GATE 1990 Electronic Devices (EDC) Video solutions

1.  In a uniformly doped abrupt PN junction, the doping level of the N-side is four times the doping level of the P-side. Then the ratio of the depletion layer widths is......

Answer:

W_n / W_p = 1 / 4

Solution :  http://www.youtube.com/watch?v=ddZ7Kuje6E8


2. In a semiconductor, under high electric fields, with increasing electric fields, mobility of charge carriers ............. and velocity of charge carriers ..............

Answer: Reduces, Gets saturated so remains constant.

Solution : http://www.youtube.com/watch?v=8cjTRCdTpbk


3. In a forward biased photo diode, with increase in incident light intensity, the diode current
       a.  increases
       b.  remains constant
       c.  decreases
       d.  remains constant, but the voltage drop across the diode increases.

Answer: A

Solution :  https://www.youtube.com/watch?v=dBkgMBzNyYA

4.  In a junction diode,
       a. the depletion capacitance increases with increase in the reverse bias
       b. the depletion capacitance increases with decrease in the reverse bias
       c. the diffusion capacitance increases with increase in the forward bias
       d. The diffusion capacitance is much higher than the depletion capacitance, when it is   
           forward biased.

Answer: B, C, D

Solution :  https://www.youtube.com/watch?v=2Vmo8zKg4vY

5. SCR can be turned On by
       a. Applying anode voltage at sufficiently fast rate
       b. Applying sufficiently large anode voltage
       c. Increasing the temperature of SCR to a sufficiently large value
       d. Applying sufficiently large gate current

Answer: All are correct, most accurate is D

Solution :   https://www.youtube.com/watch?v=U0x31gFcggQ

6. The pinch off voltage of a JFET is 5.0 volts. Its cutoff voltage is

Answer: 5 volts

Solution :  https://www.youtube.com/watch?v=YZcn5Y_9p14

7.  Which of the following effects can be caused by a rise in the temperature ?
       a. Increase in MOSFET current
       b. Increase in BJT current
       c. Decrease in MOSFET current
       d. Decrease in BJT current

Answer: B,C

Solution : https://www.youtube.com/watch?v=BoRoLQ2eN6o

8. (a) Two ideal and identical junction diodes (with ideality factor Ƞ = 1) connected in series as shown below.

Show that exp(eV1/KT) + exp(-eV2/KT) = 2, where V1 and V2 are the voltage drop across the diodes D1 and D2.

(b) Assuming that the current flowing through the reverse biased diode is saturated at I_CO, calculate the voltage drop across the forward biased diode. Assume KT = 26 meV.

Answer: (b) 0.018 volts

Solution :   https://www.youtube.com/watch?v=qz6ZD9Y5nUo

9. In a semiconductor at room temperature, the intrinsic carrier concentration and resistivity are 1.5 X 10¹⁶ m^-3 and 2 X 10⁵ Ω-m respectively. It is converted into an extrinsic semiconductor with a doping concentration of 10²⁰ per 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 doubles.

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

Answer:

a) 2.25 x 10¹² /m³

b) 60 Ω-m

c) 0.228 eV

d) 9 x 10¹² /m³

Solution :    https://www.youtube.com/watch?v=1E491IgBz-I