Photoconductive detectors time response

Thermal and photoconductive detectors are used to measure radiation intensities, but all have relatively slow responses and are subject to drift. The lead sulfide or telluride photoconductive cell has a response time of about 0.5 ms, but sensitivity decreases sharply above 2900 cm" for the sulfide and above 1700 cm- for the telluride. Thermal detectors are employed at longer wavelengths. The simplest of these is the thermocouple, which has a relatively slow response (about 60 ms), and several are usually linked to form a thermopile. Bolometers... 

The speed of response of an intrinsic photoconductive detector is essentially the same as the longest photoexcited carrier lifetime. One can shorten the response time of a detector of this kind by biasing it as far as possible into the sweepout mode, since the effective minority carrier lifetime r/(z) is reduced in proportion to the bias field. 

The time response of a photoconductive detector is limited by the carrier lifetimes. PbS detectors for instance have time constants between 0.1 - 1 ms while InSb detectors reach a few us. For measurements of fast transient signals photodiodes are more suitable (see Sect.4.5.8). 

Recently, Baumann et al.(43) have measured time-resolved photoconductivity in PDA-TS-6 crystals as well as polyacetylene excited by 25 ps pulses of a Nd YAG laser (ftU) = 2.3 eV). The response time of the detector was 200 ps. The transient signal shown in fig.5 reveals a fast initial peak with instrument-limited pulse-shape followed by a slower decaying tail. The field dependence of the peak height (fig.6) parallels that of the carrier generation process and is in accord with what Donovan and Wilson have found on a 20 ns time resolution. The quantum efficiency associated with the fast photocurrent peak is 1.5x10 times the dc-quantum efficiency measured at hu) = 2.7 eV. 

MCT detectors may be operated in the photoconductive (PC) or photovoltaic (PV) mode, with PC operation being the more common. PV-MCT detectors are used when the response time of the measurement must be very short. Because the D of most PC-MCT detectors is approximately constant between modulation frequencies from 1 kHz to 1 MHz, it is difficult to drive the interferometer mirror... 

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