CUET Physics MCQs Unit IX Electronic Devices

Question : The forbidden energy gap for germanium crystal at 0 K is
(a) 0.071 eV 
(b) 0.71 eV
(c) 2.57 eV 
(d) 6.57 eV

Answer: B

Question : In semiconductors, at room temperature
(a) the conduction band is completely empty
(b) the valence band is partially empty and the conduction band is partially filled
(c) the valence band is completely filled and the conduction band is partially filled
(d) the valence band is completely filled

Answer: C

Question : Reverse bias applied to a junction diode
(a) increases the minority carrier current
(b) lowers the potential barrier
(c) raises the potential barrier
(d) increases the majority carrier current

Answer: C

Question : The transistor are usually made of 
(a) metal oxides with high temperature coefficient of resistivity
(b) metals with high temperature coefficient of resistivity
(c) metals with low temperature coefficient of resistivity
(d) semiconducting materials having low temperature coefficient of resistivity

Answer: A

Question : For a common base amplifier, the values of resistance gain and voltage gain are 3000 and 2800 respectively. The current gain will be
(a) 1.1
(b) 0.98
(c) 0.93
(d) 0.83

Answer: C

Question : In a semiconductor
(a) there are no free electrons at 0 K
(b) there are no free electrons at any temperature
(c) the number of free electrons increases with pressure
(d) the number of fre electrons is more than that in a conductor

Answer: A

Question : The electrical conductivity of pure germanium can be increased by
(a) increasing the temperature
(b) doping acceptor impurities
(c) doping donor impurities
(d) All of the above

Answer: D

Question : What is the resistivity of a pure semiconductor at absolute zero ?
(a) Zero
(b) Infinity
(c) Same as that of conductors at room temperature
(d) Same as that of insulators at room temperature

Answer: B

Question : The relation between number of free electrons (n) in a semiconductor and temperature (T) is given by
(a) n ∝T 
(b) n ∝ T2
(c) n ∝ √T 
(d) n ∝T3/2

Answer: D

Question : An electric field is applied to a semiconductor. Let the number of charge carriers be n and the average drift speed be v. If the temperature is increased
(a) both n and v will increase
(b) n will increase but v will decrease
(c) v will increase but n will decrease
(d) both n and v will decrease

Answer: A

Question : A semiconductor device is connected in a series circuit with a battery and a resistance. A current is found to pass through the circuit. If the polarity of the battery is reversed, the current drops to almost zero. The device may be a/an
(a) intrinsic semiconductor
(b) p-type semiconductor
(c) n-type semiconductor
(d) p-n junction diode

Answer: D

Question : Forward biasing is that in which applied voltage
(a) increases potential barrier
(b) cancels the potential barrier
(c) is equal to 1.5 volt
(d) None of these

Answer: B

Question : In a transistor
(a) the emitter has the least concentration of impurity
(b) the collector has the least concentration of impurity
(c) the base has the least concentration of impurity
(d) all the three regions have equal concentrations of impurity

Answer: C

Question : In forward biasing of the p–n junction
(a) the positive terminal of the battery is connected to p–side and the depletion region becomes thick
(b) the positive terminal of the battery is connected to n–side and the depletion region becomes thin
(c) the positive terminal of the battery is connected to n–side and the depletion region becomes thick
(d) the positive terminal of the battery is connected to p–side and the depletion region becomes thin

Answer: D

Question : Bridge type rectifier uses
(a) four diodes 
(b) six diodes
(c) two diodes 
(d) one diode

Answer: A

Question : The electrical conductivity of pure germanium can be increased by
(a) increasing the temperature
(b) doping acceptor impurities
(c) doping donor impurities
(d) All of the above

Answer: D

Question : In the half wave rectifier circuit operating from 50 Hz mains frequency, the fundamental frequency in the ripple would be
(a) 25 Hz
(b) 50 Hz
(c) 70.7 Hz
(d) 100 Hz

Answer: B

Question : An electric field is applied to a semiconductor. Let the number of charge carriers be n and the average drift speed be v. If the temperature is increased
(a) both n and v will increase
(b) n will increase but v will decrease
(c) v will increase but n will decrease
(d) both n and v will decrease

Answer: A

Question : A transistor has three impurity regions. All the three regions have different doping levels. In order of increasing doping level, the regions are
(a) emitter, base and collector
(b) collector, base and emitter
(c) base, emitter and collector
(d) base, collector and emitter

Answer: D

Question : NAND and NOR gates are called universal gates primarily because they
(a) are available universally
(b) can be combined to produce OR, AND and NOT gates
(c) are widely used in Integrated circuit packages
(d) are easiest to manufacture

Answer: B

Question :  A p-type semiconductor is
(a) positively charged
(b) negatively charged
(c) uncharged
(d) uncharged at 0K but charged at higher temperatures

Answer: C

Question : Number of electrons in the valence shell of a pure semiconductor is
(a) 1 
(b) 2 
(c) 3 
(d) 4

Answer: D

Question : In a p-type semiconductor, the acceptor valence band is
(a) close to the valence band of the host crystal
(b) close to conduction band of the host crystal
(c) below the conduction band of the host crystal
(d) above the conduction band of the host crystal

Answer: A

Question : One serious drawback of semi-conductor devices is
(a) they do not last for long time.
(b) they are costly
(c) they cannot be used with high voltage.
(d) they pollute the environment.

