Space charge control

When the NCS is sufficiently thick, the high field effects should be determined by the uniform field in the bulk. The independence of the current on the polarity of the applied voltage is usually taken as an experimental proof that conduction is not determined by nonuniform fields near the contact. This requirement is, however, not sufficient since most processes, discussed in the previous chapter, are insensitive to the parameters of the electrode material. The injection mechanisms are essentially controlled by the properties of the semiconductor too and Queisser et al (1971) showed that ohmic conduction can be space charge controlled over a wide voltage range in a relaxation case semiconductor. Moreover, the rectification ratio of NCS contacts are expected to be small even for blocking... 

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

Relation (18) for the potential-dependent PMC signal is a reasonably good approximation only for the depletion region, where the space charge layer is controlled by the presence of fixed electron donors (Afo). It would become even more complicated if bimolecular or even more complicated kinetic reaction steps were considered. 

The schemes in Figs. 44 and 45 may serve to summarize the main results on photoinduced microwave conductivity in a semiconductor electrode (an n-type material is used as an example). Before a limiting photocurrent at positive potentials is reached, minority carriers tend to accumulate in the space charge layer [Fig. 44(a)], producing a PMC peak [Fig. 45(a)], the shape and height of which are controlled by interfacial rate constants. Near the flatband potential, where surface recombination... 

The maximum number of ions in a cylindrical QIT is limited to about 105 before space-charge effects seriously affect the performance, so the dynamic range is rather poor. The poor dynamic range can sometimes be compensated for by using automatic gain control. The linear QIT has a larger volume and can store more ions before space-charge affects the performance. 

Another type of non-spectral matrix effect, associated with the oxidation state of the analyte, was proposed by Zhu et al. (2002). Figure 14 plots the relative Fe(II) to total Fe ratio of ultra pure Fe standard solutions versus the difference between the 8 Fe value of the mixed valence state standard and the 5 Fe value of the Fe(III) only standard. The oxidation state of these standards was not quantified by Zhu et al. but based on colorimetric methods using 2,2 -bipyridine the relative Fe(ll) to total Fe ratios of these standards are well known. This matrix effect appears to exert a signihcant control on isotope accuracy, where for example if a reduced ferrous solution was compared to an oxidized ferric standard, the accuracy of the 5 Fe value could be affected by up to l%o. This matrix effect associated with oxidation state is unlikely to be a result of space charge effects because the mass of an electron is unlikely to produce a large change in the mass of the ion beam. Perhaps this matrix effect may be associated with ionization properties in the plasma. 

As shown in Fig. 9-9, the interfacial double layer of semiconductor electrode consists of a space charge layer with the potential of in the semiconductor and a compact layer with the potential of at the electrode interface. The potential 4+sc across the space charge layer controls the process of ionization of smface atoms (Eqn. 9-24) whereas, the potential across the compact layer controls the process of transfer of surface ions (Eqn. 9-33). The overvoltage iiac across the space charge layer and the overvoltage t b across the compact layer are eiq)ressed, respectively, in Eqn. 9-34 ... 

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