Photoconductive gain

A turning point in the study of amorphous semiconductors was reached with the discovery that the addition of hydrogen to amorphous silicon could dramatically improve the material s optical and electrical properties. Unlike pure amorphous silicon, which is not photoconductive and cannot be readily doped, hydrogenated amorphous silicon (a-Si H) displays a photoconductive gain of over six orders of magnitude and its dark conductivity can be changed by over ten orders of magnitude by n-type or p-type... 

The photoconductive gain G, defined as the number of charge carriers passing through the sample per absorbed photon, can be derived from Eq. (13) ... 

The photoconductive gain G as measured from saturated current-voltage curves is of the order of unity in several dyes 3,50,51) .g. in malachite green G =0.2, pinacyanol G=0.37 and merocyanine A 10 7 G=0.6. 

Moreover, in agreement with the given relationships, the photoconductive gain G increases considerably with decreasing electrode spacing at constant field strength and layer thickness (cf. Table 1 50>). 

From these relationships it can be seen that, e.g. in low-resistivity photoconductors with equal response times, or by introduction of deep traps that increase the transition voltage Gscl, photoconductive gains greater than unity may be obtained. For example, in merocyanine dyes doped with electron-acceptor compounds a quantum yield G=2.3 has been measured 14>53>. 

Traps and recombination centers which depend on purity, crystal defects and preparation, can exert an influence, and electrode contacts, carrier injections, and other factors can interfere with measurements. Yet there is no doubt that the photoconductive gain (quantum yield) G can be reproduced by different methods. As in the case of dark conductivity, the photoconductivity properties are related to the electronic and structural behavior of pure and doped organic compounds, also those in the polycrystalline state. 

Like dark conductivity, photoconductivity increases rapidly with the concentration of the doping compound [see Eq. (51)]. Suitable doping agents can give photoconductive gains G > 1 52>. 

The enhancement of the photoconductive gain G (Eq. [(22)] with decreasing layer thickness, as shown in Table 14 50 90>, should also be mentioned here. 

Photoconductivity gain, G, may be defined as the number of interelectrode transits that can be made by an electron until the photogeiierated hole is eliminated by recombination. For the case treated here, namely where one type of carrier predominates, the gain, G, can be stated as ... 

In the photoconductive sensor, the electrode is ohmic where carrier replenishment occurs. Electron photoconductivity is the dominant mode. In this case, the photocurrent is a secondary current. Photocurrent and photoconductive gain are given by (Bube, 1961)... 

The results of photoconductive gain and excitation profile are remarkable. They show complete agreement of the spectral dependencies of photoproduction of solitons and photoconductive gain. They provide further support that the photoproduction of solitons occurs at energy lower than the threshold for the band-to-band absorption. They also show that the charge carriers are the soliton in the /-PA. 

The reverse bias p-i-n sensor is a primary photoconductor in which injection from the contacts is prevented by the junction. A secondary photoconductor allows charge to flow in from the contacts and offers the possibility of photoconductive gain. Gain occurs when the... 

The photoconductive gain is given by the ratio of free carrier lifetime to transit time. 

The first two terms within the parentheses in (4.12) represent gr noise due to the background photon flux and to thermal equilibrium carriers in the semiconductor, respectively these terms are derived in Appendix A from the more familiar expression for gr noise. The third term within the parentheses in (4.12) represents the Johnson noise, where R is the photoconductor resistance, which in this case can depend upon the background photon flux density. The photoconductive gain G in (4.12) is generally a function of the bias voltage applied to the photoconductor. At high bias voltages carrier sweepout effects occur which will be discussed later, and the gr noise terms must be multiplied by a numerical factor a, where l/2[Pg.105]

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]. 

The relationship between structure and photoelectrical behavior can further be described by the parameters G, quantum yield (or photoconductive gain) and the mean distance of the carrier drift (or Schubweg ). The quantum yield G is defined as the number of carriers passing through the outer circuit (Iph/e) per number of light quanta absorbed by the photoconductor during the same period of time (gV) ... 

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