Answer: C

Question : Carbon, Silicon and Germanium atoms have four valence electrons each. Their valence and conduction bands are separated by energy band gaps represented by (Eg)C, (Eg)Si and (Eg)Ge respectively. Which one of the following relationship is true in their case?
(a) (Eg)C > (Eg)Si 
(b) (Eg)C < (Eg)Si
(c) (Eg)C = (Eg)Si 
(d) (Eg)C < (Eg)Ge

Answer: A

Question : In V-I characteristic of a p-n junction, reverse biasing results in
(a) leakage current
(b) the current barrier across junction increases
(c) no flow of current
(d) large current

Answer: A

Question : In a semiconductor diode, the barrier potential offers opposition to
(a) holes in P-region only
(b) free electrons in N-region only
(c) majority carriers in both regions
(d) majority as well as minority carriers in both regions

Answer: C

Question : When p-n junction diode is forward biased then
(a) both the depletion region and barrier height are reduced
(b) the depletion region is widened and barrier height is reduced
(c) the depletion region is reduced and barrier height is increased
(d) Both the depletion region and barrier height are increased

Answer: A

Question : The average value of output direct current in a full wave rectifier is
(a) I0/π 
(b) I0/2
(c) π I0/2
(d) 2 I0/π

Answer: A

Question : If a small amount of antimony is added to germanium crystal
(a) it becomes a p–type semiconductor
(b) the antimony becomes an acceptor atom
(c) there will be more free electrons than holes in the semiconductor
(d) its resistance is increased

Answer: C

Question : Choose the correct option
Assertion : A pure semiconductor has negative temperature coefficient of resistance.
Reason : In a semiconductor on raising the temperature, more charge carriers are released, conductance increases and resistance decreases.
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.

Answer: A

Question : The current gain of a transistor in common base mode is 0.995. The current gain of the same transistor in common emitter mode is
(a) 197
(b) 201
(c) 198
(d) 199

Answer: D

Question : Electric conduction in a semiconductor takes place due to
(a) electrons only
(b) holes only
(c) both electrons and holes
(d) neither electrons nor holes

Answer: C

Question : In an insulator, the forbidden energy gap between the valence band and conduction band is of the order of
(a) 1 MeV 
(b) 0.1 MeV
(c) 1 eV 
(d) 5 eV

Answer: D

Question : At absolute zero, Si acts as
(a) non-metal 
(b) metal
(c) insulator 
(d) None of these

Answer: C

Question : A strip of copper and another of germanium are cooled from room temperature to 80K. 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

Question : The diffusion current in a p-n junction is from the
(a) n-side to the p-side
(b) p-side to the n-side
(c) n-side to the p-side if the junction is forward-biased and in the opposite direction if it is reverse-biased
(d) p-side to the n-side if the junction is forward-biased and in the opposite direction if it is reverse-biased

Answer: B

Question : Filter circuit
(a) eliminates a.c. component
(b) eliminates d.c. component
(c) does not eliminate a.c. component
(d) None of these

Answer: A

Question : The cause of the potential barrier in a p-n junction diode is
(a) depletion of positive charges near the junction
(b) concentration of positive charges near the junction
(c) depletion of negative charges near the junction
(d) concentration of positive and negative charges near the junction

Answer: D

Question : In a half wave rectifier, the r.m.s. value of the a.c. component of the wave is
(a) equal to d.c. value 
(b) more than d.c. value
(c) less than d.c. value 
(d) zero

Answer: B

Question :  The peak voltage in the output of a half-wave diode rectifier fed with a sinusoidal signal without filter is 10V. The d.c. component of the output voltage is
(a) 20/p V
(b) 10/Ö2 V
(c) 10/p V
(d) 10V

Answer: C

Question : When npn transistor is used as an amplifier
(a) electrons move from collector to base
(b) holes move from emitter to base
(c) electrons move from base to collector
(d) holes move from base to emitter

Answer: D

Question : Let nh and ne be the number of holes and conduction electrons in an extrinsic semiconductor. Then
(a) nh > ne 
(b) nh = ne 
(c) nh < ne
(d) nh ≠ ne

Answer: D

Question : Temperature coefficient of resistance of semiconductor is
(a) zero 
(b) constant
(c) positive 
(d) negative

Answer: D

Question : When an impurity is doped into an intrinsic semiconductor, the conductivity of the semiconductor
(a) increases 
(b) decreases
(c) remains the same 
(d) becomes zero

Answer: A

Question : Diffusion current in a p-n junction is greater than the drift current in magnitude
(a) if the junction is forward-biased
(b) if the junction is reverse-biased
(c) if the junction is unbiased
(d) in no case

