Carriers, electron and hole

As discussed previously, inherent disorder possessed by i -SiH alloy limits the mobiUty of the free carriers (electrons and holes) to about 10 cm /( V) this is compared with crystalline Si, in which the electron mobiUty is 1500 cm /( V). However, crystalline Si is expensive to manufacture and its size is limited to about 20 cm in diameter. Many appHcations discussed have either emerged or been identified which preclude the use of crystalline Si because of cost, size, or both. The basic commonality in these appHcations is the abiHty to fabricate devices on areas much larger than can be addressed by crystalline Si Furthermore, these appHcations are not demanding in terms of speed, which then provides i -SiH alloy with a distinct competitive advantage. 

The simplest polymer-based EL device consists of a single layer of semiconducting fluorescent polymer, c.g., PPV, sandwiched between two electrodes, one of which has to be transparent (Fig. 1-1). When a voltage or bias is applied to the material, charged carriers (electrons and holes) are injected into the emissive layer and these earners arc mobile under the influence of the high (> 105 V enr1) elec-... 

The electronic properties of silicon are essential in the understanding of silicon as an electrode material in an electrochemical cell. As in the case of electrolytes, where we have to consider different charged particles with different mobilities, two kinds of charge carriers - electrons and holes - are present in a semiconductor. The energy gap between the conduction band (CB) and the valence band (VB) in silicon is 1.11 eV at RT, which limits the upper operation temperature for silicon devices to about 200 °C. The band gap is indirect this means the transfer of an electron from the top of the VB to the bottom of the CB changes its energy and its momentum. 

Both electrons and holes are mobile charge carriers in semiconductors. The mobile charge carrier whose concentration is much greater than the other is called the majority carrier, and the minority carrier is in much smaller concentrations. In n-type semiconductors, the mcgority carriers are electrons in the conduction band and the minority carriers are holes in the valence band. The product of the concentrations of majority and minority carriers (electrons and holes) in a semiconductor of extrinsic type (containing impurities) equals the square of the concentration of electron-hole pairs, ni, in the same semiconductor of intrinsic type (containing no impurities) ... 

Thus, lattice defects such as point defects and carriers (electrons and holes) in semiconductors and insulators can be treated as chemical species, and the mass action law can be applied to the concentration equilibrium among these species. Without detailed calculations based on statistical thermodynamics, the mass action law gives us an important result about the equilibrium concentration of lattice defects, electrons, and holes (see Section 1.4.5). 

Because both carriers, electrons and holes, can be mobile in a-Si H, blocking contacts for both electrodes are needed to prevent carrier injection from the electrodes. In this case the photocurrent is a primary current that saturates with unity collection efficiency when ptxE > d, where n is the mobility of the photocarriers that drift in a-Si H, r the lifetime of photocarriers, E is the electric field in a-Si H, and d the thickness of a-Si H. In the saturation photocurrent region, the photocurrent is not very dependent on a-Si H film quality and increases linearly with increasing light intensity. 

This equation states that the electrical conductivity due to a free carrier is the product of the charge on the carrier, q, its concentration in the solid, and its mobility, fx. Since semiconductors have two different types of mobile charge carriers, electrons, and holes, the total sample conductivity, a, is simply the sum of the individual conductivities due to each carrier type. It should be noted that the conductivity depends only on the absolute number of carriers, and therefore is not affected by the signs of the carriers themselves. Carrier mobilities for electrons and holes in a variety of semiconductors can be measured experimentally. These values have been tabulated in various reference books and are available for many semiconductors of interest. Doping of a semiconductor therefore allows precise control over the conductivity of the semiconductor sample. 

Accordingly, many of these studies examined the primary photocatalytic events that involve (a) absorption of light, (b) formation of free carriers (electrons and holes) and/or trapped carriers (electrons as Ti in the case of Ti02, and holes as OH radicals), and (c) reaction of preadsorbed acceptor or donor molecules with the relevant trapped carriers. This approach was reasonable when the only purpose of photocatalysis was elimination of undesirable environmental pollutants. However, when we queried how to render a process more efficient, it became evident that we needed to address more fundamental questions, namely the nature of the primary events that follow photoexcitation of the photocatalyst. In most instances, our... 

Finally, a low work function metal is vacuum deposited as the cathode for electron injection. Upon application of a voltage, both types of charge carriers (electrons and holes) are injected and move towards each other by hopping [1], Once a hole and an electron meet on an active site they combine to form an exciton, which can relax to... 

Tributsch and co-workers [93, 94] have pioneered the application of modulated microwave reflectivity measurements to the study of the semiconductor electrolyte interface. The method is based on the fact that the microwave reflectivity of a material changes when its dielectric constant is perturbed. In the case of a semiconductor, perturbation of the density of mobile carriers (electrons and holes) by changes in potential in illumination intensity influence the conductivity and hence the imaginary component of the dielectric constant at microwave frequencies. For small perturbations, the change AR, in microwave reflectivity becomes a linear function of the change in conductivity. A full discussion is given in [176]. 

Electron-transfer processes at the semiconductor/electrolyte interface are strongly affected by the density of available carriers (electrons and holes) in the semiconductor at the interface. The observed i-E behavior differs from that at metals and carbon (Chapter 3), where there is always a large density of carriers in the conductor. In the dark, electron-transfer processes involving species in solution with energy levels in the band gap of the semiconductor (Figure 18.2.5Z ) are usually dominated by the majority carrier. Thus, moderately doped w-type materials can carry out reductions, but not oxidations. That is, there are electrons available in the conduction band to transfer to an oxidized solution species, but few holes to accept an electron from a reduced species. The current for a reduction of species O at an w-type semiconductor is given by... 

These potentials are a result of charge carrier (electron and hole) migration in the solid phase attributable to the electrical fields entering the solid surface from an adjacent charged body. In our case, the virus develops electrical charge resulting from the ionization of prototropic groups in aqueous solutions. 

In all the foregoing theories, the electrode has been assumed to be a metal in which there is no potential distribution within the metal. When the electrode is a semiconductor, the concentration of charge carriers (electrons and holes) in it is considerably lower than in the case of a metal. Thus, similar to the diffuse layer region in electrolytes in dilute solutions, there is a diffuse or space charge region within the semiconductor 12-14). 

As we have seen above, absorption of radiation does not necessarily take place in the luminescent center itself, but may also occur in the host lattice. It is obvious to make a simple subdivision into two classes of optical absorption transitions, viz. those which result in free charge carriers (electrons and holes), and those which do not. Photoconductivity measurements can distinguish between these two classes. 

Although this picture is simple, it reveals an important feature. It is found that the vacancies left in the valence band when electrons are promoted to the conduction band also contribute to the conduction process. To a good approximation, these vacancies can be equated to positive electrons, and move in the opposite direction to the electrons in an applied field. They are called positive holes, or just holes. Semiconductors are characterised by an increase in conductivity with temperature because the number of mobile charge carriers, electrons and holes, will increase as the temperature increases. 

The electronic properties of an extrinsic semiconductor are determined by the number of mobile charge carriers, electrons and holes, both intrinsic and extrinsic, and by the position of the Fermi energy (Figure 13.10). The total numbers of mobile electrons and holes is related to and pi, the numbers of intrinsic electrons, and holes by ... 

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