With 15 FiguresPhotovoltaic and photoconductive effects in solids are widely used for detecting infrared radiation. These detectors offer very high detectivities, although they must often be cooled to achieve such performance. Their performance is high and continues to improve because of the development of highly purified, singlecrystal semiconductors as their active materials. However, several different materials appear to be competing for dominance, and it is not clear whether photovoltaic or photoconductive effects should be emphasized. We will attempt to put the situation into better perspective.Thus we will review recent progress in these detectors, assess their present status, and analyze prospects for their future improvement, emphasizing the relationships of detector performance parameters to semiconductor material parameters and to fundamental limits. There shall be little discussion of detector fabrication technology. Of particular interest shall be the potential performance of the various detector materials if their properties can be optimized, and the comparison of photovoltaic and photoconductive effects in these materials. We will treat only infrared detectors; detectors of optical radiation in the visible region of the electromagnetic spectrum were reviewed recently by Seib and Aukerman I-4.1]. Useful recent general references for this chapter are the reviews of in frared detectors and their applications by Dimmock [4.2] and of narrow-gap semiconductors by Harman and Melngailis [4.3].The basic theory of photovoltaic and photoconductive detectors shall be presented in Section 4.1 in a unified form convenient for intercomparison of the two effects and of the various detector materials. Then Sections 4.2, 4.3, and 4.4 shall cover photovoltaic, intrinsic photoconductive, and extrinsic photoconductive detectors, respectively, each of these sections including first a subsection in which the general theory of Section 4.1 is specialized to that class of detector, and then a subsection in which specific materials suitable for that class of detector are evaluated in terms of the theory. Finally in Section 4.5 we will draw some conclusions about the status and prospects ofphotovoltaic and photoconductive infrared detectors. Symbols used in this chapter which are not defined in the text are defined in where 2 is the photon wavelength, a photon detector is sensitive only to photons with 2~2co , where 2~o is a "cutoff" wavelength given from (4.1) byThe excitation can occur either from the valence band to the conduction band to create an electron-hole pair (intrinsic excitation) or from a discrete crystal-defect (dopant) energy level to either band to create a conduction electron or hole (extrinsic excitation).
Photocurrent, Gain, and ResponsivityI f • photons/cm2-s o f 2 ~ 2co are incident on a photon detector, r/q~ are absorbed and converted into photoexcited conducting electrons, where t/is the quantum efficiency. Either a photovoltaic or a photoconductive effect is used to convert these photoexcited electron...
