Vellore Institute of Technology 2011 B.Tech Electrical and Electronics Engineering Physics for semiconductor devices - Question Paper

School of Advanced Sciences - Physics
Model ques. Paper – Winter 2010 -11
Course: B.Tech Sem: II Semester
Subject: Physics of Semiconductor Devices Sub Code: PHY104
Time: 3 hours Max. Marks: 100

Part A (8×5 = 40 Marks)
ans any 8 ques.

1) Draw the wave function and probability distribution in ground state for a particle in 1
dimensional box.
2) Distinguish ranging from direct and indirect band gap semiconductors. provide examples.
3) Explain the process of generation and recombination of carriers in semiconductors.
4) What do you mean by diffusion of charge carriers? Deduce the transport equations.
5) Why the effective masses of holes and electrons are smaller in GaAs than in Si?
6) An abrupt (step) junction made of silicon has NA = 1017 cm-3 and ND = 1015cm-3. The diode is at 300K and has area = 10-5 cm2. The relative permittivity of Si is 11.8. compute (i) built in voltage (ii) the maximum value of electric field intensity.
7) Sketch the flat band diagram in metal semiconductor contact and discuss it.
8) Why is the Schottky-barrier diode much faster, in switching, than the PN diode?
9) What do you mean by the term Quasi-Fermi level in P-N junction
10) Mention the significance of Possion's formula in metal semiconductor junction.

Part B (5×12 = 60 Marks)
ans All the ques.

11. (a) i) find the time independent Schrodinger wave formula.
ii) find the expression for the density of states.
iii) discuss the experimental technique used to obtain Hall coefficient. (4+5+3 = 12 Marks)
OR
(b) discuss with suitable diagrams the conduction band, valence band and forbidden energy gap. Write a short note on formation of conduction band, valence band and forbidden energy band gap on basis of Kronig-Penny model, considering silicon atom as an example. (5+7 = 12 Marks)
12. (a) i) Write a short note on drift and diffusion phenomena in semiconductors.
ii) describe the term 'Density of states of electron'. find an formula for density of electrons in terms of Fermi energy at 0 K. (4+2+6= 12 Marks)
OR
(b) i)Write a short note on carrier scattering in semiconductor.
ii) find continuity formula for charge carriers in an n-type semiconductor.
(4+8 = 12 Marks)
13. (a) i) Derive an expression for built in potential for a p-n junction.
ii) compute the theoretical barrier hight, built in barrier hight and maximum electric field in a metal semiconductor diode for zero applied bias. Consider a contact ranging from tungsten and n-type Si doped to Nd = 1016 cm-3 at 300K. provided that work function of tungsten 4.55 eV and electron affinity of Si is 4.01 eV (6+6 = 12 Marks)
OR
(b) i) A sample of n-type Si has dark resistivity of 1KO-cm at 300 K. The sample is illuminated uniformly to to generate 1021 electron-holes pairs per cm3 per sec. The hole life time in the sample is 1µsec. compute the sample resistivity and the percentage change in the conductivity after illumination owing to majority and minority carriers.
ii) Write short notes on: Contact potential and space charge phenomena (6+6 = 12 Marks)
14. (a) i) discuss why a depletion region is not formed in the metal at a metal-semiconductor junction
ii) The silicon in an Aluminium-Silicon Schottky diode has ND = 1015 cm-3. Determine the built in voltage (6+6 = 12 Marks)
OR
(b) Write short notes on metal semiconductor contacts: (i) Ohmic contacts (ii) tunnel contacts (iii) Schottky contacts. (4+4+4 = 12 Marks)
15 (a)Describe 1 method for fabricating diode. (12 Marks)
OR
(b) Write short notes on (i) Gunn diode, (ii) Trapatt diode (iii) Impatt diode
(4+4+4 = 12 Marks)

Earning:   Approval pending.