Question

Which of the following is an example of a direct band gap intrinsic semiconductor?

• A. Silicon
• B. Germanium
• C. Gallium Arsenide
• D. All of these

Answer 1

The correct option is C

Gallium Arsenide

The explanation for the correct option:

In the case of option (C),

1. A direct band-gap semiconductor is one in which the maximum energy level of the valence band aligns with the minimum energy level of the conduction band with respect to momentum.
2. The band gap represents the minimum energy difference between the top of the valence band and the bottom of the conduction band, However, the top of the valence band and the bottom of the conduction band are not generally at the same value of the electron momentum.
3. In a direct band gap semiconductor, the top of the valence band and the bottom of the conduction band occur at the same value of momentum, as in the schematic above.
4. In a direct band gap, the top energy level of the valance band which is maximum is aligned to the bottom energy level of the conduction band.
5. So, the transition of electrons from the valance band to the conduction band takes place directly.
6. It occurs at the same level of momentum.
7. Gallium Arsenide has a direct bandgap unlike many other semiconductors implying it can emit light with high efficiency. Being a direct bandgap material, it is resistant to radiation damage enabling its use in optical windows and space electronics in high-power applications.
8. Thus, gallium arsenide represents the direct band gap intrinsic semiconductor.

The explanation for incorrect options:

In the case of option (A),

1. Silicon is an indirect band gap semiconductor with a band gap of $1.12\mathrm{eV}$.
2. In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a different value of momentum to the minimum in the conduction band energy as shown in the schematic diagram above.
3. In an indirect band gap, there is a difference in the top energy level of the valance band and the bottom energy level of the conduction band.

In the case of option (B),

1. Germanium is also an indirect band gap semiconductor with a difference of $140\mathrm{meV}$ between its direct and indirect band gap.

Hence, option (C) is correct.