Explain the difference in recombination process in
direct and indirect band gap semiconductors. And
explain its effect on the charge earner life time and
luminescent properties for a semiconductor.
In direct band gap semiconductors, the lowest energy level for electrons in the conduction band and the highest energy level for holes in the valence band occur at the same momentum in the electronic band structure. This means that the recombination process can occur directly, without any change in momentum. As a result, direct band gap semiconductors have a higher recombination rate, and hence shorter charge carrier lifetime, compared to indirect band gap semiconductors.
In indirect band gap semiconductors, the lowest energy level for electrons in the conduction band and the highest energy level for holes in the valence band occur at different momenta in the electronic band structure. This means that the recombination process cannot occur directly and requires a change in momentum. This change in momentum can be achieved through interactions with lattice vibrations (phonons) or defects in the crystal structure. As a result, indirect band gap semiconductors have a lower recombination rate, and hence longer charge carrier lifetime, compared to direct band gap semiconductors.
The difference in recombination processes between direct and indirect band gap semiconductors also affects their luminescent properties. Luminescence refers to the emission of light from a material. In direct band gap semiconductors, the recombination of an electron-hole pair can occur directly, leading to efficient radiative recombination and the generation of light. This makes direct band gap semiconductors more suitable for optoelectronic devices such as LED (light-emitting diodes) and lasers.
In contrast, in indirect band gap semiconductors, the recombination process involves a change in momentum, which is less likely to result in the emission of light. Instead, a significant portion of the recombination processes in indirect band gap semiconductors are non-radiative, causing the energy to be dissipated as heat. This makes indirect band gap semiconductors less efficient for light emission compared to direct band gap semiconductors.
In summary, the difference in recombination processes between direct and indirect band gap semiconductors affects both the charge carrier lifetime and luminescent properties of the semiconductor. Direct band gap semiconductors have shorter charge carrier lifetimes, higher recombination rates, and are more efficient in light emission. In contrast, indirect band gap semiconductors have longer charge carrier lifetimes, lower recombination rates, and are less efficient in light emission.