowing charge carriers
(D) semi-conducting properties will be lost

(6) For p-n junction, which statement is incorrect
(A) Donor atoms are depleted of their holes in junction
(B) No net charge exists far from junction
(C) Barrier potential VB is generated
(D) Energy VB is to be surmounted before any charge can flow across junction

(7) The intrinsic semi-conductor has :
(A) a finite resistance which does not change with temperature
(B) infinite resistance which decreases with temperature
(C) Finite resistance which decreases with temperature
(D) Finite resistance which does not change with temperature

(8) The behaviour of Ge as semi-conductor is due to width of :
(A) Conduction band being large
(B) Forbidden band being large
(C) Conduction band being small
(D) Forbidden band being small and narrow

(9) Which of the following is not the advantage of PN junction diode over tube valve ?
(A) Unlimited life
(B) No warming-up time after switching
(C) Large efficiency
(D) Low consumption of Power

16) At 0 K temp, a N - type semi-conductor :
(A) does not have any charge carriers (B) has few holes but no free electrons
(C) few holes and few electrons (D) has equal number of holes and electrons

(17) In Si-crystal, impurity donor atom have valency.
(A) 2 (B) 3 (C) 4 (D) 5

(18) A N-P-N transistor conducts when collector is and emitter is with respect to base.
(A) positive, negative (B) positive, positive
(C) negative, negative (D) negative, positive

(19) A full wave rectifier is operating at 50Hz, 220V the fundamental frequency of ripple will be .
(A) 50 Hz (B) 75 Hz (C) 110 Hz (D) 100 Hz

(20) Reverse bias applied on a junction diode :
(A) raises the potential barrier (B) increases majority charge carrier current
(C) lowers the potential barrier (D) increases the temperature of junction

(31) How many NAND gates are used to form AND gate ?
(A) 1 (B) 2 (C) 3 (D) 4

(32) Ripples are .
(A) A.C. mixed with D.C (B) D.C. mixed with output
(C) D.C. output (D) A.C. output

(33) In an P.N.P transistor circuit, the collector current is 10 mA. If 90% of the electrons emitted reach the collector :
(A) I_E = 9mA (B) I_E = 10mA (C) I_B = 1mA (D) I_B = -1 mA

(34) When a P-type semi-conductor is heated :

(A) number of holes increases while that of electrons decreases

(B) number of electron increases while that of hole decreases

(C) number of electrons and holes remains same

(D) number of electrons and holes increases equally

(35) The depletion layer in PN junction diode is caused by
(A) drift of holes (B) Diffusion of impurity ions
(C) diffusion of charge carriers (D) drift of electrons

(44) In Ge sample, traces of gallium are added as impurity. The resultant sample would behave like :
(A) a conductor (B) a P-type semiconductor
(C) an N-type semiconductor (D) an insulator

(45) A light emitting diode has a voltage drop of 2V across it when 10mA current is passed. If this LED is to be operated with 6V battery the value of limiting resistor would be .
(A) 400Ω (B) 4000Ω (C) 40kΩ (D) 300Ω

(46) NAND gate is .
(A) A basic gate (B) Not a universal gate
(C) A universal gate (D) Multipurpose gate

(47) The number of holes and electrons in an intrinsic conductors are x and y respectively at room temperature. Which of the following options are true ?
(A) x > y (B) y > x (C) x = y (D) x << y

(48) How will you increase the resistivity of Ge semi-conductor ?
(A) On adding donor impurity (B) On adding acceptor impurity
(C) On making UV light incident on Ge crystal (D) On decreasing the temperature 

(49) What is type of material, for the energy band diagram shown in the figure ?

(A) N - type semi-conductor

(B) P - type semi-conductor

(C) Insulator

(D) Intrinsic semi-conductor

(50) From the following semi-conductor devices, operates in forward bias only.
(A) Varactar diode (B) Zener diode (C) Light emitting diode (D) photo-diode

(51) device is the odd-one out.
(A) solar-cell (B) Varactor diode (C) Photodiode (D) Zener diode

(52) The value of depletion capacitance on decreasing the reverse bias on varactor diode
(A) decreases (B) increases (C) becomes zero (D) does not change

(53) Which of the following statement is correct for transistor LC oscillator circuit ?
(A) It works with negative feed back

(B) The phase difference between output and input signal is π radian

(C) To start oscillation external signal is required

(D) The frequency of output signal is independent of the components used in feed back circuit

(64) A zener diode used as voltage regulator is connected _____ .

(i) in forward bias (ii) in reverse bias (iii) in parallel with load (iv) in series with load

(A) (i) and (ii) are correct (B) (ii) and (iii) are correct

(C) only (i) is correct (D) only (iv) is correct

Directions : Question numbers (65), (66) and (67) are based on following passage.

