Surface space charge barrier

Fig. 19. The space charge barrier in an n-type semiconductor in exhaustion where the major donor site energy lies well below the conduction-band edge. The system parameters are, with respect to the Fermi level, E] (energy of the CB in the bulk) = 0.2 eV, E (CB at surface) = 0.6 eV, Ld = 20.A. , (vs. EF) values are given against each curve.

Since the SPV method has been intensively used for diffusion length measurement in undoped a-Si H, we shall discuss the theory of the method in some detail. The approach is to contrast and compare the theory to that already given for conventional semiconductors. The differences arise from two basic facts (1) Undoped a-Si H is a photoconductive semi-insulator rather than an extrinsic semiconductor (2) The thickness of the surface space-charge region (the surface barrier) may be comparable to the diffusion length, whereas in the SPV theory for conventional semiconductors it is assumed that W c Lp. 

Space-Charge-Limited Diode. This method is equally applicable to electropositive and electronegative adsorbed layers. In a diode the maximum current which can be drawn from the cathode is given by the emission formula in Sec. V,A. Electrons emitted from the cathode at low current densities build up an electron atmosphere or space charge at the surface, and this space charge presents an electrostatic barrier to electrons flowing to... 

If the space charge in the semiconductor arises from the ionization of impurities only, as in the model we have used, the surface barrier is termed a Schottky barrier. The barrier region near the surface of the crystal is sometimes called the exhaustion region, as the mobile electrons have been removed from this region (16). 

There have been several proposed mechanisms for the operation of these sensors (Gopel, 1985 Franke et al., 2006). They all seem to converge on the existence and modulation of the Schottky barrier heterojunctions formed between the grains of the polycrystalline layer. They are equivalent to a chain of resistive elements connected in series. The density of surface states affects the depth of the Schottky barrier and depends on the interaction with the adsorbate (Fig. 8.8). The size of the grains apparently plays a major role. As the diameter of the grains decreases to below 5 nm, the space charge is smeared and the relative response of the sensor increases (Fig. 8.9). 

The VDR behaviour in ZnO varistors is governed by electron states that are formed on the surfaces of crystals as a consequence of the discontinuity. These surface states act as acceptors for electrons from the n-type semiconductor. Electrons will be withdrawn from the region near the surface and replaced by a positive space charge. In this way oppositely oriented Schottky barriers will be created at the surfaces of neighbouring crystals so that a high resistance will be offered to electron flow in either direction (Fig. 4.10(a)). The situation with an applied field is shown in Fig. 4.10(b). With low applied fields small thermally... 

One other problem which arises in dealing with deep space charge region is the part played by the barrier in limiting the recombination of minority carriers at the surface. In the papers of Bardeen and Brattain (6), Stevenson and Keyes (16),... 

Many (17), and Wang and Wallis (18), it is explicitly assumed that in adapting the Shockley-Read model (19) of recombination via traps to the surface the volume and surface portions of die conduction bands are in good thermodynamic equilibrium. Garrett (20) has shown, however, that in the case where the region of space charge is much wider than the mean free path of the carriers, the barrier may become the limiting factor. 

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