Question

Q1

The mobility of carriers in a lightly doped silicon sample will decrease if :

Select one:

a. The sample is doped N-type instead of P-type

b. None of these

c. All of these

d. The temperature is increased

e. The number of dopant atoms is decreased

Q2

Electron mobility in silicon is typically lower than hole mobility.

Select one:

True

False

Q3 If a P-type silicon sample with a hole mobility of 370 cm^2/Vs has an electric field of 18558 V/cm applied to it, then what is the magnitude of the hole drift velocity in cm/s?

Q4

Which of the following is true?

Select one:

a. Indirect band gap semiconductors recombine through deep traps in the band gap caused by impurities

b. Indirect band gap semiconductors can radiate energy in the form of light to recombine

c. The recombination time for an indirect band gap semiconductor can vary from 10 milliseconds to 1 second

d. None of these

e. Gallium Arsenide (GaAs) has an indirect band gap, while Silicon (Si) has a direct band gap

Q5

The average time a carrier travels between collisions in the crystal is typically less than a picosecond.

Select one:

True

False

Answer 1

1) Mobility of carriers refers to both electron and hole mobility. Thus in lightly doped silicon,

a) P type and N- type doping will result in mobility of hole and electron respectively . Thus the option "Sample is doped N-type instead of P-type" is not correct.

d) In semiconductor, there temperature dependence of mobility is due to impurity scattering and lattice scattering. In impurity scattering free electron gets scattered with impurity atoms and gets ionised. This increases free electrons, thus as temperature increases more and more free electrons are generated. This leads to increase in mobility. But above certain temperature all the impurities are ionised and there are no more ionisation to take place. Now the free electron will scatter among each other and mean free path is reduced .This is known as lattice scattering. The mobility decreases. in lattice scattering

Since in this case silicon is lightly doped thus the lattice scattering will dominate therefore, the mobility decreases with increase in temperature. Option d is the answer.

2) False.

The mobility is defined as

where m* is the effective mass. The effective mass of hole is higher than the effective mass of electron thus , the mobility of electron is higher than that of mobility of hole.

3)The drift velocity is defined as

where u is the mobility and E is the electric field.

Thus

4) a) True.

In indirect band gap the momentum (k) between the electron in conduction band is different from that of holes in valence band. In this case the recombination has to assisted by phonon to account for this momentum due to conservation of momentum rule. Thus in indirect semiconductor like silicon, as recombination center has to be present in between the and to mediate the recombination of electron and hole. Trap states could be one of the recombination center.

b) False.

Photon has zero momentum. In indirect band gap semiconductor, there is change in momentum while recombination which cannot be accounted by zero momentum photo and conservation of momentum gets violated. Therefore, the indirect band gap cannot radiate photon.

c) False

d) False

e) False. GaAs is direct band gap whereas silicon is indirect band gap.

5) True.

, taking the above parameters,