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