Exclusion Photoconductors

Exclusion of minority carriers is a nonequilibrium transport effect occurring as a consequence of the application of electric field to an isotype contact, i.e., to a junction between semiconductors of the same type, but with different dopant concentrations and/or bandgaps. In n-type semiconductors exclusion is provided by 

An isotype junction does not represent a barrier for the flow of majority carrier. However, in order to minimize the space charge the electron concentration will also drop, and its level will be defined by the dopant concentration. Since according to (1.38) the level of the Auger processes is proportional on carrier concentration, it will also drop. 

Of course, an equivalent consideration is also valid for isotype junctions in p-type material. 

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Minority carrier exclusion was chronologically the first nonequilibrium effect to be described within the context of its use in infrared detectors for Auger processes suppression. In 1985 British researchers Ashley and Elliott published a paper [356] that introduced the concept of nonequiUbrium suppression of Auger processes (including generation-recombination noise) at near-room temperatures. In that paper they proposed the use of the exclusion effect to that pmpose. In the same year White published a detailed numerical and experimental analysis of exclusion devices [50]. In that paper he presented an approximate analytical model for determination of most important parameters of exclusion photoconductors. 

Figure 3.6b shows a modification of this structure, a vertical exclusion photoconductor. In this case infrared radiation passes through an n" layer that at the same time serves as a passive Burstein-Moss filter in a manner also proposed for conventional cooled InSb photodiodes [359]. The main advantage of this stmcture is a higher ratio of the active area to the area where surface recombination appears, so that the relative portion of noise due to surface states to the total noise of the device is significantly reduced. 

Fig. 3.8 Dependence of the exclusion length on injected current density in an n v exclusion photoconductor for various concentrations of donors in the excluded region. T = 300 K, material is HgCdTe with a composition of 0.165...

Fig. 3.9 Electron concentration profile along an n v mercury cadmium telluride exclusion photoconductor with gradient isojunction for various values of reverse bias voltage (shown in figure) at 295 K. Dashed line shows donor concentration...

Figure 3.9 shows electron concentration profiles, and Fig. 3.10 hole distribution for various reverse bias voltages for the case of an n exclusion photoconductor with gradient junction. The length of the whole structure was 5 pm, material HgCdTe with composition x = 0.205 at a temperature of 295 K. This composition is optimal for an IR wavelength of 7.5 pm. 

Fig. 3.11 Concentration profile for electrons (solid) and holes (dashed) along a mercury cadmium telluride exclusion photoconductor of n v type with an abrupt junction at 185 K for various reverse bias voltages. Dotted line presents electron concentration at 0.4 Vm but with a donor gradient of 3] = 10 3[i tanh(6 X 10 y] + 5 x 10 ...

For a comparison. Fig. 3.18 shows the current-voltage characteristics of an n" v exclusion photoconductor at a temperature of 190 K. Several differences are readily noticeable the exclusion threshold Tcnee is much more clearly defined than in the previous case. The dynamic resistance is larger. Both of these results are a consequence of lower operating temperature. A negative properly of the presented device is that the threshold current density dramatically increases with cutoff wavelength, i.e., the consequence is similar to the case of the dopant concentration increase. 

Fig. 3.19 Theoretical noise current of an exclusion photoconductor versus total bias current, r = 180 K, material HgCdTe, x = 0.185...

Fig. 3.21 Specific detectivity versus bias current for exclusion photoconductor,...

Fig. 3.22 Transit time in an exclusion photoconductor versus operating temperature for different dopant concentration. Material Hgi xCdxTe, X = 0.186...

Figure 3.22 shows a dependence of the transit time of a mercury cadmium telluride exclusion photoconductor on bias current density for different temperatures. The cadmium molar fraction was kept constant in all cases, i.e., the compositions are not constant for the given temperatures. 

The performance of the diode structures utilizing the effects of minority carrier extraction is much better than that of the exclusion photoconductors. Their leakage currents are much lower, and Auger suppression much larger at significantly lower electric fields. 

As an illustration of this approach we consider the case when only electric and magnetic fields are used to enhance the simplest photoconductive and photovoltaic structures. Even in this case two generic combinations appear obvious, together with their subversions an exclusion photoconductor and extraction-exclusion photodiode, placed into magnetic field so that the Lorentz force promotes the depletion of the low-doped active area. 

Fig. 3.74 Schematic presentation of a hybrid magnetoconcentration-exclusion photoconductor...

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