Tuesday, 11 June 2013

Video Solutions for GATE 1992 ECE : Five Mark Questions ( Electron Devices )

1. A p-n-p transistor shown in figure has uniform doping in the 

    emitter , base and collector regions, where in the doping 

    concentrations are 1019 per cm3 , 1017 per cm3 and 1015 per cm3

    respectively. The minority carrier diffusion lengths in the emitter 

    and the base regions are 5 microns and 100 microns,   


    Assuming low level injection conditions and using law of the 

     junction, calculate the collector current density and the base 

     current density due to base recombination. 

     [suitable approximations may be made if required].

    In all the regions of the transistors DP = 8 cm2/sec, 

    Dn = 16 cm2/sec, ni = 1.5x1010 /cm3, KT/q = 26 mV 

    and q = 1.6x10-19 C.

2. An  n-channel MOSFET having a VT of 2 volts is used in the 

    circuit shown in figure. Initially, transistor is OFF and is in 

    steady state. At time t = 0, a step voltage of magnitude of

    4 volts is applied to the input so that the MOSFET turns ON 

    instantaneously. Draw the equivalent circuit and calculate the 

    time taken to the output Vo to fall to 5 volts. 

    The device constant of the MOSFET, K = 5 mA/V2, RDS = ∞, 

    CDS = 0, CDG = 0.

 Ans : 138 nSec

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