Isotype junction

Both anisotype and isotype junctions, based on n-GaAs, were fabricated with pentacene (Pn) and lead phthalocyanine (PbPc) as organic semiconductor of p-type, and methyl perylene pigment (MPP) as OS of n-type. 

An equivalent approach to the derivation of boundary conditions is also utilized in the case of isotype junctions, again using the corresponding minority carrier concentrations and expressions (3.71)-(3.72) or (3.73)-(3.74). 

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. 

White presented a generalization of the above expression [50] for the case of a gradient isojunction, where the concentration gradient is represented as n = n +exp (-x/d) (the parameter a describes the gradient nature of the concentration profile starting from the interface between the n" and the v region). In this case the term an +/2 ) g is added into the bracketed term of (3.94). This is an intuitively expected result, because it shows that the influence of the n" region is less felt in the detector performance if the isotype junction is more abmpt. 

Current-voltage characteristics of mercury cadmium telluride exclusion devices are shown in Figs. 3.16, 3.17, 3.18. All of the presented curves are obtained for reverse bias of the isotype junction, but only the absolute values of current and voltage are shown in the diagrams. This is a convention used throughout this work. 

With an increase of the width of the active area the effect of the negative resistance becomes smaller and finally completely disappears. According to Sect. 3.5 the thickness of the pseudoextrinsic layer must be comparable to the diffusion length in the given material (in the considered case about 2.7 pm). If the thickness is much larger, the useful effect disappears altogether and the device behaves as two separate junctions in a series, one of them a hi-lo junction and the other an isotype junction. 

Figure 7a-c shows the band profiles of an isotype Nn and anisotype Np and Pn heterojunctions, respectively. As we can see, all junctions have band discontinuities at the interface. The nature and effect of such discontinuities on the device properties depends on its application. Let us consider the case of Np heterojunction in detail. Example of this would be an n-type AlGaAs grown epitaxially on p-type GaAs. As shown in Fig. 7b, the band lineup at... 

Semiconductor Junctions, Soiid-Soiid Junctions, Fig. 7 Band diagrams of heterojunction formed between AlGaAs and GaAs for various doping types, (a) Nn isotype heterojunction (b) Np, and (c) Pn anisotype heterojunctions... 

The second type of structures are those with a gradient of chemical potential. These are various kinds of semiconductor junctions (isotype or p-n, homo- or heterojunclion, abmpt, or graded). 

The terms anti-barrier and non-directing junction are used in the further text both for isotype semiconductor homo- or heterojunctions and for the corresponding Schottky junctions, while the terms barrier and directing junction are used to denote p-n junctions (both homo- and heterojunctions) and Schottky diode junctions. 

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... 

Figures 3.30 and 3.31 show the concentration profiles in a three-layer HgCdTe extraction-exclusion p jin photodiode at a temperature of 250 K for various values of bias voltage. The active region width was 2.7 pm, the p" layer was 0.65 pm wide, the n" layer 0.5 pm. Material composition throughout the whole diode was Xcd = 0.187. Donor concentration in the n" region was lO cm, and the concentration of acceptors in the p layer had the same value. The p-dopant concentration in the active region was 10 " cm . Both the isotype and the p-n junction were assumed to be abrupt.

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