New method to enhance the low-level detection limit of radiometric-quality photovoltaic and photoconductive IR detectors

摘要

Abstract: The low level detection of photoconductive and photovoltaic detectors is influenced by their dark current and their impedance, by the change of these parameters during a measurement in connection with the attached electronic circuit. Photoconductive detectors are generally biased by a constant voltage through a resistance. Without radiation the voltage on the detector produces dark current by the help of thermally generated and injected carriers. When incident light falls on the surface of the detector carriers are generated either by band-to-band transitions or transitions involving forbidden gap energy levels. This changes the conductivity and by it the voltage drop on the detector which results in a change of the thermal load of the detector and that of the original dark current. This way a distinction between the change of the dark current the effect of impedance on the measuring circuit and the radiation-generated current can not be made. Photovoltaic detectors for radiometric quality measurements are used in the short circuit mode. Light absorption in a photodiode produces electron-hole pairs. Pairs generated either in the depletion region or in a region from where a carrier can eventually diffuse to the depletion region will produce a current flow in the external circuit. The same is produced by thermally generated carriers, too. Generally a non-ideal short circuit is used, a change in the measured current slightly changes the voltage on the photodiode resulting in a small deterioration of the field structure within the diode. This changes both the recombination rate at the different layers of the diode and the collection factor causing a change in the dark current. The change of the dark current means the change of the impedance and this deviation can strongly influence the measurement results. At low level measurements this change of the dark current and that of the impedance can at either type of detector be critical. The bootstrapping of the detectors practically eliminates this problem, this solution is analyzed. !7