Bulk semiconductors

The apphcations of high purity gases are primary in the semiconductor industries. From 1991 to 1995, the North American semiconductor bulk gas sales increased from U.S. 214 to 252 million (aimual growth rate of 4.2%) and specialty gas sales increased from U.S. 78 million to 169 million (aimual growth rate of 21.4%). 

Direct splitting of water can be accomplished by illuminating two interconnected photoelectrodes, a photoanode, and a photocathode as shown in Figure 7.6. Here, Eg(n) and Eg(p) are, respectively, the bandgaps of the n- and p-type semiconductors and AEp(n) and AEF(p) are, respectively, the differences between the Fermi energies and the conduction band-minimum of the n-type semiconductor bulk and valence band-maximum of the p-type semiconductor bulk. lifb(p) and Utb(n) are, respectively, the flat-band potentials of the p- and n-type semiconductors with the electrolyte. In this case, the sum of the potentials of the electron-hole pairs generated in the two photoelectrodes can be approximated by the following expression ... 

The conclusions from these considerations are that semiconductor photoelectrodes can be used to effect either reductions (p-type semiconductors) or oxidations (n-type semiconductors) in an uphill fashion. The extent to which reaction can be driven uphill, Ey, is no greater than Eg, but may be lower than Eg owing to surface states between Eqb and Eye or to an Inappropriate value of Ere(jox. Both Eg and Epg are properties that depend on the semiconductor bulk and surface properties. Interestingly, Ey can be independent of Ere(jox meaning that the choice of Ere(jox and the associated redox reagents can be made on the basis of factors other than theoretical efficiency, for a given semiconductor. Thus, the important reduction processes represented by the half-reactions (3)-(5) could, in principle, be effected with the same efficiency at a Fermi level pinned (or... 

Fig. 16.2 Simplified kinetic model of the photocatalytic process. ps represents the light absorbed per unit surface area of the photocatalyst, e b and h+b are the photogenerated electrons and holes, respectively, in the semiconductor bulk, kR is the bulk recombination rate constant and /R the related flux, whatever recombination mechanism is operating A is the heat resulting from the recombination kDe and kDh are the net first-order diffusion constants for fluxes Je and Jh to the surface of e b and h+b in the semiconductor lattice, respectively e s and h+s are the species resulting from...

These cells operate under illumination in combination with a bias, which serves to either drive electrolytic reactions for which the photon energy is insufficient or to increase the rate of chemical energy conversion by reducing electron-hole recombination in the semiconductor bulk. Most commonly an electrical bias is provided to drive the reactions [36-41]. 

In conclusion let us recall that the position of the electrochemical potential of electrons in a semiconductor, i.e., F relative to that of vacuum, is determined by the thermodynamic work function for the semiconductor wT, and that the position of F relative to the edges of the bands Ec and v in the semiconductor bulk is given by... 

Let us note one vital point, which is of methodological importance. It has been traditionally accepted in electrochemistry to choose the positive direction of the electrode potential

positive electrode charge. Here the zero potential is assumed to be that of the reference electrode, which coincides, within a constant, with the potential in the solution bulk (— oo). On the other hand, in physics of semiconductor surface the potential is usually reckoned from the value in the semiconductor bulk ( ) the enrichment of the surface with electrons, i.e., the formation of a negative space charge, corresponding to the positive potential of the surface. In particular, this statement directly follows from the Boltzmann distribution for electrons and holes in the space-charge region in a semiconductor ... 

The maximum value of the minority-carrier current, which may flow under steady state conditions from the semiconductor bulk to the surface, is called the limiting current. It is determined by bulk generation of electron-hole pairs. As simple calculation shows (see, for example, Myamlin and Pleskov, 1967), the absolute value of the limiting current of minority carriers, holes for illustration, is... 

Assume for simplicity that the potential drop in the Helmholtz layer does not change under illumination, so that the position of the band edges at the surface is fixed with respect to the system of energy levels in the solution ( band pinning at the surface see Section 3). At the same time, the position of the Fermi level F relative to the band edges Ec and Ey is strictly determined in the semiconductor bulk. Therefore, the bands unbend under illumination and pull the Fermi level, so the latter shifts with respect to its position in the nonilluminated semiconductor (Fig. 16b). This shift can be measured as the photopotential (ppb = —AF/e [cf. Eq. (8)]. 

Here c is the velocity of light, n is the complex refractive index in the semiconductor bulk... 

Argon [7440-37-1] -in magnesium manufacture [MAGNESIUM AND MAGNESIUM ALLOYS] (Vol 15) -as semiconductor bulk gases [HIGH PURITY GASES] (Supplement)... 

Let us now turn to the thickness of the conducting channel. The concept of thickness is not that obvious, because the actual distribution of charge-carriers decreases continuously from the insulator-semiconductor interface to the semiconductor bulk, so one can more sensibly speak of an effective thickness. The distribution can be estimated by resolving Poisson s equation (Eq. 3) ... 

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