AlGaN/GaN heterostructure devices for hydrogen gas sensor applications at high temperature were fabricated and characterized. Over the large range of temperatures from 25°C to 800°C, AlGaN/GaN heterostructure devices, both Schottky diodes and field effect transistors (HFETs), demonstrated reliable hydrogen sensing operation with long term thermal stability. Due to the reduction of Schottky barrier height with exposure to H2 containing ambient, the device currents are increased. The hydrogen detection limits of Pt/AlGaN/GaN devices are tens of ppb level. As temperature increases, the device modulation by H2 is increased due to the more active dissociation of H2 on the catalytic metal at higher temperatures, and the detection limit is also enhanced. HFETs show better sensing performance compared to Schottky diodes due to the advanced device management. By the appropriate operation of gate and drain biases at HFETs, two to three orders higher sensitivity and few times higher detection limit are observed. The magnitude of HFET sensitivity is drastically influenced by gate and drain biases and it shows a peak value at the bias of gate threshold voltage and drain knee voltage in the sensing gas.
The thermodynamics of hydrogen adsorption process at Pt/AlGaN is endothermic reaction with a value of enthalpy change (ΔH°) = ~24 kJ∙mole-1 and entropy change (ΔS°) = ~154 J∙(mole∙K)-1. The hydrogen adsorption time (τa) at Pt/AlGaN ranges from 1 to 5 sec. at a range of temperature from 25°C to 800°C and H2 concentration from 30 ppb to 5000 ppm. It gets shorter with increasing the H2 concentration and temperature. The hydrogen desorption time (τd) is longer than τa and shows an opposite trend. τd are around 5 sec to 10 sec. The magnitudes of activation energies of hydrogen adsorption and desorption at the Pt/AlGaN increase with the concentration higher than 500 ppb due to the energetic heterogeneity over the Pt/AlGaN structure.
The different catalytic metals, Pt, IrPt, and PdAg, at AlGaN/GaN Schottky diodes reveal different tendencies on the sensing performance with temperature variation. With temperature increases, the H2 detection sensitivity of Pt and IrPt diodes improves up to 800°C and 700°C, respectively, due to more effective catalytic dissociation of H2 on the catalytic metals at higher temperatures. On the other hand, the sensitivity of the PdAg diodes decreases with temperature because of the relatively poor thermal stability of PdAg. In the comparison of the catalytic efficiencies of Pt, IrPt, and PdAg, IrPt exhibits better response to H2 than Pt over the whole range of temperatures from 200°C to 800°C. While the sensitivity of PdAg diodes degrades as temperature increases, PdAg catalytic metal shows the most remarkable response to H2 from 200°C to 300°C.
The catalytic film thickness at Pt/AlGaN/GaN Schottky diodes influences the gas sensing performance and the thermodynamics and kinetics of gas adsorption/desorption at Pt/AlGaN structures. The device modulation by H2 is enhanced with decreasing the catalytic film thickness from 40 nm to 2.5 nm due to the increase of effective surface boundary with thinner catalytic metal. The thermodynamic analysis demonstrates that the amount of adsorbed heat (ΔH°) for the process of hydrogen dissociation and adsorption at Pt/AlGaN is not affected by Pt thickness. On the other hand, energy dispersal (ΔS°) increases with decreasing the catalytic film thickness due to the enhancement of hydrogen dissociation and adsorption at Pt/AlGaN caused by the increase of effective surface boundary with uneven thin catalytic surface. This matter of surface morphology of Pt catalytic metal also affects the kinetics of hydrogen adsorption/desorption at Pt/AlGaN/GaN. The decrease of film thickness increases the hydrogen adsorption/desorption times and their activation energies.