Response Time of an Exclusion Photoconductor

In this section, an analytical consideration of transitional response of an exclusion photodetector is given. A starting assumption is that the response time is mosdy defined by the drift transit time of photogenerated carriers across the active region. The influence of the detector circuitiy is not taken into account. 

The time of transit between the contact pads of a photoconductive device is determined as the ratio between the inter-electrode distance L and the product of electric field and the mobility of the considered carrier type, T = UE i. If we divide the detector length into infinitesimally small elements dy, the differential transit time across each of these elements will be At = AylE(y) t. Thus the total transit time across the detector will be 

The transit time between some other two points, determined by the position of the signal readout contacts that may in a general case differ from the bias contacts, is obtained in an almost identical manner, by integrating the expression on T within different limits. 

It can be seen that shorter transit times are obtained for higher temperatures, which is a desirable property, since the basic application of nonequilibrium devices is to elevate the operating temperature of photodetectors. On the other hand, it appears what may seem paradoxical that a bias increase influences detrimentally the transit time. Actually the electric field increases carrier velocity, but at the same time it increases the length of the exclusion zone, so that a carrier takes longer time to cross the whole zone. A transit across a part of the excluded zone with a constant length would, naturally, show an increasingly shorter transit time with a bias increase. 

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