Electrolyte-insulator-semiconductor reference electrode

Another impedance-based imaging technique for laterally resolved characterization of thin films or electrochemical systems is Scanning Photo-induced Impedance Microscopy (SPIM) [44]. It is based on photocurrent measurements at field-effect structures. In their simplest arrangement, field-effect structures consist of a semiconductor substrate with a thin insulator, and a gate electrode. This gate electrode can be a metal film resulting in the structure Metal Insulator Semiconductor (MIS) or, alternatively. Electrolyte Insulator Semiconductor structures are used, in which the electrolyte is in direct contact with the insulator, and a reference electrode is required to fulfill the function of the gate electrode. 

To a first approximation, the BLM can be considered to behave like a parallel plate capacitor immersed in a conducting electrolyte solution. In reality, even such a thin insulator as the modified BLM (designated by and R, in Fig. 108) could block the specific adsorption of some species from solution and/or modify the electrochemical behavior of the system. Similarly, System C may turn out to be a semiconductor(l)-insulator-semiconductor(2) (SIS ) rather than a semiconductor(l)-semiconductor(2) (SS ) junction. The obtained data, however, did not allow for an unambiguous distinction between these two alternative junctions we have chosen the simpler of the two [652], The equivalent circuit describing the working (Ew), the reference (Eg), and the counter (Ec) electrodes the resistance (Rm) and the capacitance (C of the BLM the resistance (R ) and capacitance (Ch) of the Helmholtz electrical double layer surrounding the BLM as well as the resistance of the electrolyte solution (RSO ) is shown in Fig. 108a [652],... 

The complete ac equivalent circuit of an EIS is complex, as it involves components such as the bulk resistance and space-charge capacitance of the semiconductor, the capacitance of the gate insulator, the interface impedance at the insulator-electrolyte interface, the double-layer capacitance, the resistance of the bulk electrolyte solution and the impedance of the reference electrode [58-60]. However, considering usual values of insulator thickness ( 30-100 nm), the ionic strength of the electrolyte solution (>10 -10 M) and low frequencies (<1000 Hz), the equivalent circuit of an EIS structure can be simplified as a series connection of insulator capacitance and space-charge capacitance for the semiconductor, which is similar to the MIS capacitor [58-60]. Therefore, the capacitance of the EIS structure may be expressed in terms of the electrolyte solution/ insulator interface potential (cp) as ... 

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