n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors

Selectivity has been the central issue in the research area of semiconducting metal oxide gas sensors and still remains a challenge for the researchers. In the present work, we report a promising hydrogen (reducing gas) sensing characteristics of nanocrystalline indium doped zinc oxide thin film sensing elements. At sensor operating temperature in the range (200-300) °C, 'n' to 'p' type carrier reversal is detected in the measured resistance transients. Furthermore, for a given operating temperature the type of carriers remain unaltered for each concentrations of gas. Thus, temperature plays a crucial role for this transition irrespective of the gas concentration. However, such carrier reversal is not observed in NO2 (oxidizing gas) environment even with the variation of operating temperature and gas concentration. This is attributed to the gas sensing mechanism related to the surface conductivity and the underlying variations of the carrier type. It is argued that, it can pave the way for selective detection of hydrogen at lower operating temperatures.