When the position of the silicon in the bottom of the conduction band and the top of the valence band are different, the electron absorption light cannot jump beyond the energy band. This is because when the wavelength of light is 600 nm, the wave number k-2u/2 is about 10 m. On the other hand, the electronic wave number is 12 of the reciprocal character, so about 10"mr

Thus, the number of light waves is three orders of magnitude smaller than the number of electron waves, and because momentum =, the conservation of momentum does not hold.

Therefore, the use of the power of phonons (quantums of character vibration) in silicon makes it possible to convert between bands. This is called an indirect transition. Such a semiconductor is called an indirect transition semiconductor.

Because indirect transitions have poor absorption of light, the fabrication of solar cells requires very thick materials. On the other hand, for example, GaAs, the bottom of the conduction band and the top of the valence band are at the same wavenumber position, and the law of conservation of momentum is established, and the valence band electron absorption light can directly transition to the guide band. This is why silicon and calcium absorb different light intensities.