Hydrogen, one of the cleanest energies, is very attractive in the near future. However, it could be hazardous to store, transport and use hydrogen gas because leakage can cause explosion if sparks appear. Therefore, it is essential to develop sensors to detect the hydrogen leakage in order to prevent potential accidents. In this research, Metal-Insulator-Semiconductor (MIS) Schottky-diode hydrogen sensors with different gate insulators (Ta2O5, La2O3, LaTiON, and HfTiO) or substrates (Si, SiC, and InGaN/GaN MQW) were prepared in order to study their hydrogen sensing performances. Firstly, two sensors based on Si and SiC with Ta2O5 as gate insulator were prepared and compared. Owing to high permittivity (~25), good thermal stability and low electrical defects, Ta2O5 was chosen as the insulator. The differences in sensitivity and response time between the two sensors were ascribed to the difference in the surface morphology of Ta2O5 between the SiC sensor (mean surface roughness was 0.39 nm) and its Si counterpart (mean surface roughness was 0.22 nm). Secondly, due to the high permittivity (~25) and good thermal stability of La2O3, the high permittivity (~20), low interface-state density, and low leakage current of LaTiON, Si sensors with these two dielectrics as gate insulator were developed. The sensitivity of the La2O3 sensor could exceed 7.0 at 150 oC, and the sensor exhibited good hydrogen sensing performance at up to 250 oC. On the other hand, the maximum sensitivity of the LaTiON sensor could reach 2.5 at 100 oC. For the LaTiON sensor, the Poole-Frenkel model controlled the carrier transport at high temperatures (150 ~ 200 oC) while the thermionic emission was the dominant conduction mechanism at lower temperatures (from room temperature to 150 oC). For the La2O3 sensor, the hydrogen reaction kinetics was confirmed, and an activation energy of 10.9 kcal/mol was obtained for this sensor. Thirdly, the La2O3 gate insulator used in the previous work was applied to make MIS sensor on SiC substrate for higher-temperature applications. Its maximum sensitivity and response time at high temperature (260 oC) are 4.6 and 20 s, respectively. The electrical conduction mechanisms were explained in terms of Fowler-Nordheim tunneling (below 120 oC) and the Poole-Frenkel effect (above 120 oC). Finally, in order to see whether the unique structure of InGaN/GaN multiple quantum wells (MQWs) can be utilized for the MIS Schottky-diode hydrogen sensor, three sensors were made on InGaN/GaN MQWs substrate, one without gate insulator, one Finally, in order to see whether the unique structure of InGaN/GaN multiple quantum wells (MQWs) can be utilized for the MIS Schottky-diode hydrogen sensor, three sensors were made on InGaN/GaN MQWs substrate, one without gate insulator, one In summary, the quality of the gate insulator plays an important part in the performance of the hydrogen sensors. SiC and InGaN/GaN MQW substrates are suitable for high-temperature (from ~200 to ~500 oC) applications while the low-cost sensors based on Si substrate can function well below about 200 oC. Hydrogen sensors with these high-k materials (Ta2O5, La2O3, LaTiON, and HfTiO) as gate insulator can produce good electrical characteristics, high sensitivity, and fast response. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy