**GATE 2015 (Electron Devices)**

1. Which
one of the following processes is preferred to form the gate dielectric (SiO_{2})
of MOSFET?

a. Sputtering

b. Molecular
Beam Epitaxy

c. Wet
Oxidation

d. Dry
Oxidation

3. If
the base width in a bipolar junction transistor is doubled, which one of the following
statements will be TRUE?

a. Current
gain will increase

b. Unity
gain frequency will increase

c. Emitter
base junction capacitance will increase

d. Early
voltage will increase

5.

The
electric field profile in the depletion region of a PN junction in equilibrium is
shown in the figure. Which one of the following is NOT TRUE?

a. The
left side of the junction is N-type and the right side is P-type

b. Both
the N-type and P-type depletion regions are uniformly doped

c. The
potential difference across the depletion region is 700 mV

d. If the
P-type region has a doping concentration of 10^{15} cm^{-3},
then the doping concentration in the N-type region will be 10^{16} cm^{-3}

^{ }

** Solution : ** https://www.youtube.com/watch?v=LNjwCHJv3UE
6.

In the circuit shown, the both the enhancement mode NMOS transistors
have the following characteristics: K

_{n} = µ

_{n}.C

_{ox}(W/L)
= 1 mA/V

^{2}, V

_{TN} = 1 volt.

Assume that the channel length
modulation parameter λ is zero and body is shorted to source. The minimum
supply voltage VDD (in volts) needed to ensure that transistor M1 operates in
saturation mode of operation is ___________________

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

7.

The current in an enhancement mode NMOS transistor biased in saturation
mode was measured to be 1 mA at a drain to source voltage of 5 volts. When the
drain – source voltage was increased to 6 volts, while keeping gate-source voltage
same, the drain current increased to 1.02 mA. Assume that drain to source saturation
voltages is much smaller than the applied drain source voltage. The channel
length modulation parameter λ (in V

^{-1}) is __________

** Solution : ** https://www.youtube.com/watch?v=4AebD_Jv5C8

8.

An NPN BJT having reverse saturation current I

_{s} = 10

^{-15}
A is biased in the forward active region with V

_{BE} =700 mV. The thermal
voltage (V

_{T}) is 25 mV and the current gain (β) may vary from 50
to 150 due to manufacturing variations. The maximum emitter current (in µA) is
____________________________

** Solution : ** https://www.youtube.com/watch?v=-Ff_pmSDNlQ

9.

A
silicon sample is uniformly doped with donor type impurities with a
concentration of 10

^{16} cm

^{-3}. The electron and hole mobilities
in the sample are 1200 cm

^{2}/V-sec and 400 cm

^{2}/V-sec
respectively. Assume complete ionization of impurities. The charge of an
electron is 1.6x10

^{-19} C. The resistivity of the sample (in Ω-cm)
is _______________

** Solution : ** https://www.youtube.com/watch?v=Zo6ei-B4gX8

10. A region of negative differential
resistance is observed in the current voltage characteristics of a silicon PN
junction if

a. Both
the P-region and N-region are heavily doped

b. The N-region
is heavily doped compared to P-region

c. The
P-region is heavily doped compared to N-region

d. An intrinsic
silicon region is inserted between the P-region and the N-region

13.

For the NMOSFET in the
circuit shown, the threshold voltage is V

_{th} greater than zero. The source
voltage V

_{SS} is varied from 0 to V

_{DD}. Neglecting the
channel length modulation, the drain current I

_{D} as a function of V

_{SS}
is represented by

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

14.

For a silicon diode
with ling P and N regions, the acceptor and donor impurity concentrations are
1x10

^{17} cm

^{-3} and 1x10

^{15} cm

^{-3}
respectively. The lifetimes of electron in P-region and holes in N-region are
both 100 µs. The electron and hole diffusion coefficients are 49 cm

^{2}/sec
and 36 cm

^{2}/sec respectively. Assume thermal voltage is 26 mV, the
intrinsic carrier concentration is 1x10

^{10} cm

^{-3} and q =
1.6x10

^{-19} C. When a forward voltage of 208 mV is applied across the
diode, the hole current density (in nA/cm

^{2}) injected from P-region
to N-region is ___________

** Solution : ** https://www.youtube.com/watch?v=aR7Vz7Fm3co

15.

A MOSFET in saturation
has a drain current of 1 mA for V

_{DS} = 0.5 volts. If the channel
length modulation coefficient is 0.05 V

^{-1}, the output resistance (in
kΩ) of the MOSFET is ________

** Solution : ** https://www.youtube.com/watch?v=uFmH59NNz6s

16.

The built in potential
of an abrupt PN junction is 0.75 volts. If its junction capacitance (C

_{J})
at a reverse bias (V

_{R}) of 1.25 volts is 5 pF. The value of C

_{J}
(in pF) when V

_{R} = 7.25 volts is _____

** Solution : ** https://www.youtube.com/watch?v=dk28aPHYdnY

17.

A piece of silicon is
doped uniformly with phosphorous with a doping concentration of 10

^{16}
per cm

^{3}. The expected value of mobility verses doping concentration
for silicon assuming full dopant ionization is shown below. The charge of an
electron is 1.6x10

^{-19} C. The conductivity (in Simons/cm) of the
silicon sample at 300

^{o}K is _______________

** Solution : ** https://www.youtube.com/watch?v=6im7x-zbWH4

18.

An N-type silicon
sample is uniformly illuminated with light which generates 10

^{20}
electron-hole pairs per cm

^{3} per second. The minority carrier lifetime
in the sample is 1 µs. In the steady state, the hole concentration in the sample
is approximately 10

^{x}, where x is an integer. The value of x is
_________________

** Solution : ** https://www.youtube.com/watch?v=Cd_4k0VGFXg

19.

In MOS capacitor with
an oxide layer thickness of 10 nm. The maximum depletion layer thickness is 100
nm. The permittivity’s of the semiconductor and the oxide layer are ε

_{s}
and ε

_{ox} respectively. Assuming ε

_{s}/ε

_{ox}
= 3, the ratio of the maximum capacitance to the minimum capacitance of this
MOS capacitor is ____________

** Solution : ** https://www.youtube.com/watch?v=eIk6sCXiKbY

21.

A DC voltage of 10
volts is applied across an N-type silicon bar having a rectangular cross section
and length of 1 cm as shown. The donor doping concentration N

_{D} and
the mobility of electron µ

_{n} are 10

^{16} cm

^{-3} and
1000 cm

^{2}/V-sec respectively. The average time (in µs) taken by the electrons
to move from one end of the bar to the other end is _______________

** Solution : ** https://www.youtube.com/watch?v=kOhp3iqtlSk

22.

The diode in the
circuit given below has V

_{ON} = 0.7 volts but is ideal otherwise. The current
(in mA) in the 4 kΩ resistor is __________________

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