Intrinsic Photoconductors

Intrinsically conducting polymers, 13 540 Intrinsic bioremediation, 3 767 defined, 3 759t Intrinsic detectors, 22 180 Intrinsic fiber-optic sensors, 11 148 Intrinsic magnetic properties, of M-type ferrites, 11 67-68 Intrinsic photoconductors, 19 138 Intrinsic rate expressions, 21 341 Intrinsic semiconductors, 22 235-236 energy gap at room temperature, 5 596t Intrinsic strength, of vitreous silica, 22 428 Intrinsic-type detectors, cooling, 19 136 Intrinsic viscosity (TV), of thermoplastics, 10 178... 

In some intrinsic photoconductors one type of thermally excited carrier is present in far greater numbers than the other. If electrons dominate, and if their mobility is greater than that of the holes (the usual case), then (2.44) and (2.45) reduce to... 

In the analysis below we shall consider first the theory underlying (4.43) for an intrinsic photoconductor and then the theory of G RA. We shall analyze the simple geometrical model of a photoconductor shown in Fig. 4.8 there is no loss of generality in the results from assuming this regular geometry, and photoconductive detectors nearly always have this configuration anyway. 

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]. 

Sweepout effects can occur in extrinsic photoconductors also [4.26,27], but we shall not explicitly include them in this model they are more difficult to understand, but not generally as important in practice as in intrinsic photoconductors. The essential result is that whereas sweepout of carriers can limit the... 

The left side of (4.88) is plotted vs N — N in Fig. 4.10 for several possible operating temperatures we have used the values of B and NJg given in Appendix F, as well as t = 5 x 10 cm to be consistent with condition 4 below. By comparing these curves with that for Hg gCd jTe in Fig. 4.10 and with the abscissa of Fig. 4.1, we see that this extrinsic Si photoconductive detector requires considerably lower operating temperatures than the intrinsic photoconductor for comparable performance. This result is a well-known disadvantage of an extrinsic photoconductor [4.31]. The curves for a p-type example would lie... 

As indicated earlier, condition 4 is not easily satisfied by these detectors. Let us require that f=2.5a so that if >0.9, use maximum value for the shallow dopant of this example at which the ionization energy nearly equals the low doping level value then f 2.5 x 10 cm is the required detector thickness. We had used this thickness in the above evaluation of condition 2. Thus even in this "best case" estimate, the Si extrinsic photoconductor must be about 50 times thicker than the comparable Hg<, 8Cdo.2Te intrinsic photoconductor. 

Nearly all the development of Hg, Cd Te alloys as intrinsic photoconductor materials to date has involved the composition range 0.18 5 x 0.4, corresponding to cutoff wavelengths of 3 30 pm. Alloys of the xci 0.2 composition... 

The photoconductor, as shown in Fig. 7, depends upon the creation of holes or electrons in a uniform bulk semiconductor material, and the responsivity, temporal response, and wavelength cutoff are unique to the individual semiconductor. An intrinsic photoconductor utilizes across-the-gap photoionization or hole-electron pair creation. An extrinsic photoconductor depends upon the ionization of impurities in the material and in this case only one carrier, either hole or electron, is active. The same is true for a quantum-well photoconductor, in which electrons or holes can be photoexcited from a small potential well in the narrower band-gap regions of the semiconductor. The quantum efficiency for the structure in the figure is determined by the absorption coefficient, o, and may be written 2isrj = (l — / )[ — where R is the reflection coefficient at the top surface. Carriers produced by the radiation, P, flow in the electric field and contribute to this current flow for a time, r, the recombination time. The value of the current is... 

The simplest model of the material which can account for the major performance features of these photoconductors is the following. The semiconductor is assumed -type with riQ = N — and P p, although one could use the same model for p-type material by exchanging ns and p s. This is an extrinsic semiconductor while also an intrinsic photoconductor. The material contains centers which can trap holes (minority carriers) from the valence band generally The traps may be defects in the bulk crystal, surface... 

Most intrinsic photoconductors are made of indium antimonide (InSb), cadmium sulfide (CdS), or lead sulfide (PbS). Figure 4.79 shows the spectral sensitivity of these materials. While PbS detectors can be used also at room temperature with detectivities of 5x 10 cm Hz W InSb detectors... 

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