MODELING THE GAS-SENSING BEHAVIOR OF SNO2-GATE FETS

An attempt to create a first-order model of the operation mechanisms o f SnO2-gate FETs sensitive to ammonia is reported in this work. In pre vious work a strong dependence of the sensing effect on the relative h umidity of the ambient has been reported. On these grounds, the presen ce of adsorbed positive NH+ ions and negative OH- ions on the SnO2 sur face is assumed and the model is built up on the lateral transportatio n of these ions under the combined effect of the transversal gate-to-c hannel electric field and the surface source-drain field. The transver sal field is supposed to weaken electrostatically the adsorption bonds of the OH- ions over the channel and of the NH+ ions over the saturat ed surface area. This enables lateral transportation and redistributio n of the ions along the surface source-drain field. Due to the strong non-uniformity of the channel this redistribution results in equivalen t addition to the gate voltage applied and, thus, in channel current m odulation. Expressions for the floods of positive and negative ions ov er the channel are derived. The calculated curves for the gate voltage variation according to these expressions are in good agreement with e xperimental data.