In class it was stated that the mobility of electrons is hig

In class it was stated that the mobility of electrons is higher than the mobility of holes, but the reason for this was not given. Based on your understanding of how electons and holes move in a semiconductor, explain why it is reasonable to expect that holes will have lower overall mobility compared with electrons. If the reason does not present itself after you think about it for a while, you may wish to explore this topic using sources available on the Wed.

Solution

The mobility is proportional to the carrier relaxation time and inversly proportionnal to the carrier effective mass.
The electron mobilty is often greater than hole mobility because quite often, the electron effective mass is smaller than hole effective mass.
The relaxation times are often of the same order of magnitude for electrons and holes and therefore, they do not make too much difference.
In order to increase the speed of a device one has to choose materials with small electron and hole effective masses and long relaxation times i.e. where the electrons and holes do not have to experience too much collissions on crystal imperfections, impurities, ...i.e. one has to choose materials with high crystal quality.

In a semiconductor the mobility of electrons (referring to ‘conduction electrons’ or ‘free-electrons’) is greater than that of a holes (indirectly referring to ‘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. In an applied 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.

Holes are created by the elevation of electrons from innermost shells to higher shells or shells with higher energy levels. Since holes are subjected to the stronger atomic force pulled by the nucleus than the electrons residing in the higher shells or farther shells, holes have a lower mobility.

In an intrinsic silicon, at temperature 300 K:
Electron mobility = 1500 cm2/(Vs)
Hole mobility = 475 cm2/(Vs)