Intrinsic photoconductivity

Le. those which form a charge-transfer complex with the monomer units of the transporting polymer. The charge transfer complex then becomes the carrier generating species and can also contribute to the transport. The electron acceptor can also be chemically attached to the polymer. In such a case the polymer may become intrinsically photoconductive. 

The intrinsic photoconductivity of BaCP crystals sets in at a photon energy of about... 

Fig. 21. a. Action spectra for photopolymerization of neat (triangles) and partially polymerized (circles) multilayer systems. AOD is the change in optical density at a given irradiation dose b Action spectrum of intrinsic photoconductivity of a fully polymerized multilayer assembly from Ref. 

The interband absorption of semiconductors produces free electrons and holes in the conduction and valence bands. These free carriers produce intrinsic photoconductivity above the band gap in adequate structures, and several types of infrared photoconductors have been built on this principle [43]. 

The intrinsic photoconductive property was found for a cobaltacyclopentadine polymer, 32.68 Photoresponse of current-voltage (I-V) characteristics for ITO/32/ITO indicated that the polymer had a low conductivity in the dark and the photocurrent was four times larger than the dark current. It was proposed that the metal -character orbitals localized at cobalt sites, plus their energy level lying between valence and conduction bands acted as the trapping sites of holes generated by photoactivation of electrons from the valence band to the conduction band. 

Intrinsic and Extrinsic. Photoconductivity can be observed in virtually all semiconductors. Intrinsic photoconductivity requires the excitation of a free... 

No intrinsic photoconductivity occurs for radiation of wavelength greater than Aq. a convenient expression to determine the long wavelength limit in micrometers of an intrinsic photoeffect for a semiconductor whose energy gap is expressed in electron volts is... 

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 of photovoltaic and photoconductive infrared detectors. Symbols used in this chapter which are not defined in the text are defined in Table 4.1. 

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. 

The valence band in any of these semiconductors has a maximum at the center of the Brillouin zone with relatively large effective mass, whereas the conduction band minimum at the zone center has a much smaller effective mass [4.3,34,40]. Several consequences of this band structure are important in intrinsic photoconductivity. The electron mobility for virtually any scattering mechanism is considerably greater than the hole mobility because of the mass difference i.e., = 1. The conduction band has a low density of states... 

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