## Thursday, 13 June 2013

### Video Solutions for GATE 1991 ECE : One Mark Questions ( Electron Devices )

1. A silicon sample is uniformly doped with 1016 phosphorous

atoms / cm3 and 2x 1016 boron atoms/cm3 . if all the dopants are

fully ionized, the material is

(a)  n-type with carrier concentration of 1016 /cm3

(b)  p-type with carrier concentration of 1016 /cm3

(c)  p-type with carrier concentration of 2x 1016 /cm3

(d)  n-type with carrier concentration of 2x1016 /cm3

2. An n-type silicon sample, having electron mobility µn twice

the hole mobility µp, is subjected to a steady illumination such

that the electron concentration doubles from its thermal

equilibrium value. As a result, the conductivity of the sample

increases by a factor of……….

Ans : 2

3. The small signal capacitance of an abrupt P+N junction is

1 nF/cm2 at zero bias. If the built in voltage is 1 volt, the

capacitance at a reverse bias voltage of 99 volts is equal to …..

Ans :    0.1 nF /Cm2

4. Referring to the figure, the switch S is in position 1 initially and

steady state conditions exist from time t=0  to t = t0. The switch

is suddenly thrown into position 2. The current I flowing through

the 10K resistor as function of time from t=0, is …………

(Give the sketch showing the magnitudes of the current at t=0,
t=t0 and t=)

5. Discrete transistors T1 and T2 having maximum collector

current rating of 0.75 Amps are connected in parallel as shown

in the figure. This combination is treated as a single transistor to

carry a total current of 1 Amp, when biased with self bias

circuit. When the circuit is switched on, T1 draws 0.55 Amps

and T2 draws 0.45 Amps. If the supply is kept on continuously,

ultimately it is very likely that

(a)   Both T1 and T2 get damaged

(b)   Both T1 and T2 will be safe

(c)   T1 will get damaged and T2 will be safe

(d)   T2 will get damaged and T1 will be safe

6. The built in potential of the gate junction of an-channel JFET

is 0.5 volts. The drain current saturates at VDS = 4.0 volts

when VGS = 0. The pinch off voltage is ______________

Ans :  - 4 volts

7. In figure, all transistors are identical and have a high value of

beta (β). The voltage VDC is equal to _____

Ans :  5 volts

8. In figure the input Vi is a 100 Hz triangular wave having a

peak to peak amplitude of 2 volts and an average value of

zero volts. Given that the diode is ideal, the average value

of the output Vo is ______    Ans : 0.3 Volts

9. In figure, the n-channel MOSFETs are identical and their current

voltage characteristics are given by the following expressions:

Ans :  1 mA

1. 2. 