Doping concentrations, semiconductor

A semiconductor laser takes advantage of the properties of a junction between a p-type and an n-type semiconductor made from the same host material. Such an n-p combination is called a semiconductor diode. Doping concentrations are quite high and, as a result, the conduction and valence band energies of the host are shifted in the two semiconductors, as shown in Figure 9.10(a). Bands are filled up to the Fermi level with energy E. ... 

The study of metals and metal surfaces is rapidly gaining general interest, mainly because of its importance in industrial research. Semiconductors 136> and especially the doping of semiconductors, S8> are of prime interest. Studies of oxydation states were for instance carried out for W—V—O-phases 122) or for a series of carbides 123>. An example is shown in Fig. 10, where the state of the niobium and nitrogen present in very low concentrations in steel, had to be detected. 

The spectacular success of the semiconductor industry is based on the production of materials selectively designed for specialized applications in electronic and optical devices. By carefully controlled doping of semiconductors with selected impurities—electron donors or electron acceptors—the conductivity and other properties can be modulated with great precision. Fig. 12.8 shows schematically how doped semiconductors work. In an intrinsic semiconductor (a), conducting electron-hole pairs can only by produced by thermal or photoexcitation across the band gap. In (b), addition of a small concentration of an electron donor creates an impurity band just below the conduction band. Electrons can then Jump across a much-reduced gap to the conduction band and act as negatively-charged current carriers. This produces a n-type semiconductor. In (c), an electron acceptor creates an empty impurity band just above the valence band. In this case electrons can jump from the valence band to leave positive holes. These can also conduct electricity, since electrons falling into positive holes create new holes, a sequence... 

In general, in the absence of an oxide the partition of the applied potential across the space charge layer and the Helmholtz layer depends on doping concentration and current range. There are also two different potential distributions depending on whether it is under a forward bias or a reverse bias. Under a forward bias for an anodic process on ap-type semiconductor electrode the current density can be described as follows ... 

Activation and conductivity at room temperature are problems that can be addressed by the incorporation of other electronic structures that increase carrier transport. Crystal morphology is an important parameter in the boron doping process to determine uncompensated acceptors (Aa-Ad) for different crystal facets as a function of doping concentration. The temperature coefficient of resistance for a CVD diamond film can be changed by boron doping. As conductivity depends on the crystal phase, the combined electromechanical properties can be exploited in sensor applications both for resistive temperature detectors and for pressure transdu-cers. " As electrical conductivity is related linearly with boron concentration, a better-controlled process may allow for the development of better semiconductor devices improving crystal quality and operating limits. ... 

As was remarked as early as chap. 1, many of the properties of materials are not intrinsic in the sense that they are highly dependent upon the structure, purity and history of the material in question. The electrical conductivity is one of the most striking examples of this truth. Nowhere is this more evident than in considering the role of doping in semiconductors. This is hinted at in fig. 7.2 where it is seen that the conductivity of Si ranges over more than 6 orders of magnitude as the concentration of impurities is varied. In fig. 7.3 this effect is illustrated concretely with the variation in resistivity of Si as a function of the concentration of impurities. 

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