Non-resistive sensors

Various non-resistive gas sensors using semiconductors have been proposed. As described below, these sensors, constructed based on various principles, provide useful information to learn how receptor function and transducer function are generated and combined together into gas sensors, though most of the sensors are yet to be exploited further for use in practice. 

10 (a) Structure of Pd-gate FET. (b) Drain current (/ ) characteristics observed Vq, gate voltage, l/[,s (source-drain voltage). 

Solid electrolyte-gate field effect transistor 

Three-phase contact between metal, solid electrolyte and gas is known to act as an active site for electrochemical reactions (half cell reaction). If the solid electrolyte is a proton conductor, for instance, the following reaction takes place in the presence of H2, and the half cell equilibrium is expressed by the following Nernst equation  

Similarly, devices sensitive to NO2 or CO2 can be fabricated by attaching NaN02 (Na+ ionic conductor) or Li2C03-based composite salt (Li+ ionic conductor) to the gate, respectively. The response mechanisms involved can be understood in the same way. In the NO2 device, for example, the half cell reaction is expressed as follows  

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