Question

Why hʋ is greater than energy gap in some material and µe usually greater than µh (µe is electron mobility , µh mobility of holes)?

Answer 1

photon energy is higher than the energy gaps of a semiconductor material when the material achieved High absorption coefficient.

It was observed that, SnSe thin films of thickness 148 nm had the highest absorption coefficient while thin films with a thickness of 112 nm had the least absorption coefficient. This is because of reduction in stacking faults and lattice defects as film thickness increased.

and absorption coeffiecient

where is absorption coefficient

is incident photon energy

is constant

In a semiconductor the mobility of electrons ( ‘conduction electrons’ or ‘free-electrons’) is greater than that of a holes (or ‘valence electrons’) because of different band structure and scattering mechanisms of these two carrier types.

Conduction electrons (free-electrons) travel in the conduction band and valence electrons (holes) travel in the valence band.

if we applied an electric field, valence electrons cannot move as freely as the free electrons because their movement is restricted. The mobility of a particle in a semiconductor is larger if its effective mass is smaller and the time between scattering events is larger.

“Free electrons are more mobile under an electric field rather than the bounded electrons, or, ‘holes'. So, the mobility of electron is considered more than the holes.”