Conductance changes, semiconductor

An interesting special application has been proposed by Schlichthorl and Peter.31,41 It aims at deconvolution of electrochemical impedance data to separate space charge and surface capacitance contributions. The method relies on detection of the conductivity change in the semiconductor associated with the depletion of majority carriers in the space charge region via potential-modulated microwave reflectivity measurements. The electrode samples were n-Si(lll) in contact with fluoride solution. 

Several demonstrations of this concept have recently been published The first one is based on the pH dependence of redox transitions in oxide semiconductors that are connected with conductivity changes. If the bridging polymer layer in Fig. 6 is WO3 sputtered onto the electrode array or electrochemically deposited Ni(OH)j the transistor amplification is a function of the pH of the... 

Expressions (1.45) and (1.46) which are valid in case of applicability of above assumptions indicate on availability of direct proportionality between the value of the change of the surface-adjacent conductivity of semiconductor adsorbent and concentration of chemisorbed particles on its surface, the latter being in charged form. This results in the fact that when the surface is covered by adsorbed particles at degrees lower... 

Let us dwell on existing key models describing chemisorption induced response of electric conductivity in semiconductor adsorbent. Let us consider both the stationary values of electric conductivity attained during equilibrium in the adsorbate-adsorbent system and the kinetics of the change of electric conductivity when the content of ambient atmosphere changes. Let us consider the cases of adsorption of acceptor and donor particles separately. In all cases we will pay a special attention to the issue of dependence of the value and character of signal on the structure type of adsorbent, namely on characteristics of the dominant type of contacts in microcrystals. 

The fair agreement of expressions (2.67) and (2.71) with experimental data as well as agreement of independently obtained experimental data concerning kinetics of the change of a with the data on equilibrium enabled the author of paper [89] to conclude that the proposed mechanism of effect of hydrogen on electric conductivity of semiconductors can be one of active mechanisms. The heat of total reaction (2.63) calculated from the values found was about 4.6 kcal. 

The diversity of EEP reactions on a solid surface can be illustrated by the survey if interaction between excited atoms of mercury and zinc oxide [186]. When atoms of Hg get to an oxidized surface of ZnO at room temperature, an increase in the semiconductor electrical conductivity take place (Fig. 5.3, curve 2). The electrical conductivity change signal is irreversible, and in case of an increase in the temperature, after the Hg flux is disabled, an additional increase in the electrical conductivity (curves 3 and 4) takes place. One can logically suppose that we are dealing here with partial reduction of zinc oxide according to the scheme... 

The sensor detection of EEPs is methodically more complicated than the detection of atoms and radicals. With atoms and radicals being adsorbed on the surface of semiconductor oxide films, their electrical conductivity varies merely due to the adsorption in the charged form. If the case is that EEPs interact with an oxide surface, at least two mechanisms of sensor electrical conductivity changes can take place. One mechanism is associated with the effects of charged adsorption and the other is connected with the excitation energy transfer to the electron... 

The conductivity changes in a discontinuous fashion at a composition of approximately x = 0.12 at which composition the metal becomes a poor semiconductor. The formula is now Li [LiJi2Ti044Tut4]O4. This transition is reversible. Donor doping of insulating Li4/3Ti5/304 will return the compound to the metallic state. 

There is a fundamental difference between electron-transfer reactions on metals and on semiconductors. On metals the variation of the electrode potential causes a corresponding change in the molar Gibbs energy of the reaction. Due to the comparatively low conductivity of semiconductors, the positions of the band edges at the semiconductor surface do not change with respect to the solution as the potential is varied. However, the relative position of the Fermi level in the semiconductor is changed, and so are the densities of electrons and holes on the metal surface. 

Energy band gaps for selected semiconductors are summarized in Table I. On the basis of the nature of the transition from the valence band to the conduction band, semiconductors are classified as direct or indirect. In a direct semiconductor, the transition does not require a change in electron momentum, whereas in an indirect semiconductor, a change in momentum is required for the transition to occur. This difference is important for optical devices such as lasers, which require direct-band-gap materials for efficient radiation emission (7, 8). As indicated in Figure 7, Si is an indirect semiconductor, whereas GaAs is a direct semiconductor. 

The Situation in Doped Semiconductors. There is an increasing belief amongst workers in the field that the M-NM transition is continuous, based on experimental measurements carried out at low temperatures down to 3 mK. In Figure 12, we show the experimental evidence in P-doped Si. Note that at a fixed (very low) temperature, the conductivity changes continuously with, for example, donor concentration. In addition, the extrapolated zero-temperature value of the conductivity (o(0)) varies continuously with impurity concentration. An example showing the variation of the extrapolated zero-temperature conductivity41 in the case of B-doped Si is... 

The electrical conductivity of electrons in a solid depends on the ability of an electron to move to a higher energy level when accelerated by an electric field. The energy change is very small, so that only partially filled bands can conduct. In semiconductors thermal energy will promote a few valence-band electrons into the conduction band. These electrons can now move in the field. So can the electrons in the valence band whose energies are just below the levels of the promoted electrons. 

Radha and Swamy (278) proposed a possible mechanism for the dehydrogenation of 2-propanol over La2MnM06 (M = Co, Ni, Cu). These authors found that admission of H2, together with the alcohol, does not have any influence on the reaction rate however, admission of acetone with 2-propanol decreases the reaction rate at all partial pressures. It can be inferred that H2 acts as a mere diluent whereas acetone has an inhibiting effect that may be due to its slow desorption. They also measured the conductivity changes of the catalyst in the presence of the reactants or products of the dehydrogenation. As a result of these studies it was concluded that the catalyst surface is covered predominantly with acetone under reaction conditions. Because acetone adsorbs by a donor-type mechanism, as shown by the decrease of the conductivity on its adsorption, its desorption involving electron transfer from the p-type semiconductor catalyst to the adsorbed species can be expected to be the slow process. 

There are several types of gas sensors in current use. Semiconductor-type sensors are sensitive to either a single gas or group of gases that modify electrical properties of the solids. A classical example is ZnO whose n-type conductivity changes proportionally to /- o 4, although this effect only becomes measurable at high temperatures due to diffusion limitations. 

Semiconductor- Perovskite-type oxide Conductivity change by 10 -10 ppm 2-3 min. Air conditioner [29[, [31]... 

Semiconductor gas sensor is based on electrical conduction changes, caused... 

Inchcations that Mott s rule on the doping insensitivity of amorphous semiconductors may not be strictly obeyed in amorphous silicon could be taken already from the work of Chittick et al. (1969). For a-Si films prepared by glow-discharge decomposition of SiH4 (monosilane) these authors reported a rise in the conductivity by factors of 10 and—10 when 200 ppm and 4% PH3 (phosphine), respectively, were added to the silane. The observed instability of these films, however, sheds doubt about whether this was a true doping effect or rather an eflfect of defects, which were known to cause even higher conductivity changes (Beyer and Stuke, 1972). 

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