## Friday, 28 June 2013

### Video Solutions for GATE 1990 ECE : Five Mark Questions ( Electron Devices )

1.       (a) Two ideal and identical (ideality factor ƞ = 1)
junction diodes are connected in series as

shown in figure.

Show that
exp (eV1/KT)  +  exp (-eV2/KT) = 2

where V1 and V2 are the voltage drops across
the diodes D1 and D2.

(b) Assuming that the current through the reverse
biased diode is saturated at Io.

Calculate the voltage drop across the forward
biased diode

Assume KT = 26meV.

2.       In a semiconductor at room temperature (300oK), the intrinsic carrier concentration and resistivity are 1.5 X 1016 m3 and 2 X 10 Ω-m respectively. It is converted into an extrinsic semiconductor with a doping concentration of 1020/m3. For the extrinsic semiconductor, calculate the

a.       Minority carrier concentration

b.      Resistivity

c.       Shift in Fermi level due to doping

d.      Minority carrier concentration when its temperature is increased to a value at which the intrinsic carrier concentration np doubles.

Assume the mobility of minority and majority carriers to be the same and KT = 26 meV at room temperature.