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 μn compared to Hole mobility μp.
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.
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