Engineering Questions with Answers - Multiple Choice Questions

# MCQs on Charge Densities in a Semiconductor impurities

1 - Question

Is n/p=ni2 is a correct formula?
a) True
b) False

Explanation: The correct formula is n*p=ni2.

2 - Question

Calculate the number of electrons is the number of holes are 15*1010?
a) 15*1010
b) 1.5*108
c) 1.5*109
d) 1.5*1010

Explanation: n*p=(1.5*1010)2
n*15*1010=1.5*1.5*1010*1010
n=1.5*109 electrons.

3 - Question

For which type of material, the number of free electron concentration is equal to the number of donor atoms?
a) P type semiconductor
b) Metal
c) N-type semiconductor
d) Insulator

Explanation: The n-type semiconductor has equal concentration of free electron and donor atoms.

4 - Question

Identify the correct condition for a semiconductor to be electrically neutral.
a) Nd+p=Na+n
b) Nd-p=Na+n
c) Nd+p=Na-n
d) Nd-p=Na-n

Explanation: The sum of the number of donors and the holes is equal to the sum of the number of the acceptors and the electrons.

5 - Question

Do the Fermi energy level changes in a semiconductor?
a) True
b) False

Explanation: The Fermi energy level changes as the electron and hole concentrations change because of the formula which defines the position of the Fermi level depending on the concentration of holes and electrons.

6 - Question

Consider a silicon wafer having Nc=2.8*1019cm-3 and the Fermi energy is .25eV below the conduction band. Calculate the equilibrium concentrations of electrons at T=300K?
a) 18*1016cm-3
b) 1.8*1016cm-3
c) 1.8*1014cm-3
d) 180*1016cm-3

Explanation: n0=Nc*exp(-Eg/KT)=2.8*1019*exp(-0.25/0.0259)
=18*1016cm-3.

7 - Question

If Ef>Efi, then what is the type of the semiconductor?
a) n-type
b) P-type
c) Elemental
d) Compound

Explanation: For n-type, the Fermi energy level is greater than the intrinsic Fermi energy level because in an energy band, Fermi level of donors is always greater than that of the acceptors.

8 - Question

The 1-fF (E) increases in which of the following band for n type semiconductor?
a) Conduction band
b) Donor band
c) Acceptor band
d) Valence band

Explanation: For an n-type semiconductor, the probability of fF (E) decreases in the valence band. The probability of finding the electron in the conduction band is more.

9 - Question

The fF (E) decreases in which of the following band for p-type semiconductor?
a) Conduction band
b) Donor band
c) Acceptor band
d) Valence band

Explanation: The probability of finding the electron in the conduction band decreases for a p-type semiconductor because in a p-type semiconductor, the holes will be in conduction band rather than the electrons.

10 - Question

Do the intrinsic Fermi energy level changes with the addition of dopants and acceptors?
a) True
b) False