CBSE Class 12 Physics Electronic Devices Assignment Set A

 Important Questions for NCERT Class 12 Physics Electronic Devices

Question. 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

Question. In a diode, when there is a saturation current, the plate resistance will be 
(a) data insufficient
(b) zero
(c) some finite quantity
(d) infinite quantity

Answer : D

Question. The value of current gain a of a transistor is 0.98. The value of b will be
(a) 490
(b) 4.9
(c) 59
(d) 49

Answer : D

Question. To a germanium sample, traces of gallium are added as an impurity. The resultant sample would behave like : 
(a) a conductor
(b) a p-type semi-conductor
(c) an n-type sem-iconductor
(d) an insulator

Answer : B

Question. A Ge specimen is doped with Al. The concentration of acceptor atoms is ~10²¹ atoms/ m3. Given that the intrinsic concentration of 22. Which of the following logic gates is an universal gate ? 
(a) OR
(b) NOT
(c) AND
(d) NAND

Answer : A

Question. 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 1mA, what will be the change in emitter current
(a) 1.1 mA
(b) 1.01 mA 
(c) 0.01 mA
(d) 10 mA

Answer : B

Question. In n-p-n transistor circuit the collector current is 10 mA. If 90% of the electron reach the collector, then emitter current will be 
(a) 1 mA
(b) 0.1 mA
(c) 2 mA
(d) nearly 11 mA

Answer : D

Question. A transistor is a/an
(a) chip
(b) insulator
(c) semiconductor
(d) metal

Answer : C

Question. An LED (Light Emitting Diode) is constructed from a p–n junction based on a certain Ga–As– P semi–conducting material whose energy gap is 1.9 eV. What is the wavelength of the emitted light? 
(a) 650 nm
(b) 65Å
(c) 800 nm
(d) 8000 Å

Answer : A

Question. When using a triode, as an amplifier, the electrons are emitted by
(a) grid and collected by cathode only
(b) cathode and collected by the anode only
(c) anode and collected by cathode only
(d) anode and collected by the grid and by cathode.

Answer : B

Question. For amplification by a triode, the signal to be amplified is given to
(a) the cathode
(b) the grid
(c) the glass envelope
(d) the anode 

Answer : B

Question. For an electronic valve, the plate current I and plate voltage V in the space charge limited region are related as
(a) I is proportional to V 3/2
(b) I is proportional to V 2/3
(c) I is proportional to V
(d) I is proportional to V 2 

Answer : A

Question. When a triode is used as an amplifier the phase difference between the input signal voltage and the output is
(a) 0
(b) π
(c) π/2
(d) π/4.

Answer : B

Question. Choose the only false statement from the following.
(a) In conductors the valence and conduction bands overlap.
(b) Substances with energy gap of the order of 10 eV are insulators.
(c) The resistivity of a semiconductor increases with increase in temperature.
(d) The conductivity of a semiconductor increases with increase in temperature. 

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 relationships 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 semiconductors at a room temperature
(a) the valence band is partially empty and the conduction band is partially filled
(b) the valence band is completely filled and the conduction band is partially filled
(c) the valence band is completely filled
(d) the conduction band is completely empty.

Answer : A

Question. C and Si both have same lattice structure; having 4 bonding electrons in each. However, C is insulator whereas Si is intrinsic semiconductor. This is because
(a) in case of C the valence band is not completely filled at absolute zero temperature
(b) in case of C the conduction band is partly filled even at absolute zero temperature
(c) the four bonding electrons in the case of C lie in the second orbit, whereas in the case of Si they lie in the third
(d) the four bonding electrons in the case of C lie in the third orbit, whereas for Si they lie in the fourth orbit.

Answer : C

Question. At absolute zero, Si acts as
(a) non metal
(b) metal
(c) insulator
(d) none of these.

Answer : C

Question. For a p-type semiconductor, which of the following statements is true?
(a) Electrons are the majority carriers and pentavalent atoms are the dopants.
(b) Electrons are the majority carriers and trivalent atoms are the dopants.
(c) Holes are the majority carriers and trivalent atoms are the dopants.
(d) Holes are the majority carriers and pentavalent atoms are the dopants. 

