Some General Properties of Nonequilibrium Photodetectors

Nonequilibrium detectors share a number of similar or even identical characteristics, independently on a particular structure or process. This is a consequence of similar basic principles utilized for nonequilibrium operation. All of the Auger-suppressed devices utilize relatively strong external or internal fields to decrease minority carrier concentration in a given volume and thus operate in the mode of large deviations from equilibrium. The degree of suppression is proportional to the intensity of the applied fields. The structures with the decreased concentration of minority carriers are conventional intrinsic photonic detector strucmres. 

In all nonequilibrium devices presented until now the active region is fabricated in weakly doped v or t material. If a nonequilibrium mechanism is applied the majority concentration in this region decreases near to extrinsic concentration. To maintain electroneutrality, the minority carrier concentration drops several orders of magnitude more. Thus, a nonequilibrium and stationary carrier distribution is reached and dynamically maintained by means of external fields. In such a mode semiconductor behaves again as an extrinsic one. This means that Auger-suppressed devices operate in nonequilibrium mode. 

The next phenomenon connected with nonequilibrium methods is negative luminescence [337], also called induced absorption [338]. Actually, a semiconductor with nonequilibrium decrease of carrier concentration emits less electromagnetic radiation than it should according to the Planck law. 

Finally, there is a phenomenon observed in practically all cases of nonequilibrium devices reported in the literature and limiting their application. It is the appearance of excessive 1/f (flicker) noise. This noise is proportional to the current through detector, and all the nonequilibrium methods utilized until now, independently on a particular mechanism, required increased current densities to reach nonequilibrium mode of operation. It is maintained that 1/f noise is a consequence of current flow through barriers at the surface or in the bulk of semiconductor and at Ohmic contacts and that convenient technological procedures could reduce it. However, in this moment it still appears to be a serious problem of the operation of nonequilibrium devices. 

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