Minority-carrier device

The photovoltage is esentially determined by the ratio of the photo- and saturation current. Since io oomrs as a pre-exponential factor in Eq. 1 it determines also the dark current. Actually this is the main reason that it limits the photovoltage via Eq. 2, The value of io depends on the mechanism of charge transfer at the interface under forward bias and is normally different for a pn-junction and a metal-semiconductor contact. In the first case electrons are injected into the p-region and holes into the n-region. These minority carriers recombine somewhere in the bulk as illustrated in Fig. 1 c. In such a minority carrier device the forward current is essentially determined... 

In the model presented here, only recombination in the bulk of the semiconductor has been considered. It is well known from solid state junction (minority carrier devices) that also recombination within the space charge layer can taken place. In this case, a quality factor n, ranging between 1 and 2, is introduced. Then Eq. (45) has to be replaced by... 

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)... 

Fig. 2.8 Energy diagram of a mctal-semiconductor junction for a minority carrier device, a) Hole injection into the valence band of an n-type semiconductor under forward bias b) hole extraction from the valence band under reverse bias...

In principle the same process occurs in minority carrier devices. In all cases the photocurrent is proportional to the light intensity and is independent of the applied potential. Accordingly, the photocurrent occurs in the diode equation (Eqs. (2.18) or (2.31) or (2.37)) as an additive term, so that we have... 

As mentioned above, the slope of the log y l jn-potential dependence given in Fig. 7.35, has a slope of 92 mV/decade. According to the Shockley-Read recombination model (see Section 1.6) which is valid for a recombination in the bulk of a semiconductor, a slope of 60 mV/decade would be expected. In addition, recombination can occur within the space charge layer of a semiconductor which leads to a slope of 120 mV/ decade [52] (see also Section 2.3). With most p-n junctions which are minority carrier devices, a slope between 60 and 120 mV has been found. Therefore, the slope of 92 mV... 

In solid state devices such as p-n junctions, luminescence is created by forward polarization in the dark. In such a minority carrier device, electrons move across the p-n interface into the p-type and holes into the n-type regions, where they recombine with the corresponding majority carriers. As already pointed out in Section 2.3, this kind of luminescence has not been found with semiconductor-metal junctions (Schottky junctions), as nobody has succeeded in producing a minority carrier device because of Fermi level pinning. Since the latter problem usually does not occur with semi-... 

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. 

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. 

Deep levels influence a variety of device parameters. For example, in minority carrier devices they influence the recombination and generation lifetimes. The lifetime in turn controls junction currents and refresh times in dynamic random access memories. For this reason we discuss lifetimes. Defects... 

Fig. 7a. Computed I-V characteristics of the MIS tunnel diode demonstrating "nonequilibrium" effects for minority-carrier devices. The substrate is 2-Q p-type <100> silicon, ( )ini is equal to 3.2 eV, and the variable parameter is d, the thickness of the oxide layer. (Ref 23)...

Equation (2.36) is the famous Shockley equation which is the ideal diode law [16]. According to Eq. (2.35), we have obtained again the same basic current-voltage dependence as already derived for majority and minority carrier devices with semiconductor-metal junctions (see Eqs. (2.18) and (2.31)). As already mentioned, the physical difference occurs only in the pre-exponential factor / g. The general shape of a complete j-U curve in a linear and semilog plot has already... 

Because the magnitude of both the forward and reverse currents in a homojunction diode depend upon the minority carrier properties, the devices are referred to as minority carrier devices. This is critical to the relationship between diodes and defects in the material as defects affect the minority carriers far more than the... 

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