What do you mean by effective mass of carriers? how does it depend on energy bands?
Answers
In solid state physics, a particle's effective mass (often denoted m*) is the mass that it seems to have when responding to forces, or the mass that it seems to have when interacting with other identical particles in a thermal distribution. One of the results from the band theory of solids is that the movement of particles in a periodic potential, over long distances larger than the lattice spacing, can be very different from their motion in a vacuum. The effective mass is a quantity that is used to simplify band structures by modeling the behavior of a free particle with that mass. For some purposes and some materials, the effective mass can be considered to be a simple constant of a material. In general, however, the value of effective mass depends on the purpose for which it is used, and can vary depending on a number of factors.
Thus, the electrons in the conduction band are free to move about via the many available empty states.
Corresponding problem of charge transport in the valence band is slightly more complex.
Current transport in the valence band can be accounted for by keeping track of the holes themselves.
In a filled band, all available energy states are occupied.
For every electron moving with a given velocity, there is an equal and opposite electron motion somewhere else in the band.
Under an applied electric field, the net current is zero, since for every electron j moving with a velocity , there is a corresponding electron moving with a velocity -.
In a unit volume, the current density J can be given by
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