Answer: A

Question : Barrier potential of a P-N junction diode does not depend on
(a) doping density 
(b) diode design
(c) temperature 
(d) forward bias

Answer: B

Question : The average value of output direct current in a half wave rectifier is
(a) I0/π 
(b) I0/2
(c) π I0/2
(d) 2 I0/π

Answer: A

Question :  The drift current in a p-n junction is from the
(a) n-side to the p-side
(b) p-side to the n-side
(c) n-side to the p-side if the junction is forward-biased and in the opposite direction if it is reverse biased
(d) p-side to the n-side if the junction is forward-biased and in the opposite direction if it is reverse-biased

Answer: A

Question : The gate for which output is high if atleast one input is low?
(a) NAND
(b) NOR
(c) AND
(d) OR

Answer: D

Question : In a semiconductor, the forbidden energy gap between the valence band and the conduction band is of the order is
(a) 1 MeV 
(b) 0.1 Mev
(c) 1 eV 
(d) 5 eV

Answer: C

Question : If a small amount of antimony is added to germanium crystal
(a) it becomes a p–type semiconductor
(b) the antimony becomes an acceptor atom
(c) there will be more free electrons than holes in the semiconductor
(d) its resistance is increased

Answer: C

Question : Zener diode is used for
(a) amplification 
(b) rectification
(c) stabilisation 
(d) all of the above

Answer: C

Question : Electric conduction in a semiconductor takes place due to
(a) electrons only
(b) holes only
(c) both electrons and holes
(d) neither electrons nor holes

Answer: C

Question : Hole in semiconductor is
(a) an anti – particle of electron
(b) a vacancy created when an electron leaves a covalent bond
(c) absence of free electrons
(d) an artificially created particle

Answer: D

Question : The mobility of free electrons is greater than that of free holes because
(a) they are light
(b) they carry negative charge
(c) they mutually colllide less
(d) they require low energy to continue their motion

Answer: A

Question : The ratio of forward biased to reverse biased resistance for pn junction diode is
(a) 10–1 : 1 
(b) 10–2 : 1
(c) 104 : 1 
(d) 10–4 : 1

Answer: D

Question : The resistivity of a semiconductor at room temperature is in between
(a) 10–2 to 10–5 Ω cm 
(b) 10–3 to 106 Ω cm
(c) 106 to 108 Ω cm 
(d) 1010 to 1012 Ω cm

Answer: B

Question : By increasing the temperature, the specific resistance of a conductor and a semiconductor
(a) increases for both 
(b) decreases for both
(c) increases, decreases 
(d) decreases, increases

Answer: C

Question : In the middle of the depletion layer of a reverse- biased on junction, the
(a) electric field is zero 
(b) potential is maximum
(c) electric field is maximum
(d) potential is zero

Answer: C

Question : The impurity atoms with which pure silicon may be doped to make it a p-type semiconductor are those of
(a) phosphorus 
(b) boron
(c) antimony 
(d) nitrogen

Answer: B

Question : If the two ends of a p-n junction are joined by a wire
(a) there will not be a steady current in the circuit
(b) there will be a steady current from the n-side to the pside
(c) there will be a steady current from the p-side to the nside
(d) there may or may not be a current depending upon the resistance of the connecting wire

Answer: A

Question : In a semiconductor
(a) there are no free electrons at 0 K
(b) there are no free electrons at any temperature
(c) the number of free electrons increases with pressure
(d) the number of fre electrons is more than that in a conductor

Answer: A

Question : In Boolean algebra, Y = A + B implies that
(a) output Y exists when both inputs A and B exist
(b) output Y exists when either input A exists or input B exists or both inputs A and B exist
(c) output Y exists when either input A exists or input B exists but not when both inputs A and B exist
(d) output Y exists when both inputs A and B exists but not when either input A or B exist

Answer: A

Question : In reverse biasing
(a) large amount of current flows
(b) potential barrier across junction increases
(c) depletion layer resistance increases
(d) no current flows

Answer: B

Question : If the forward bias on p-n junction is increased from zero to 0.045 V, then no current flows in the circuit. The contact potential of junction i.e. VB is
(a) zero                   
(b) 0.045 V
(c) more than 0.045 V
(d) less than 0.045 V

Answer: C

Question :   To use a transistor as an amplifier, emitter-base junction is kept in ...X... and base-collector junction is kept in ...Y... Here, X and Y refer to
(a) forward bias, forward bias
(b) reverse bias, reverse bias
(c) reverse bias, forward bias
(d) forward bias, reverse bias

Answer: D

Question : In a P -N junction
(a) the potential of P & N sides becomes higher alternately
(b) the P side is at higher electrical potential than N side.
(c) the N side is at higher electric potential than P side.
(d) both P & N sides are at same potential.

Answer: B

Question : In an n-type semiconductor, donor valence band is
(a) above the conduction band of the host crystal
(b) close to the valence band of the host crystal
(c) close to the conduction band of the host crystal
(d) below the valence band of the host crystal'

Answer: C