Electrochemically Gated Devices

Recently, this method has been modified to accommodate also an electrochemical gate [68] in the experimental setup - a situation that resembles a three-terminal device. This setup was used to study the electronic transmission properties of... 

Field-Effect Transistors (FETs) are part of all modern pH meters. With the introduction of ion-sensitive field-effect transistors, they have both been brought to the attention of chemists. In order to understand the principles of operation of these new electrochemical devices, it is necessary to include the FET in the overall discussion of the electrochemical cell. The outline of the operation of an insulated gate field-effect transistor is given in Appendix C. 

As a result of the effects of nonideal structures, second-order effects in parameters, and the numerous approximations made in the derivation of the current-voltage equations, (C.27) and (C.30) can only serve as a qualitative description of the actual device each individual design must be experimentally characterized. For these reasons it is advantageous to operate the FET in the constant drain current mode in which case a suitable feedback circuit supplies the gate voltage of the same magnitude but of the opposite polarity to that produced by the electrochemical part of the device. 

Due to the inability to deposit eleetrochemically stable gate-insulating materials for GaAs, another approach was developed based on amorphous silicon (a-Si), prepared as a thin layer for LAPS devices on transparent glass substrates [40]. The diffusion length in this material was reported to be as small as 120 nm [41] and a resolution down to 1 gm has been demonstrated, which was mainly limited by the optical set-up. The electrochemical properties of the a-Si-based structures were investigated later with a LAPS device thus, the above results for SPIM were transferred back and proved for the LAPS, too [42,43]. 

Potential or current step transients seem to be more appropriate for kinetic studies since the initial and boundary conditions of the experiment are better defined unlike linear scan or cyclic voltammetry where time and potential are convoluted. The time resolution of the EQCM is limited in this case by the measurement of the resonant frequency. There are different methods to measure the crystal resonance frequency. In the simplest approach, the Miller oscillator or similar circuit tuned to one of the crystal resonance frequencies may be used and the frequency can be measured directly with a frequency meter [18]. This simple experimental device can be easily built, but has a poor resolution which is inversely proportional to the measurement time for instance for an accuracy of 1 Hz, a gate time of 1 second is needed, and for 0.1 Hz the measurement lasts as long as 10 seconds minimum to achieve the same accuracy. An advantage of the Miller oscillator is that the crystal electrode is grounded and can be used as the working electrode with a hard ground potentiostat with no conflict between the high ac circuit and the dc electrochemical circuit. 

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