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Reg. No. : ..................................... Name : ..........................................
Third Semester B.Tech. Degree Examination, November 2009 (2008 Scheme) 08.302 : SOLID STATE DEVICES (TA) Time : 3 Hours
Max. Marks : 100 PART – A
Answer all questions. Each question carries 4 marks. 1. Draw three crystal planes equivalent to (100). 2. Derive an expression for the effective mass of a free electron in a semiconductor. 3. How do the electron concentration in an n type semiconductor vary with temperature ? 4. Two ideal and identical pn junction diodes with reverse saturation current of 10nA are connected in series across a 1.2 V battery such that both of them are forward biased. What will be the current through the diodes at 300 K ? 5. A P+n silicon diode requires a breakdown voltage of 150 V at 300 K. If the critical field strength for avalanche breakdown is 3Ă—105 V/cm, determine the doping on the n side of the diode. 6. Draw the energy band diagram of a hetero junction diode under thermal equilibrium condition. 7. Define injection efficiency and transport factor of a BJT. What are the factors on which these parameters depend ? 8. Plot the CV diagram of an ideal MOS capacitor and explain. 9. Explain hot electron effect in MOSFETs. 10. For a MOS structure with silicon substrate of doping 1016 Arsenic atoms/cm 3, what is the surface potential at the onset of strong inversion, at 300 K. P.T.O.
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PART – B Answer any two questions from each Module. Each Module carries 20 marks. Module – I 11. a) Derive an expression for the electron diffusion current in a semiconductor. b) The conductivity of intrinsic silicon at 300 K is 4.4×10–4 ( Ω cm)–1. Determine the conductivity at 310 K, if Eg (300 K)=1.11 e V. 12. Derive the ideal diode equation stating the approximations used. Assume that the length of the neutral regions towards the n and p sides are Wn and Wp respectively.
13.a) Derive an expression for the depletion layer capacitance of an abrupt p-n junction. b) A silicon sample doped with 1018 cm–3 arsenic atoms is optically excited such that gop=1021 EHP/cm3/s . Defermine sample resistivity before and after illumination. Assume μn = 3μp = 1200 cm 2 / V.s . Module – II 14. Derive expressions for the terminal currents of an n-p-n. BJT in terms of its dimensions, doping etc. State the approximations used. 15. a) Plot the input and output characteristics of a p-n-p BJT in common emitter configuration and explain. What are the effects of base width modulation on the characteristics. b) A Schottky barrier diode with n type silicon has ND=2×1015 cm–3 of the electron affinity of silicon is 4.15 eV and the work function of the metal is 4.9 eV, determine : i) The built in voltage ii) The width of depletion layer.
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16. a) Draw the energy band diagram of a metal n type semiconductor ohmic contact under thermal equilibrium. b) A silicon bipolar junction transistor has base doping of ND = 8×1015cm–3 and the collector is heavily doped. The metallurgical width of the base is 2 μm and the critical field strength for avalanche breakdown is 3×105 V/cm. Determine the breakdown voltage of the collector base junction (avalanche breakdown) and the punch through voltage. Module – III 17. Derive expression for the drain current of a JFET. 18. a) Explain the principle of operation of an IGBT. b) Determine the low frequency and high frequency capacitance of a MOS capacitor with oxide of thickness 100A°, relative permittivity 3.9 and substrate doping of NA = 1015cm–3 . 19. a) Explain the principle of operation of a UJT. b) A silicon n channel JFET has ND = 1015cm –3, NA = 10 18 cm –3 and a = 2 μm . Determine the built in voltage and pinch off voltage.
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