  Nontrivial capacitance behavior, including a negative capacitance (NC)
effect, observed in a variety of semiconductor devices, is discussed
emphasizing the physical mechanism and the theoretical interpretation of
experimental data. The correct interpretation of NC can be based on the
analysis of the time-domain transient current in response to a small voltage
step or impulse, involving a self-consistent treatment of all relevant physical
effects (carrier transport, injection, recharging etc.). NC appears in the case
of the non-monotonic or positive-valued behavior of the time-derivative of the
transient current in response to a small voltage step. The time-domain
transient current approach is illustrated by simulation results and
experimental studies of quantum well infrared photodetectors (QWIPs). The NC
effect in QWIPs has been predicted theoretically and confirmed experimentally.
The huge NC phenomenon in QWIPs is due to the non-equilibrium transient
injection from the emitter caused by the properties of the injection barrier
and the inertia of the QW recharging.
