The Formation of a p-n Junction

As stated in the introduction to this chapter, semiconductor detectors operate like ionization counters. In ionization counters (see Chap. 5), the charges 

An n-type semiconductor has an excess of electron carriers. A p-type has excess holes. If a p-type and an n-type semiconductor join together, electrons and holes move for two reasons  

Both electrons and holes will move from areas of high concentration to areas of low concentration. This is simply diffusion, the same as neutron diffusion or diffusion of gas molecules. 

Under the influence of an electric field, both electrons and holes will move, but in opposite directions because their charge is negative and positive, respectively. 

The potential Vq (Fig. 7.17a) depends on electron-hole concentrations and is of the order of 0.5 V. If an external voltage V/, is applied with the positive pole 

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A lilhium-drifted detector is formed by vapor-depositing lithium on the surface of a p-doped silicon crystal. When the crystal is heated to 4(X) C to 5(X) C the lithium diffuses into the crystal. Because lithium easily loses electrons, its presence converts the p-lype region to an n-type region. While still at an elevated temperature, a dc voltage applied across the crystal causes withdrawal of Ihe electrons from the lithium layer and holes from Ihe p-type layer. Current across thepn junction causes migration, or drifting, of lithium ions into the p layer and formation of the intrinsic layer, where the lithium ions replace the holes lost by conduction. When the crystal cools, this central layer has a high resistance relative to the other layers because the lithium ions in this medium are less mobile than the holes they displaced. 

Figure 4.21. Schematic of carrier flow within a p-n junction. Shown are (a) the diffusion of free electrons from n- to p-type Si resulting in recombination with holes, (b) formation of a depletion zone that repels the flow of electrons and holes from n- and p-type regions, respectively, (c) application of a reverse bias, resulting in widening the junction potential, eVo, (d) application of a forward bias, which lowers eVo and results in an exponential voltage spike once a threshold bias of Vd is applied, (e).

Figure 10. Effect of fixed oxide/insulator charge on the effective junction space-charge region (a) flat-band condition (b) positive fixed charge causes inversion of the p side and formation of a n-type surface channel (c) negative fixed charge causes accumulation of the p side and a narrow field induced junction at the surface (d) larger amount of negative fixed charge causes inversion of the n side and formation of a p-type surface channel. (With permission of Academic Press.)...

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