# Semiconducter Assignment 3 Answers :- Semiconducter Devices And Circuit

In this blog we are providing semiconducter assignment 3 answers of NPTEL/SWAYAM. We will provide answers of every week of nptel courses.

1.Given an energy band diagram, how can one find the electric field?

a ) It is proportional to Ec

b) It is proportional to minus Ec

c) It is proportional to the slope of Ec

d) It is proportional to the minus of the slope of 𝐸c

Ans :- c) It is proportional to the slope of Ec

2. Consider an n-type semiconductor at 0 K temperature. Where do you expect the Fermi
level to be?

a) Inside the valence band.

b) Inside the conduction band.

c) Near the middle of the band-gap.

d) Between the donor level and the conduction band.

Ans :- d) Between the donor level and the conduction band.

3. An intrinsic Si wafer is doped with 5 × 10^16 /𝑐𝑐 phosphorous atoms and 6 × 10^16 / 𝑐𝑐 boron atoms. Assume that at 300K, the intrinsic carrier concentration in Si is 10^10 /cc and all the dopant atoms are ionized. What is the location of the Fermi level with respect to the conduction band edge at 300K? (Assume the effective density of states at the conduction band edge, 𝑁𝐶 = 2.82 × 10^19 /𝑐𝑐).

a) 0.2 eV

b) 0.56 eV

c) 0.16 eV

d) 0.92 eV

Ans :- d) 0.92eV

4. As temperature increases from 0K to high temperature, the carrier concentration goes through three regions. In what order does the transition occur?

a) Freezeout, extrinsic, intrinsic

b) Intrinsic, extrinsic, freezeout

c) Freezeout, intrinsic, extrinsic

d) Intrinsic, freezeout, extrinsic

Ans :- a) Freezeout, extrinsic, intrinsic

5. An intrinsic germanium wafer is doped with a shallow acceptor density of 3ni/2, where ni is the intrinsic carrier concentration. At temperature T, all the acceptors are ionized. Calculate the hole density at temperature T.

a)ni

b)ni/2

c)2ni

d)3ni/2

Ans :- c) 2ni

6. Under which condition the position of the Fermi level in an intrinsic bulk semiconductor lies exactly at the middle of the band-gap at a non-zero temperature T?

a) Fermi level of an intrinsic semiconductor will always lie at the middle of the band-gap.

b) When mobility of electrons > mobility of holes.

c) When the energy band-gap > 2kT.

d)  When effective mass of electrons = effective mass of holes.

Ans :- d)  When effective mass of electrons = effective mass of holes.

7. Assume two semiconductors A and B have the same effective density of states both at the conduction band edge (𝑁𝐶) and the valence band edge (𝑁𝑉). The intrinsic carrier concentration of A and B at 300 K are respectively 1.5𝑒10 𝑐𝑚−3 and 3.2𝑒7 𝑐𝑚−3. What is the band-gap of semiconductor B if the semiconductor A has a band-gap of 1.12 eV?

a) 1.12 eV

b) 1.44 eV

c) 1.92 eV

d) 0.74 eV

Ans :- b) 1.44 eV

8. The probability of a state being filled by an electron at energy 𝐸c − 𝑘𝑇 is equal to the probability of a state being filled by a hole at energy 𝐸c − 3𝑘𝑇. Where is the Fermi level located?

a) 3kT above the valence band edge.

b)2kT below the conduction band edge.

c) 2kT above the conduction band edge

d) 3kT below the valence band edge.

Ans :- b) 2kT below the conduction band edge.

9. A silicon wafer is doped with 10^16 cm−3 arsenic atoms. The donor level 𝐸𝑑 is located at 54 meV below 𝐸c. What percentage of the donor atoms are ionized at T = 77K?  [Assume, 𝑁c(𝑇 = 77𝐾) = 4.2 × 10^18 𝑐𝑚−3 and the degeneracy factor of donor level =2 ]

a) 100 %

b) 65.5 %

c) 22.3 %

d) 10.9 %

Ans :- c) 22.3 %

10. For a certain semiconductor the densities of states in the conduction and valence bands are gC(E) = A. (E−EC)and gV(E) = B. (Ev−E) respectively where A and B are two non−zero constants. Assume Boltzmann distribution for both types of carrier. Consider the reference potential energy level at the valence band edge i.e. EV =0, EC is the energy at the conduction band edge. If A =2B, compute the intrinsic Fermi energy at 300K.

a)EC/2−9meV

b)EC/2+9meV

c)EC−26meV

d)EC+26meV

Ans :- a) EC/2−9meV