Electrolyte-insulator-silicon

The measurement of changes of the surface potential Vo at the interface between an insulator and a solution is made possible by incorporating a thin film of that insulator in an electrolyte/insulator/silicon (EIS) structure. The surface potential of the silicon can be determined either by measuring the capacitance of the structure, or by fabricating a field effect transistor to measure the lateral current flow. In the latter case, the device is called an ion-sensitive field effect transistor (ISFET). Figure 1 shows a schematic representation of an ISFET structure. The first authors to suggest the application of ISFETs or EIS capacitors as a measurement tool to determine the surface potential of insulators were Schenck (15) and Cichos and Geidel (16). 

Vfb The flat-band voltage of an electrolyte/insulator/silicon (EIS) capacitor ... 

The concept of light addressable potentiometric sensors (LAPS) was introduced in 1988 [67], LAPS is a semiconductor-based sensor with either electrolyte-insulator-semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively. Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure. A semiconductor substrate (silicone) is covered with an insulator (Si02). A sensing ion-selective layer, for instance, pH-sensitive S3N4, is deposited on top of the insulator. The whole assembly is placed in contact with the sample solution. 

Electrolyte-insulator-semiconductors are the basis for silicon field-effect chemical sensors and include ISFETs/ChemFETs, light-addressable potentiometric sensors (LAPSs) and capacitive sensors. The first ISFET was invented in 1970. ... 

A Schottky diode is always operated under depletion conditions flat-band condition would involve giant currents. A Schottky diode, therefore, models the silicon electrolyte interface only accurately as long as the charge transfer is limited by the electrode. If the charge transfer becomes reaction-limited or diffusion-limited, the electrode may as well be under accumulation or inversion. The solid-state equivalent would now be a metal-insulator-semiconductor (MIS) structure. However, the I-V characteristic of a real silicon-electrolyte interface may exhibit features unlike any solid-state device, as... 

The easiest way to have different parts of the electrode surface under different bias is to disconnect them by an insulator. This method is elucidated by an experiment in which an electrochemical etch-stop technique has been used to localize defects in an array of trench capacitors. In a perfect capacitor the polysilicon in the trench is insulated from the substrate whereas it is connected in a defect capacitor, as shown in Fig. 4.15 a. If an anodic bias is applied the bulk silicon and the polysilicon in the defect trench will be etched, while the other trenches are not etched if an aqueous HF electrolyte is used, as shown in Fig. 4.15b. The reverse is true for a KOH electrolyte, because the only polysilicon electrode in the defect trench is passivated by an anodic oxide, as shown in Fig. 4.15 c. 

The measurement of properties such as the resistivity or dielectric constant of PS requires some kind of contact with the PS layer. Evaporation of a metal onto the PS film-covered silicon sample produces a metal/PS/Si sandwich, which behaves like an MIS structure with an imperfect insulator. Such sandwich structures usually exhibit a rectifying behavior, which has to be taken into account when determining the resistivity [Si3, Bel4]. This can be circumvented by four-terminal measurements of free-standing PS films, but for such contacts the applied electric field has to be limited to rather small values to avoid undesirable heating effects. An electrolytic contact can also be used to probe PS films, but the interpretation of the results is more complicated, because it is difficult to distinguish between ionic and electronic contributions to the measured conductivity. The electrolyte in the porous matrix may short-circuit the silicon filaments, and wetting of PS in-... 

Combustion products can affect sensitive electronic equipment. For example, hydrogen chloride (HCI) is formed by the combustion of PVC cables. Corrosion due to combusted PVC cable can be a substantial problem. This may result in increased contact resistance of electronic components. Condensed acids may result in the formation of electrolytic cells on surfaces. Certain wire and cable insulation, particularly silicone rubber, can be degraded on exposure to HCI. A methodology for classifying contamination levels and ease of restoration is presented in the SFPE Handbook... 

The stability of silicon electrodes contacting an aqueous electrolyte is a severe problem in regenerative solar systems. As mentioned previously, the standard electrode potential of a silicon element is negative enough to induce an electrochemical reaction mechanism, giving rise to an insulating surface silicon oxide in the absence of complexing reactants. On the... 

Semiconductors like silicon or germanium are an intermediate case. Their electrons are not as tightly bound as in insulators so that at any given time a small fraction of them will be mobile. In a perfect germanium crystal, for instance at 25°C, about 3 x 1019 electrons per m3 are free. This corresponds to a concentration of 5 x 10-8 M or 50 nM. It is much lower than the concentration of charge carriers (cat- and anions) in an aqueous electrolyte solution. Despite this small concentration, the conductivities are of the same order of magnitude, because the electrons in a semiconductor are typically 108 times more mobile than ions in solution. 

The key was the insertion of a zirconia buffer layer between the silicon substrate and the superconducting film on top of it. (Zirconia is a white crystalline compound used as an insulator in enamels and as an electrolyte in fuel cells.) The buffer, deposited with superconducting film onto the silicon by electron-beam evaporation, served as an effective barrier, preventing the elements from intermingling during the annealing process. 

Other experimental conditions may also affect OCP. A 150-mV increase in OCP was observed when a limited amount of electrolyte was confined between the silicon surface and an insulating material. " This effect may be caused by the changing composition of the confined solution. In NH3 the OCP does not change with flow rate of the solution. ° ... 

Silicon is highly unstable in aqueous electrolytes due to the formation of an insulating oxide film which prevents the use of n-Si as photoanode. On the other hand, the silicon electrode has poor kinetics for hydrogen evolution which is not desirable for its use as a photocathode. Many methods have been explored to stabilize Si electrodes in aqueous solutions for possible applications as photochemical cells. They include coating the surface with noble metals, metal oxides, metal silicides, or organic materials as shown in Table 6.6. Also, some redox species, the reduction of which can favorably compete with the oxidation of silicon, can be used to stabilize silicon anodes... 

Jaffrezic-Renault, N., et al., Study of the silicon nitride/aqueous electrolyte interface on colloidal aqueous suspensions and on elecholyte/insulator/semiconductor structures, Colloids Surf., 36, 59, 1989. 

Ion Sensitive Field-Effect Transistors (ISFETs) developed to measure pH and to sense a variety of analytes in solution are well known in the silicon technology field [11]. In these devices the gate insulator is in direct contact with the electrolyte solution (see Fig. 6.1). 

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