## 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,

respectively.

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