Conduction in conductors and semiconductors

Conduction in conductors and semiconductors

Mobility μ: In good conductors like metals, free electrons exist in abundance. They are supposed to be accelerated under the influence of electric or magnetic field as per ballistic (dynamics) laws. But in practice it is found that the electrons move with a constant velocity proportional to the field. The reason for this is the random nature of the electron movement involved in repeated collisions. The loss of energy during collisions is supplemented due to acceleration caused by the applied field E. Thus it is observed that the random motion of electrons when resolved in the direction of the field, the electrons acquire a constant speed called the drift speed v that is proportional to the field E (V /m) and velocity v is in metres/second.

where μ is the constant of proportionality. μ is called as mobility. It is measured as m²/V-s. Mobility of electrons and Holes due to the influence of electric field is given in Eq. (2.10). Because of the lighter mass of electrons, electrons have large values of mobility μ_n compared to Hole mobility μ_p.

For a given excitation energy to electrons (due to applied field strength), electrons move faster in Germanium semiconductor when compared to Silicon semiconductor, because of small forbidden band-gap energy in Germanium semiconductors. So Germanium semiconductor devices find their use in high-frequency applications.