Majority-carrier device

The forward current at a semiconductor-metal junction is mainly determined by a majority carrier transfer i.e. electrons for n-type, as illustrated in Fig. 1 d. In this majority carrier device the socalled thermionic emission model is applied according to which all electrons reaching the surface are transferred to the metal. In this case we have ... 

Since the reorientationnergy X varies in the range of 0.5-2 eV the half width can be in the order of the bandgap of the semiconductor. Assuming that in the dark the electron transfer occurs entirely via the conduction band (majority carrier device) the current-potential dependence can be derived as follows ... 

It is evident from Eq. (94) that the maximum photovoltage depends critically on the exchange current Jo- In the case of pn-junctions, jo is determined by the injection and recombination (minority carrier device). Whereas in Schottky-type of cells jo can be derived from the thermionic emission model (majority carrier device). The analysis of solid state systems has shown that jo is always smaller for minority carrier devices [20,21]. Using semiconductor-liquid junctions, both types of cells can be realized. If in both processes, oxidation and reduction, minority carrier devices are involved, then jo is given by Eq. (37a), similarly as... 

A system in which only majority carriers (electrons in n-type) carry the current, is frequently called a majority carrier device . On the other hand, if the barrier height at a semiconductor-metal junction reaches values close to the bandgap then, in principle, an electron transfer via the valence band is also possible, as illustrated in Fig. 2.8a. In this case holes are injected under forward bias which diffuse towards the bulk of the semiconductor where they recombine with electrons ( minority carrier device ). It is further assumed that the quasi-Fermi levels are constant across the space charge region i.e. the recombination within the space charge layer is negligible. In addition Boltzmann equilibrium exists so that we have according to Eqs. (1.57) and (1.58)... 

Eq. (2.31) is identical to Eq. (2.18) derived for a majority carrier device (thermionic emission model). Accordingly, the same type of current-voltage curve is expected as that given in Fig. 2.7. The characteristics of the models occur only in the preexponential factors, which indeed are different in both cases (compare Eqs. 2.17 and 2.30). As mentioned before the jo of the majority carrier device is only determined by the barrier height and some physical constants (Eq. 2.19), whereas the y o of the minority carriers depends on material-specific quantities such as carrier density, diffusion constant and diffusion length. 

Sometimes an ideality factor of greater than 1 is also reported for a majority carrier device. In this case, however, there is no physical basis for an ideality factor of n > 1 and any deviation from n = 1 must have technological reasons. 

Eq. (11.1) is also valid for pure solid state devices, such as semiconductor-metal contacts (Schottky junctions) and p-n junctions, as described in Chapter 2. The physics of the individual systems occurs only in y o- The main difference appears in the cathodic forward current which is essentially determined by /o. In this respect it must be asked whether the forward current is carried only by minority carriers (minority carrier device) or by majority carriers (majority carrier device). Using semiconductor-liquid junctions, both kinds of devices are possible. A minority carrier device is simply made by using a redox couple which has a standard potential close to the valence band of an n-type semiconductor so that holes can be transferred from the redox system into the valence band in the dark under cathodic polarization. In this case, the dark current is determined by hole injection and recombination (minority carrier device) and /o is given by Eq. (7.65), i.e. 

Weizmann Institute in Israel has studied these systems in detail (see ref. [34] and literature cited there). Since the standard potential occurs somewhere in the middle of the bandgap - ef7 redox 0.8 V, Table 11.1), the forward current is carried by electron transfer from the conduction band to the redox system. Accordingly, these systems must be majority carrier devices. It has been found furthermore that etching also plays an important role here [36]. In addition, it has been observed that various alkali cations influence the power plot of corresponding cells (Fig. 11.4a). This effect was related to ion pairing for strongly hydrogenated cations such as Li", which results into a decreased activity of the active (poly)sulfide at the electrode. When Cs was used instead of Li" there was considerable improvement, not only in the electrochemical kinetics, but also in the stability [21]. 

In the photovoltaic electrolysis cell, the n-type region of the device, covered with a metal layer, becomes a cathode while the p-type region covered with a metal layer becomes an anode (i.e. it behaves like a majority carrier device) in the photochemical diode, the opposite is true, i.e. it is a minority carrier device with the n-type region acting as anode and the p-type region acting as a cathode. 

Schottky barriers occur when majority carriers are depleted when a semiconductor contacts a metal and behave very much as do diodes. They are, however, majority carrier devices with a different form of the reverse current... 

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