CBSE Class 12 Physics Electronic Devices Assignment Set C

Question. The usual semiconductors are:
(a) germanium and silicon
(b) germanium and copper
(c) silicon and glass
(d) glass and carbon

Answer: A

Question. In a good conductor, the energy gap between the valence and conduction bands is
(a) 1 eV
(b) 6 eV
(c) infinite
(d) zero

Answer: D

Question. When trivalent impurity is mixed in a pure semiconductor, the conduction is mainly due to
(a) electrons
(b) holes
(c) protons
(d) positive ions

Answer: B

Question. The example of p-type semiconductor is
(a) pure germanium
(b) pure silicon
(c) germanium doped with arsenic
(d) germanium doped with boron

Answer: D

Question. The impurity atoms to be mixed in pure silicon to form p-type semiconductor are, of
(a) phosphorus
(b) germanium
(c) antimony
(d) aluminium

Answer: D

Question. Electrical conduction in a semiconductor occurs due to
(a) electrons only
(b) holes only
(c) electrons and holes both
(d) neither electrons nor holes.

Answer: C

Question. If ne and nh are the number of electrons and holes in pure germanium, then
(a) ne > nh
(b) ne < nh
(c) ne = nh
(d) ne = finite and nh = 0

Answer: C

Question. When an electric field is applied across a semiconductor
(a) electrons move from lower energy level to higher energy level in the conduction band.
(b) electrons move from higher energy level to lower energy level in the conduction band.
(c) holes in the valence band move from higher energy level to lower energy level.
(d) holes in the valence band move from lower energy level to higher energy level.

Answer: A, C

Question. The energy gap between the valence and conduction bands of a substance is 6 eV. The substance is a:
(a) conductor
(b) semiconductor
(c) insulator
(d) superconductor

Answer: C

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

Answer: C

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

Answer: D

Question. Holes are charge carriers in
(a) intrinsic semiconductor only
(b) p-type semiconductor only
(c) intrinsic and p-type semiconductors
(d) n-type semiconductor

Answer: C

Question. 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. In the depletion region of a diode
(a) there are no mobile charges
(b) equal number of holes and electrons exist, making the region neutral.
(c) immobile charged ions exist.
(d) All of the options

Answer: D

Question. The breakdown in a reverse biased p-n junction diode is more likely to occur due to
(a) large velocity of the minority charge carriers if the doping concentration is small.
(b) large velocity of the minority charge carriers if the doping concentration is large.
(c) strong electric field in a depletion region if the doping concentration is small.
(d) strong electric field in the depletion region if the doping concentration is large.

Answer: A, D

Fill in the Blanks

Question. The number of electron (ne) is equal to the number of holes (nh) in _________________ semiconductors.
Answer: ntrinsic

Question. The number of charge carriers can be changed by doping of a suitable impurity in pure semiconductors. Such semiconductors are known as _________________ semiconductors.
Answer: extrinsic

Question. Valence band energies are ______________ as compared to conduction band energies.
Answer: low

Question. For insulators ________________, for semiconductors Eg is 0.2 eV to 3 eV while for metals Eg ≈ 0
Answer: E_g > 3 eV

Question. _________________ can be used for rectifying an ac voltage.
Answer: Diodes

Question. In reverse bias, after a certain voltage, the current suddenly increases (breakdown voltage) in a Zener diode. This property has been used to obtain ________________.
Answer: voltage regulation

Question. LED works under _________________ bias.
Answer: forward

Question. The resistance of p-n junction is _________________ when reverse biased.
Answer: high

Question. Hole density is _________________ compared to electron density in a p type semiconductor.
Answer: greater

Question. In half-wave rectification, if the input frequency is 50 Hz then the output frequency of the signal will be _______________ Hz.
Answer: 50