Intrinsic Photoconductive Detectors

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. 

We saw in Subsection 4.2.2 that satisfaction of condition 1 was almost sufficient for a semiconductor to be a satisfactory photovoltaic detector material conditions 2 through 4 placed demands mainly on device design and technology rather than on fundamental properties of the semiconductor material. The situation is more restrictive for intrinsic photoconductive detectors, because condition 3 places specific demands on fundamental material properties which eliminate some classes of semiconductors as satisfactory high-performance detector materials. Let us consider condition 3 in a qualitative way next to determine which materials may be satisfactory. We shall treat conditions 2 and 3 in terms of a quantitative example later. 

Thus the zinc blende structure semiconductors can be useful for intrinsic photoconductive detectors. Compounds such as InSb have been used as intrinsic photoconductors [4.20], as well as for photovoltaic detectors, but greater versatility of wavelength response is possible with the Hg j tCd Te alloy system. The Hgi j.Cd,Te alloys have received considerable development effort in recent years and are the most prominent intrinsic photoconductor materials they will be analyzed in this subsection. The development of Hg, Cd Te has concentrated almost entirely on n-type material since it provides high photoconductive gain however, p-type Hg, Cd,(Te crystals may be useful for intrinsic photoconductive detectors also [4.21]. 

Fig. 4,11. G RA product vs carrier concentration for n-type Hgo gCdo 2Te intrinsic photoconductive detector at 77 K...

Condition 4 provides the highest possible BLIP detectivity by requiring that the quantum efficiency approach its maximum value of unity. This condition is easily met by relatively thin photovoltaic and intrinsic photoconductive detectors. However, it is a major problem for extrinsic Si photoconductors, because limited maximum values of dopant concentrations and absorption cross sections give rather low absorption coefficients, requiring undesirably thick detectors for high quantum efficiencies. 

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