s in the band gap of a semiconductor. After [Gove84]. Physically, electron-hole pair generation occurs by the thermal excitation of an electron from the valance band to the defect level followed by its emission to the conduction band. Midgap energy levels in a depletion region are most effective at generating dark current in a device via this process. Recombination occurs when a carrier of one sign is captured at a defect, and not re-emitted before a carrier of the opposite sign is also captured. The energy may be released in the form of light (radiative recombination), or in the form of phonons (i.e., lattice vibrations) which is termed non-radiative recombination. The minority carrier lifetime, which is a key parameter in device performance, is determined by the recombination rate [e.g., Schr82]. Carrier trapping refers to the process whereby a carrier is captured at a defect and then released to its original band. In the case of CCDs, signal charge may be trapped only to be released after the signal packet has already passed causing the charge transfer efficiency of the device to degrade [Mohs74]. Filled traps with a net charge are more effective scattering centers thereby reducing carrier mobility. Carrier removal results when a majority carrier is trapped. Compensation is also responsible for carrier removal. As seen in the figure (for n-type material), the free electrons provided by the shallow donor levels are compensated by deep lying acceptor levels thereby reducing the net carrier concentration. For example, the resistance in a lightly doped collector of a bipolar transistor can increase as a result of this type of carrier removal. Finally, defect levels can assist tunneling through a potential barrier in the bandgap. This effect can produce increased current in a reverse biased junction, and is most significant in materials with small bandgaps and high electric fields. IV-57 The most important material parameters for the practical operation of most semiconductor devices are the minority carrier lifetim