Silicon based microfabricated tin oxide gas sensor incorporating use of Hall effect measurement

Characterization of a microfabricated sol–gel derived nano-particle tin oxide thin film on a silicon substrate, through simultaneous measurement of conductivity, Hall mobility and electron density, was accomplished in this study. A tin oxide thin film (1100 Å thick), derived by the sol–gel method, was deposited on a Si/SiO2 substrate by means of spin-coating method. It was determined that conductivity is strongly dependent on electron density level and shows very weak dependence on Hall mobility. Lack of Hall mobility sensitivity to H2 concentration suggests that conduction is grain control limited. In this regime, in which the grain size (D) is less than twice the characteristic Debye length (LD), a change in reducing gas concentration results in a nearly simultaneous change in carrier density throughout the entire grain, while the Hall mobility remains unchanged. A sensor calcined at 500°C and operated at 250°C showed maximum conductivity sensitivity to H2 in air. The sensor exhibited a high conductivity sensitivity of 10.6–100 ppm H2 in air with response time of ∼1 min and recovery time of ∼4 min. The effects of calcination temperature and operating conditions on the tin oxide structure were studied. Sensitivity decreased as average grain size increased from 7.7 to 14.7 nm, with increasing calcination temperature from 500 to 800°C.