On the hydrogen sensing properties of a Pd/GaAs transistor-type gas sensor in a nitrogen ambiance

A hydrogen sensor based on the Pd/GaAs pseudomorphic high electron mobility transistor (PHEMT) is fabricated and investigated under various hydrogen concentrations in air and N2 environments. Experimentally, in nitrogen (air) ambiances, the studied sensor exhibits a hydrogen detection limit of 4.3 ppm H2/N2 (98 ppm H2/air) at 130 °C, a high sensitivity of 1295 μA/mm-ppm H2/N2 (275.8 μA/mm-ppm H2/air) in 14 ppm H2/N2 (H2/air) at 30 °C, a fast transient response time of 2 (3) s in 9970 ppm H2/N2 (H2/air) at 130 °C, and a large initial rate of 774.4 (589.8) μA/s in 9970 ppm H2/N2 (H2/air) at 90 °C. However, the studied sensor shows a longer recovery time in nitrogen than in air due to the lack of additional desorption process. From the experimental results, it is speculated that the oxygen in air occupies the adsorption sites and reduces the adsorbed hydrogen atoms at the Pd/GaAs interface. This interprets that the studied sensor shows a larger hydrogen-induced current variation in nitrogen than in air under the same hydrogen concentration. In addition, there is no overshoot phenomenon in transient response observed in the nitrogen atmosphere due to the absence of oxygen.