Electrical and hydrogen-sensing characteristics of field effect transistors based on nanorods of ZnO and WO_2.72

Abstract

Top-gated field effect transistors (FETs) using Au-gap (5 μm) electrodes on glass substrate and SiO₂/Si as gate have been fabricated with undoped and doped nanorods of ZnO as well as with WO_2.72 nanorods as active semiconductor elements. The I–V characteristics at different gate voltages show that the nanorods are n-type semiconductors and the derived transfer characteristics show that the FET devices function in the depletion mode. Al-doping (3 at%) enhances the carrier mobility of ZnO nanorods to 128.6 cm²/V·s as against to 0.009 cm²/V·s estimated in the case of the undoped nanorods. Doping with Cd and Mg (3 at%) as well as N (∼1 at%) similarly increases the mobility although to a smaller extent. The Cd-doped ZnO nanorods exhibit the high sensitivity (defined as the ratio of the resistance in air to that in the hydrogen) (20) for 1000 ppm of hydrogen. Application of gate voltage decreases the recovery times of the nanorod sensors. FETs based on WO_2.72 nanorods also show the depletion mode type characteristics and a carrier mobility of 8.38 cm²/V·s is obtained. The WO_2.72 based FETs exhibit good sensitivity (∼10) for 1000 ppm hydrogen.