PASSAGE : A n-p-n transistor is used in common emitter made in an amplifier circuit. A change of 40μA in the base current changes the output current by 2mA and 0.04V in input voltage.

(65) The input resistance is .
(A) 1kΩ (B) 10Ω (C) 10 kΩ (D) 100Ω

(66) The current amplification factor is .
(A) 20 (B) 30 (C) 50 (D) 40

(67) If a load of 6kΩ is used, then the voltage gain of the amplifier is .
(A) 100 (B) 200 (C) 300 (D) 400

(68) An amplifier has voltage gain A_V = 1000. The voltage gain in dB is .
(A) 20 dB (B) 30 dB (C) 3 dB (D) 60 dB

(82) Zener diode is used as
(A) Full. wave rectifier (B) amplifier
(C) A.C. voltage regulator (D) D.C. voltage regulator

(83) Break down voltage of a diode is 5V. By which effect this breakdown occurs in diode ?
(A) Only avalanche effect (B) Only zener effect
(C) Avalanche or zener effect (D) None of the above

(84) When NPN transistor is used as an amplifier then
(A) electron moves from base to collector

(B) hole travels from emitter to base

(C) hole goes to emitter from base

(D) electron goes to base from collector

(87) For gate, the output is 1 only when both input are `0'
(A) AND (B) NAND (C) OR (D) NOR

(88) In VLSI circuits more than gates are contained.
(A) 1000 (B) 100 (C) 10 (D) 500

(89) The flow of valence electrons to the left means that holes are flowing.
(A) Left (B) Right (C) Either way (D) None

(90) How many free electrons does a P-type semiconductor contain ?
(A) Many (B) None
(C) Only those produced by thermal energy (D) Same number as holes

(91) Suppose an intrinsic semi-conductor at room temperature has 1 billion free electrons at room temperature.
If temperature changes to 75^o C, how many holes are there ?
(A) Fewer than 1 billion (B) 1 billion (C) More than 1 billion (D) Impossible to say

(92) Which of the following doesn't fit in the group ?
(A) Conductor (B) Semi conductor (C) Four valence electrons (D) Crystal structure

(93) What kind of device is a diode ?
(A) Bilateral (B) Linear (C) Nonlinear (D) Unipolar

(94) We want a peak load voltage of 40V out of a bridge rectifier, What is the approximate rms value of secondary voltage ?
(A) 0V (B) 14.4V (C) 28.3V (D) 56.6V

(95) The load-current is approximately constant when a zener diode is .
(A) Forward biased (B) Reverse biased
(C) Operating in breakdown region (D) Unbiased

(96) When source voltage increases in a zener diode, which of these current remians approximately constant ?
(A) Series current (B) Zener current (C) Load current (D) Total current

(97) The device associated with voltage controlled capacitance is .
(A) Light emitting diode (B) Photo diode (C) Varactor diode (D) Zener diode

(98) For normal operation of the transistor, the collector diode has to be .
(A) Forward biased (B) Reverse biased
(C) Non conducting (D) Operating in breakdown region

(99) Most of the electrons in the base of N-P-N transistor flow
(A) Out of the base lead (B) Into the collector (C) Into the emitter (D) Into the base supply

Direction for Assertion - Reason type questions

(A) If both Assertion and Reason are true and reason is the correct explanation of assertion.
(B) If both Assertion and Reason are true but Reason is not the correct explanation of assertion.
(C) If Assertion is true but Reason is false
(D) If both assertion and reason are false

(100) A : Intrinsic charge carries are thermally generated
         R : Their availability can be easily controlled
(A)   (B)   (C)   (D)

(101) A : Impurity atoms for silicon is selected from third or fourth group
         R : These Impurity atoms have same size as that of Si
(A)   (B)   (C)   (D)

(102) A : Photodiode are operated in reverse bias
          R : In reverse bias fractional change in minority charge carrier is more
(A)   (B)   (C)   (D)

(103) A : The resistivity of a semi-conductor decreases with temperature
          R : At higher temperature more co-valent bond breaks
(A)   (B)   (C)   (D)

(104) A : NAND (or NOR) gates are called digital building blocks
          R : The different combination of NAND (or NOR) gates can produce all the basic or complicated gates.
(A)   (B)   (C)   (D)

(105) A : The colour of light emitted by depends on its forward bias.
          R : The forward biasing of PN junction diode will increase the width of depletion layer
(A) (B) (C) (D)

(106) A : The ionization energy of isolated phosphorous is very large
          R : The ionization energy of phosphorous in lattice is very small
(A)   (B)   (C)   (D)

(107) A : Mostly transistor are used in common emitter configuration
R : Common emitter configuration provide more current gain and small voltage gain
(A)   (B)   (C)   (D)

(108) A : A transistor amplifier circuit in common emitter configuration has low input impednce
          R : Base - emitter junction is forward biased
(A)   (B)   (C)   (D)