Electronic Devices - Dec 2013

Electronics Engineering (Semester 3)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1 (a) Justify that space charge width increase with reverse biased voltage in a p-n junction diode(5 marks) 1 (b) Sketch low frequency capacitance versus gate voltage of a MOS capacitor with n-type substrate show individual capacitance components(5 marks) 1 (c) Sketch the IV characteristics of a PN junction solar cell(5 marks) 1 (d) Descibe construction and V-I characteristics of IGBT(5 marks) 2 (a) Derive equation of built inpotential Vbi for a p-n junction under Zero bias and hence calculate Vbi at T=300 k for Nd=10¹⁵ cm^-3 and Na=10¹⁵ cm^-3(10 marks) 2 (b) What is primary advantage of HBT over BJT? Draw and explain schematic cross section of an npn HBT structure with its energy band diagram when HBT is operated under active mode?(10 marks) 3 (a) Explain construction and V-I characteristics of Tunnel diode(10 marks) 3 (b) Explain construction, working and characteristics of N-channel JFET, explain frequency limitation factors(10 marks) 4 (a) Draw band diagrams for accumulation, depletion and inversion regions for MOS capacitor
Calculate threshold voltage for a polysilicon gate n-cahnnel MOS transistor with substrate at Zero potential with the following parameters:-
Substrate doping density N_A=10¹⁶ cm^-3
Polysilicon gate doping density N_D=2×10²⁰ cm^-3
Gate oxide thickness t_OX=500°A
Oxide-interface fixed charge density Nox=4×10¹⁰ cm^-2(10 marks) 4 (b) Describe the time delay factors in the frequency limitation of the bipolar transistor, calculate the emitter-collector transit time, cut off frequency and the beta cut off frequency of a bipolar transistor, with the following parametes, consider a silicon npn transistor at T=300 K with a low frequency common emitter current gain of ?=100. Assume the following parameters :-
IE=50?A, C_je=0.40 PF, Cμ=0.05 PF
X_B=0.5 μ, D_n=25cm²/s, X_dc=2.4 μm
r_C=20ω, C_s=0.1 ρF(10 marks) 5 (a) Describe construction, working and characteristics of :-
(i) Photodiode and
(ii) Avalanche Photodiode.(10 marks) 5 (b) Discuss the device structure and principle of operation of MESFET. Derive the equation for current-voltage characteristics for MESFET. Describe the various region of operation on V-I characteristics.(10 marks) 6 (a) Explain construction, working and characteristics of SCR.(10 marks) 6 (b) Explain the need of Hetero junction, explain the terms stradding, staggered and broke gap in relation to hetero junction. Explain the quantization of energy of an electron in a potential well in hetero junction. Explain this concept with respect to the ideal energy band diagram of an nN Ga-As-Al Ga As hetero junction in thermal equilibrium.(10 marks)