Transistor IGFET

Fig. 6.31 Insulated gate field-effect transistor (IGFET) with electronically conducting, chemically selective gate...

According to the IEEE standard test methods for the characterisation of OFETs [26], the Shockley equations for insulated gate field effect transistors (IGFETs) are used to approximate the field-indueed drain eurrent in the organic material between the drain and source contaets. It should be reeom-mended that the following assumptions form the basis of the applieability of the equations ... 

The key issue in these sensors is the interface between the ion selective membrane and the contact. The most convenient way to present this problem is in the form of the equivalent electrical circuit in which the resistances and capacitances have their usual electrochemical meaning (Fig.2). It is necessary to include the electrometer (or at least its input stage) in the analysis of these sensors. In most modern instruments the amplifier is an insulated gate field-effect transistor (IGFET) which has the input dc resistance of greater than 10 and the input capacitance on the order of picofarads. 

CHEMFET devices or chemically sensitive field-effect transistors are potentiometric devices in which the metal layer of a solid-state insulated gate field-effect transistor (IGFET) f29] is replaced with a chemically sensitive electroconductive polymer membrane film. Changes in polymer membrane potential modulate the drain impedance of the space-charge region beneath the insulator. The result is a change in drain current /o under a fixed drain voltage Vd- The potential of the membrane may be modulated in the standard way (as in po-tentiometry, above) or may be modulated by an inert electrode placed beneath the film (but isolated from both the source and drain electrodes) and connected to an efficient electrode for the candidate analyte [30]. 

Fig. 2.16. Ion-sensitive field effect transistor (intersection and symbol), (a) n-p-n transistor, (b) IGFET (MOSFET), (c) ISFET.

Let us pause and take an inventory of the situation up to this point. (1) We have a plausible mechanism of modulation of both components of WF of a selective layer (palladium) and (2) we have at least two methods of measurement of this effect, the macroscopic Kelvin probe and a field-effect transistor. However, the placement of the selective layer within the structure used for either measurement determines whether the effect is observable. In order to explain this caveat, we add another layer of the same metal M between Pd and the insulator in the structure shown in Fig. 6.33. This would correspond to the real life situation when we would try to connect a selective layer by a wire to the IGFET or a Kelvin Probe. It is not necessary to perform the same cycle as we did in Fig. 6.33. Instead, we add the individual energy contributions in the cycle, which begins and ends at the silicon Fermi level (moving again anticlockwise) ... 

IGFET insulated-gate field effect transistor... 

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