# Electronic Devices and Circuits Questions and Answers – Carrier Life time

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This set of Electronic Devices and Circuits Questions and Answers for Freshers focuses on “Carrier Life Time”.

1. What is the range of the carrier lifetime?
a) Nanoseconds to microseconds
b) Nanoseconds to hundreds of microseconds
c) Nanoseconds to tens of microseconds
d) Nanoseconds to milliseconds

Explanation: Carrier lifetime is defined as the existence of any carrier for τ seconds. Carrier lifetime ranges from nanoseconds to hundreds of microseconds.

2. What is the process number of Schokley-Read-Hall Theory processes?
Process-‘ The capture of an electron from the conduction band by an initially neutral empty trap’
a) Process1
b) Process2
c) Process3
d) Process4

Explanation: This is the first process of Schokley-Read-Hall theory of Recombination.

3. Calculate the recombination rate if the excess carrier concentration is 1014cm-3 and the carrier lifetime is 1µseconds.
a) 108
b) 1010
c) 1020
d) 1014

Explanation: The recombination rate, R=δn/τ.
So, R=1014 / 10-6
R=1020.

4. Calculate the capture rate where Cn=10, Nt=1010cm-3, n=1020 and fF (Et)=0.4.
a) 6*1030
b) 5*1030
c) 36*1030
d) 1.66*1029

Explanation: Rcn=Cn*N_t*(1-fF (Et))*n
Substituting the values,
Rcn=6*1030.

5. Calculate the emission rate where En=2.5, Nt=1010cm-3 and fF (Et)=0.6 .
a) 15*1010
b) 1.5*1010
c) 15*1011
d) 1.5*1011

Explanation: Ren=En*Nt*(fF (Et))
Substituting the values,
Ren=1.5*1010.

6. At what condition, the rate of electron capture from the conduction band and the rate of the electron emission back into the conduction band must be equal?
a) Thermal equilibrium
b) At room temperature
c) T=250K
d) At boiling temperature

Explanation: At thermal equilibrium, the electron capture rate and the emission rate will be same in the conduction band.

7. Calculate the carrier lifetime when Cp=5 and Nt=1010cm-3.
a) 2*1011
b) 2*10-11
c) 20*10-11
d) 20*1011

Explanation: τp=1/(Cp*Nt )
=1/(5*1010)
=2*10-11.

8. The number of majority carriers that are available for recombining with excess minority carriers decreases as the excess semiconductor becomes intrinsic. Is it true?
a) True
b) False

Explanation: With the increase in the number of the majority carriers, the carriers for the recombination will be decreasing with the excess minority carriers and will finally become intrinsic as the concentrations will be same.

9. Which of the following is used as the recombination agent by semiconductor device manufactures?
a) Silver
b) Gold
c) Platinum
d) Aluminium

Explanation: Gold is used as a recombination agent because of its chemical properties as it was used in the Bohr’s experiment. Thus the device designer can obtain the desired carrier lifetimes by introducing gold into silicon under controlled conditions.

10. The rate of change of the excess density is proportional to the density. Is it true of false?
a) True
b) False

Explanation: The rate of change of the excess density depends on the density of the semiconductor and the rate with respect to time is also dependent on it.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.

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