FET Structures for Ion and Gas Sensors

Two main types of devices that have been extensively investigated for chemical measurements are the single-crystal silicon field-effect transistors (FETs) (Bergueld, 1971, 1972 Matsuo and Wise, 1974 Bergueld and de Rooij, 1981 Ohta etal., 1982 Akiyama etal., 1982) and the ion-controlled diodes (ICD) (Wen et al 1978, 1979 Lauks 1981). 

Because of its particular technological characteristics, the discussion of the ICD shall remain confined to those made of c-Si therefore it will not be considered in this section. On the other hand, only those structures having a configuration adaptable to a-Si H thin-film technology will be taken into consideration. Two kinds of FETs are referred to in the literature the ion-selective FET (ISFET) and the gas-sensitive FET (CHEMFET). 

Here (f m is the work function difference between the metal and the semiconductor (Qu + + 0oxm)/Cox is the voltage contribution due to 

In the case of an ISFET, in the presence of a reference electrode and a given solution, Eq. (2) becomes 

The advent of the a-Si H as semiconductor material has opened new possibilities to produce chemical sensors in the form of FET structures suitable for both gas or ion detection. In fact, several kinds of a-Si H 

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