Sensing sensibility of surface micromachined suspended gate polysilicon thin film transistors

Suspended gate thin film transistors (SGTFT) used as gas sensor are fabricated using surface micromachining process. The suspended gate is a polycrystalline silicon micro-bridge and the process is fully compatible with both IC technology and low-cost glass substrates. Detection of gas is linked to the concentration of electrical charges located in air-gap, region between the gate and the channel of the transistor. Field effect, due to the voltage applied to the suspended gate, is enough to influence the distribution of electrical charges present in ambience, and then, the transistor electrical characteristics. Results concerning the response of polysilicon SGTFT to NH3 and NO2 detection are presented. The high sensitivity is attributed to the influence of the very high field in the narrow (0.5 μm) gap on the charge distribution in the gap as well as the surface adsorption. Behaviour of the sensor is explained by a model based on electrical charges distribution in air-gap versus gate voltage.