Gate 2005 questions on Electronic Devices

Gate 2005 (1 mark)

Question 1 

The band gap of silicon at room temperature

a)1.3 eV

b)0.7 eV

c)1.1 eV

d)1.4 eV

Answer : c

 

Question 2:

A silicon PN junction at a temperature of 20℃ has a reverse saturation current of 10 pico – Amperes (pA) . The reverse saturation current at 40℃ for the same bias approximately

a)30 pA

b)40 pA

c)50 pA

d)60 pA

Answer : b

Explanation:

For every 10℃ rise in temperature reverse saturation current doubles according to the following equation

ID(T) = Io1* 2(T- T1)/ 10

 

Question 3:

The primary reason for the widespread use of silicon in semiconductor device technology is

a)abundance of silicon on the surface of the earth.

b)large band gap of silicon in comparison to Germanium.

c)favorable properties of silicon – dioxide (SiO2).

d)lower melting point.

Answer  : a

Gate 2005 (2 marks)

Question 1

An silicon sample A is doped with 1018 atoms/ cm3 of Boron . Another sample B of identical dimensions is doped with 1018 atoms/ cm3 of Phosphorus. The ratio of electron to hole mobility is 3. The ratio of conductivity of the sample A to B is

a)3

b)1/3

c)2/3

d)3/2

Answer : b

Explanation:

σn = nqµn

σpσn = µpµn = 13

Question 2: 

A silicon PN junction diode under reverse bias has depletion region of width 10µm. The relative permittivity of silicon , εr = 11.7 and the permittivity of free space ,εo = 8.85 * 10-12 F/ m . The depletion capacitance of the diode per square meter is

a)100 µF

b)10 µF

c)1 µF

d)20 µF

Answer : b

Explanation

C = ε0εr Ad

CA = 8.85*10-12*11.710*10-6

=10.35µF 

Question 3: 

A MOS capacitor made using p – type substrate is in the accumulation mode. The dominant charge in the channel is due to the presence of

a)holes

b)electrons

c)positively charged ions

d)negatively charged ions

Answer : a

Explanation

In accumulation mode for NMOS having p – substrate VG is –ve . when negative VG is applied to gate electrode , the holes in the p – type substrate are attracted to the semiconductor oxide interface . This condition is called carrier accumulation on the surface.