Carrier sweepout

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]

Equation (4.58) represents the minority carrier sweepout effect. The total concentration dp of photoexcited holes maintained in the photoconductor by steady-state irradiation is lower the stronger the electric field, because the field sweeps out some of the holes. The quantity in (4.58) can be thought of as an... 

Theoretically, the minority carrier sweepout effect could be eliminated by having a boundary condition different at x=0 from that of (4.61). If the boundary condition were the same as that for the p — p contact proposed in Subsection 4.2.1 i.e., d(Ap)/dx=0 for the minority carriers in this n-type... 

The minority carrier sweepout effects have been observed in n-type Hg, jjCd,(Te by several investigators [4.22, 23]. The speed of response of the photoconductor is improved by biasing into the sweepout mode, as expected, and sweepout is thus a useful effect for controlling detector response time. An... 

In the above derivation we have implicitly assumed that x represents the effective carrier lifetime including the possiblity of lifetime reduction by sweepout. The theory of minority carrier sweepout is not yet rigorous, so that this assumption may be questionable. 

Recent data of Scott and co-workers [4.52] on electrical properties of p-type Hgo gCdo Te show that in rather lightly doped material (p 10 -10 cm ), Ph (170 K) 2 X 10 cm /V-s since the shape of the valence band should not change much with x, we can use these values in the lower-x example to be considered. Hole mobility data for n-type material are scarce, but one can deduce from the minority carrier sweepout data of Emmons and Ashley [4.53] for Hgo 8Cdo.2Te at 77 K that —700cmVvolt-s since p = 2x 10 cmVvolt-s in such material, the mobility ratio ft 2 300, verifying our earlier statement that bp 1. 

The second term to optimize within the D f product is the response time. This factor is very complex and in a photonic detector it is determined, among others, by the time of carrier diffusion to the depletion region, the time of transit across the active region, the carrier sweepout time (in photoconductors), the detector RC constant, and by the RC constant of external circuitry [7]. The response time is fundamentally limited by the carrier transit time [8], and this time is inversely proportional to the thickness of the active region. 

The second most important parameter in many applications is response time. It can be limited by a variety of factors, such as carrier lifetimes within the detector material, the sweepout time of photoexcited carriers, and the RC time constant of the detector and/or its associated circuitry. Detailed analysis of response times goes beyond the scope of this chapter. 

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. 

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... 

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