Impurity level

An impurity level is an electronic energy level associated with a dopant or defect in a semiconductor. A shallow donor impurity introduces a level just below the conduction band, and a shallow acceptor impurity introduces one just above the valence band. Under electrostatic band bending, these levels shift with the local band edges in the same material; their occupation is determined relative to the equilibrium chemical potential.

A sequence of bound-to-bound optical transitions of a hydrogenic donor state or hydrogenic acceptor state, converging on a photoionization threshold $R^{\ast }$. For an initially occupied $1s$ state, ordinary electric dipole transitions reach $np$ states; above the threshold, the final carrier can enter a band continuum.