Now that you have a basic understanding of how carriers behave under equilibrium
conditions, we can observe the behavior of carriers when the semiconductor
environment is changed. These changes can come in the form of externally
applied forces, like an electric field, a change in temperature, or a change
in the degree of doping. Carriers have three primary actions in a semiconductor.
These are drift, diffusion, and recombination-generation. Drift and diffusion
currents make up the total current density in the semiconductor, these
are not necessarily always present. Processes that are always present in
a semiconductor to stabilize it and return it to equilibrium conditions
are recombination and generation, where electrons and holes are destroyed
if there is an excess of a carrier, or generated if there is a deficit.