Semiconductor thermal equilibrium What precisely thermal equilibrium implies in case of a semiconductor. One thing I know is that Fermi Level remains the same. But what other things does this implies, is there net current flow in case of a thermal equilibrium?
 A: At thermal equilibrium (also assuming chemical equilibrium has been established) all process happen at the same rate in the forward and reverse direction. This is the principle of detail balance.
For example, scatting of conduction band electrons in one direction is balanced by scattering in the reverse direction. So there is no net current flow.
The emission of blackbody radiation is balanced by the absorption of blackbody radiation by the environment. So there is no net emission (or absorption) of energy.
The principle of detailed balance is very important to semiconductor physics. It crops up a lot.
You mentioned Fermi levels. The Fermi levels splits into two quasi fermi levels when the semiconductor is not in chemical equilibrium. This happens when charge carriers are injected into the material either electrically or optically.
If there is a thermal gradient across the semiconductor (not in thermal equilibrium) then the Fermi level will be different across the material.
When the material is in chemical and thermal equilibrium it is said to be in thermodynamic equilibrium.
A: Thermal equilibrium appears when the drift current is equal to the diffusion current; for example, for a semiconductor type n:
\begin{equation}
I_u + I_d = 0;\ \frac{p_n}{dt}=0
\end{equation}
\begin{equation}
en\mu E+qD_n\nabla n = 0  
\end{equation}
