A technique used to detect energy levels due to traps in the forbidden band of a semiconductor. The general technique is to use either photons, electrons, or an applied electric field to excite carriers into these levels, and to measure either the change in capacitance across the semiconductor, or transient currents produced as the equilibrium state in the material is re-established. The technique is very useful in establishing activation energies, concentrations, and capture cross sections of electron traps.