TRIBHUVAN UNIVERSITY
INSTITUTE OF SCIENCE AND TECHNOLOGY
2081 B.S.
Candidates are required to give their answers in their own words as far as practicable.
Group A
Long Answer QuestionsAttempt any Two questions.
- 1What do you mean by the contact potential? Give band scheme of a p-n Junction by illustrating: (a) Potential difference $V_c$ resulting from the positive donor ions in the n-side and negative acceptor ions in the p-side of the depletion layer. (b) Potential energy barrier faced by the majority charge carriers (electrons) in the n-side of the diode as they attempt to cross the junction. (c) Potential energy barrier faced by the majority side of the diode as they attempt to cross the junction. (10)
- 2Discuss single crystal growth by discussing following techniques: (a) Czochralski Method (b) Bridgman-Stockbarger Method (c) Floating Zone Method (d) Vapor-Phase Epitaxy (10)
- 3Describe torque on a current-carrying rectangular loop of wire on a pivot rod when placed in a magnetic field. Give alternative way of increasing the torque on the coil. (10)
Group B
Short Answer QuestionsAttempt any Eight questions.
- 4Explain group velocity. (5)
- 5Discuss effective mass of electrons and holes. (5)
- 6Set up Schrödinger equation and discuss the wavefunction. (5)
- 7An oscillating block of mass 250 g takes 0.15 sec to move between the endpoints of the motion, which are 40 cm apart. (a) What is the frequency of the motion? (b) What is the amplitude of the motion? (c) What is the force constant of the spring? (5)
- 8A proton is accelerated through a potential difference of 200 V. It then enters a region where there is a magnetic field $B = 0.5$ T. The magnetic field is perpendicular to the direction of motion of the proton. Find the force experienced by the proton. (5)
- 9A small particle of mass $10^{-6}$ g moves along the x axis; its speed is uncertain by $10^{-6}$ m/sec. (a) What is the uncertainty in the x coordinate of the particle? (b) Repeat the calculation for an electron assuming that the uncertainty in its velocity is also $10^{-6}$ m/sec. (5)
- 10A beam of hydrogen atoms is used in a Stern-Gerlach type experiment ($v = 10^4$ m/sec, region 20 cm long, $dB/dz = 3 \times 10^4$ T/m). What is the separation of the two components of the beam as they emerge from the magnet? (5)
- 11The energy gaps of some alkali halides are KCl = 7.6 eV, KBr = 6.3 eV, KI = 5.6 eV. Which of these are transparent to visible light? At what wavelength does each become opaque? (5)
- 12The output of a digital circuit ($y$) is given by this expression: $y = (AB + C'BA)(B + C)$. Draw a circuit of above equation using OR, AND and NOT gate and hence find its truth table. (5)