Answer : C

Question. In a n-type semiconductor, which of the following statement is true?
(a) Holes are minority carriers and pentavalent atoms are dopants.
(b) Holes are majority carriers and trivalent atoms are dopants.
(c) Electrons are majority carriers and trivalent atoms are dopants.
(d) Electrons are minority carriers and pentavalent atoms are dopants. 

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. Which one of the following statement is false?
(a) Pure Si doped with trivalent impurities gives a p-type semiconductor.
(b) Majority carriers in a n-type semiconductor are holes.
(c) Minority carriers in a p-type semiconductor are electrons.
(d) The resistance of intrinsic semiconductor decreases with increase of temperature.

Answer : B

Question. Depletion layer consists of
(a) mobile ions
(b) protons
(c) electrons
(d) immobile ions 

Answer : D

Question. The depletion layer in the p-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

Question. When n type semiconductor is heated
(a) number of electrons increases while that of holes decreases
(b) number of holes increases while that of electrons decreases
(c) number of electrons and holes remain same
(d) number of electrons and holes increases equally.

Answer : D

Question. In a p type semiconductor, the majority carriers of current are
(a) protons
(b) electrons
(c) holes
(d) neutrons 

Answer : D

Question. Which of the following, when added as an impurity into the silicon produces n type semiconductor?
(a) B
(b) Al
(c) P
(d) Mg 

Answer : C

Question. To obtain a p-type germanium semiconductor, it must be doped with
(a) indium
(b) phosphorus
(c) arsenic
(d) antimony. 

Answer : A

Question. When arsenic is added as an impurity to silicon, the resulting material is
(a) n-type conductor
(b) n-type semiconductor
(c) p-type semiconductor
(d) none of these. 

Answer : B

Question. 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

Question. The barrier potential of a p-n junction depends on
(1) type of semiconductor material
(2) amount of doping
(3) temperature
Which one of the following is correct?
(a) (1) and (2) only
(b) (2) only
(c) (2) and (3) only
(d) (1), (2) and (3) 

Answer : D

Question. A p-n photodiode is made of a material with a band gap of 2.0 eV. The minimum frequency of the radiation that can be absorbed by the material is nearly
(a) 1 × 10¹⁴ Hz
(b) 20 × 10¹⁴ Hz
(c) 10 × 10¹⁴ Hz
(d) 5 × 10¹⁴ Hz 

Answer : D

Question. For the flow of electrons in a vacuum tube like vacuum diode, vacuum is required in the inter-electrode space
otherwise,
(a) electrons will not be ejected from cathode
(b) extraction of electrons from remaining gas molecules or atoms will increase abruptly
(c) work function of cathode will be reduced
(d) electrons may lose their energy on collision with air molecules in their path

Answer : D

Question. A piece of semiconductor (say Si) is connected in series with a battery. On increasing the temperature of Si piece, the current in the circuit
(a) decreases
(b) increases
(c) remains unchanged
(d) first increases and then decreases

Answer : B

Question. In solid state semiconductors, the number and the direction of the flow of charge carriers can be altered by
(a) illuminating light
(b) supplying heat
(c) varying applied voltage
(d) All of the above

Answer : D

Question. Number of electrons available for conduction due to doping atom in an extrinsic semiconductor
(a) increases with increase of temperature
(b) decreases with increase of temperature
(c) is independent of change in ambient temperature
(d) reduces to zero at temperature above room temperature

Answer : C

Question. In extrinsic p- and n-type semiconductor materials, the ratio of the impurity atoms to the pure semiconductor atoms is about
(a) 1
(b) 10^-1
(c) 10^-4
(d) 10^-7

Answer : D

Question. Energy band gap of a certain material is neither very large nor zero. However, with the help of some external energy conduction takes
place due to (i.e. at room temperature)
(a) vacancies in conduction band
(b) electrons in valence band
(c) electrons in conduction band
(d) all electrons are bound to valence band

