Metal oxide semiconductor field-effect transistor, principles

The operation principle of these TFTs is identical to that of the metal-oxide-semiconductor field-effect transistor (MOSFET) [617,618]. When a positive voltage Vg Is applied to the gate, electrons are accumulated in the a-Si H. At small voltages these electrons will be localized in the deep states of the a-Si H. The conduction and valence bands at the SiN.v-a-Si H interface bend down, and the Fermi level shifts upward. Above a certain threshold voltage Vth a constant proportion of the electrons will be mobile, and the conductivity is increased linearly with Vg - Vih. As a result the transistor switches on. and a current flows from source to drain. The source-drain current /so can be expressed as [619]... 

Figure 12.12 Principle of metal-oxide-semiconductor field-effect transistor (MOS-FET). A narrow conducting channel is created when the gate is at positive voltage, allowing electrons to pass from the source to the drain.

There has recently been considerable interest in the detection and identification of air-borne volatile compounds in such diverse areas as quality control of perfume to detection of toxic gases. Miniaturization, the potential low cost of sensors and the variety of applications promise an enormous market (29, 30), Chemical sensors (29) for volatile compounds operate on varied principles and can be classified according to the method of functioning into basic groups such as electrical (field-effect transistors, metal oxide semiconductors and organic semiconductors), optical (spectrophotometric, luminescence, optothermal) and sensors that are sensitive to a change of mass (piezoelectric and acoustosurface). 

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