Electrolyte-insulator-semiconductor surface potential

One possible solution is to obtain new experimental data, which is independent of co/pH curves. The zeta potential is of course a possibility, but it suffers from the intrinsic indeterminacy of the exact location in the double layer where it occurs. Another possibility is the surface potential, Vo, which will be defined below. Variations of Vo can be measured by using electrolyte/insulator/semiconductor structures. It has been shown by Bousse et al. (14) that the Vo/pH characteristics are determined mainly by the number of charged but uncomplexed surface sites, and are insensitive to complex-ation. This means that combined consideration of tro/pH and Vo/pH characteristics should lead to a more complete and reliable determination of model parameters. 

A constant bias potential is applied across the sensor in order to form a depletion layer at the insulator-semiconductor interface. The depth and capacitance of the depletion layer changes with the surface potential, which is a function of the ion concentration in the electrolytic solution. The variation of the capacitance is read out when the semiconductor substrate is illuminated with a modulated light and the generated photocurrent is measured by means of an external circuit. 

Although the potential that is applied to microelectronic devices is normally relatively limited (< 5V), very large electric fields can result because of the very small separation distances between conductors. When condensed water and ionie contamination are present between the lines (on the surface of separating insulators), these fields produce undesirable results by means of the three separate, but related mechanisms described next and shown schematically in Figure 4. For reference, these processes are often referred to as electrolytic. An extensive review of this subject was compiled by Steppan et al. [45], The voltage driver is normally externally applied, but another source is the semiconductor junctions that exist within an IC [17], Most environmentally induced failures in ICs that are observed in practice, and especially during accelerated aging, are caused by the applied electrical bias. 

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