Answer : C

Question. In the energy band diagram of material A, conduction band is partially filled and valence band is partially empty and for material B, both conduction and valence band overlaps each other. Then, material A and B respectively are
(a) insulator and semiconductor
(b) semiconductor and conductor
(c) Both are semiconductors
(d) Both are conductors

Answer : D

Question. At an elevated temperature (T > 0 K), few of covalent bonds of Si or Ge are broken and a vacancy in the bond is created. Effective charge of the vacancy or hole is
(a) positive
(b) negative
(c) neutral
(d) either positive or negative depending upon the material

Answer : A

Question. Correct order of relative values of electrical conductivitys for different types of solid is
(a) σ_{ssemiconductor} > σ_sinsulator > σ_smetal
(b) σ_smetal > σ_{ssemiconductor} > σ_sinsulator
(c) σ_{ssemiconductor} > σ_smetal > σ_sinsulator
(d) σ_sinsulator > σ_{ssemiconductor} > σ_smetal

Answer : B

Question. The lattice structure of carbon (C), silicon (Si) and germanium (Ge) is same, then
(a) Si is an insulator
(b) C and Si are intrinsic semiconductors
(c) Si and Ge are intrinsic semiconductors
(d) only C is intrinsic semiconductor

Answer : C

Question. A Ge specimen is doped with Al. The concentration of acceptor atom is ~1021 atoms per m3. Given that the intrinsic concentration of electron-hole pairs is ~1019 per 3 m , the concentration of electrons in the specimen is
(a) 10¹⁷ per m³
(b) 10¹⁵ per m³
(c) 10⁴ per m³
(d) 10² per m³

Answer : A

Question. A hole 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 : B

Question. Three semiconductors are arranged in the decreasing order of their energy gap as follows. The correct arrangement is
(a) tin, germanium, silicon
(b) tin, silicon, germanium
(c) silicon, germanium, tin
(d) silicon, tin, germanium

Answer : C

Question. At equilibrium condition, the rate of generation of electron-hole pairs in Ge crystal
(a) is more than rate of recombination of electron and hole pairs
(b) is less than rate of recombination of electron and hole pairs
(c) equals to rate of recombination of electron and hole pairs
(d) is always zero

Answer : C

Question. The electrical resistance to the diffusion current in a p-n junction because of depletion layer, is large due to the presence of
(a) strong electric field
(b) large number of charge carriers
(c) electrons as charge carriers
(d) holes as charge carriers

Answer : A

Question. If no external voltage is applied across p-n junction, there would be
(a) no electric field across the junction
(b) an electric field pointing from n-type to p-type side across the junction
(c) an electric field pointing from p-type to n-type side across the junction
(d) a temporary electric field during formation of p-n junction that would subsequently disappear

Answer : B

Question. A diode is a
(a) three-terminal device
(b) piece of a semiconductor crystal with metallic contacts provided at two ends
(c) p-n junction with metallic contacts provided at two ends
(d) piece of a metal which is sprayed over by a semiconductor

Answer : C

Question. If an alternating voltage is applied across a diode in series with a load, then
(a) a continuous DC voltage appears across load
(b) an AC voltage appears across load
(c) a pulsating AC voltage appears across load
(d) no voltage appears across load

Answer : C

Question. LED’s are not used for room lighting (although they are used for automobile bulbs and in industrial lighting) because
(a) our eyes are not comfortable with very intense light
(b) our eyes are not comfortable with monochromatic light
(c) LED’s are much costlier than bulbs tubelights and CFL’s
(d) LED manufacture in mass production will be a very polluting process

Answer : B

Question. Diodes are most commonly used for rectification of alternating voltages. Then, which amongst the following property of diode is responsible for this?
(a) Current flow in one direction
(b) Forward bias resistance is low compared to reverse bias
(c) Reverse current is independent of voltage applied
(d) Both (a) and (b)

Answer : D

Question. Conductivity of a semiconductor increases with increase in temperature, because
(a) number density of free charge carriers increases
(b) relaxation time increases
(c) both number density of free charge carriers and relaxation time increases
(d) number density of free charge carriers increases and relaxation time decreases but effect of decrease in relaxation time is much less than increase in number density

Answer : D