# 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 šøc

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.Ā

3. An intrinsic Si wafer is doped with 5 Ć 10^16 /šš phosphorous atoms and 6 Ć 10^16 / šš 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, šš¶ = 2.82 Ć 10^19 /šš).Ā

a) 0.2 eVĀ

b) 0.56 eVĀ Ā

c) 0.16 eVĀ Ā

d) 0.92 eVĀ

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?Ā

a)Ā Freezeout, extrinsic, intrinsicĀ Ā

b)Ā Intrinsic, extrinsic, freezeoutĀ Ā

c) Freezeout, intrinsic, extrinsicĀ Ā

d) Intrinsic, freezeout, extrinsicĀ

Ans :- a)Ā Freezeout, extrinsic, intrinsicĀ Ā

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

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?

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

b) When mobility of electrons > mobility of holes.Ā Ā

c) When the energy band-gap > 2kT.Ā

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

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

7. Assume two semiconductors A and B have the same effective density of states both at the conduction band edge (šš¶) and the valence band edge (šš). The intrinsic carrier concentration of A and B at 300 K are respectively 1.5š10 ššā3 and 3.2š7 ššā3. What is the band-gap of semiconductor B if the semiconductor A has a band-gap of 1.12 eV?Ā

a) 1.12 eVĀ Ā

b) 1.44 eVĀ Ā

c) 1.92 eVĀ Ā

d) 0.74 eVĀ

Ans :- b) 1.44 eVĀ Ā

8. The probability of a state being filled by an electron at energy šøc ā šš is equal to the probability of a state being filled by a hole at energy šøc ā 3šš. Where is the Fermi level located?Ā Ā

a) 3kT above the valence band edge.Ā Ā

b)2kT below the conduction band edge.Ā Ā

c) 2kT above the conduction band edgeĀ

d)Ā 3kT below the valence band edge.Ā

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

9. A silicon wafer is doped with 10^16 cmā3 arsenic atoms. The donor level šøš is located at 54 meV below šøc. What percentage of the donor atoms are ionized at T = 77K?Ā  [Assume, šc(š = 77š¾) = 4.2 Ć 10^18 ššā3 and the degeneracy factor of donor level =2 ]Ā

a) 100 %Ā Ā

b) 65.5 %Ā Ā

c) 22.3 %Ā Ā

d) 10.9 %Ā

Ans :- c) 22.3 %

10. ForĀ aĀ certainĀ semiconductor theĀ densitiesĀ ofĀ statesĀ inĀ theĀ conductionĀ andĀ valenceĀ bandsĀ areĀ gC(E)Ā =Ā A. (EāEC)andĀ gV(E)Ā =Ā B. (EvāE)Ā respectively whereĀ AĀ andĀ BĀ areĀ twoĀ nonāzeroĀ constants. AssumeĀ BoltzmannĀ distributionĀ forĀ bothĀ typesĀ ofĀ carrier. ConsiderĀ theĀ referenceĀ potentialĀ energyĀ levelĀ atĀ theĀ valenceĀ bandĀ edge i.e.Ā EVĀ =0,Ā ECĀ isĀ theĀ energyĀ atĀ theĀ conductionĀ bandĀ edge. IfĀ AĀ =2B,Ā computeĀ theĀ intrinsicĀ FermiĀ energyĀ atĀ 300K.

a)EC/2ā9meV

b)EC/2+9meV

c)ECā26meV

d)EC+26meV

Ans :- a) EC/2ā9meV