| The combination of wide band-gap and built-in electrical polarization renders III-nitride HEMTs ideal for high-power, high-frequency, and high-breakdown microwave power amplifiers. However, III-nitrides heteroepitaxially grown on sapphire substrates contain high-density dislocations, which adversely affect device performance. Currently, the main challenge to enhance device performance is to reduce the dislocation densities in the bulk, layer interfaces, as well as the surface morphology of the AlGaN/GaN heterostructures.; This thesis focuses on enhanced DC characteristics and RF performance of III-nitride HEMTs grown on sapphire substrates by MOCVD. Firstly, the HEMTs principle, MOCVD growth, materials characterization, and initial device characteristics of conventional III-nitride HEMTs are introduced. The thermal stability of 2DEG in III-nitride HEMTs is presented. The 2DEG conductivity degraded when the AlGaN layer became partially relaxed. High frequency PECVD Si3N 4 passivationcan provided superior long-term thermal stability due to the strain solidification of the AlGaN epilayer induced by a denser Si 3N4. An increase in device performance was also realized after high frequency PECVD Si3N4 passivation due to the enhancement of strain-induced polarization by the denser Si3N 4 layer.; Some novel III-nitride HEMTs were grown and fabricated to improve the shortcomings of conventional III-nitride HEMTs. AlGaN/GaN MIS-HFET was demonstrated by incorporating a 4 nm re-grown AlN epilayer using high-temperature MOCVD. The reverse gate leakage current that was around two orders of magnitude lower was obtained in the MIS-HFET, compared to that in the AlGaN/GaN HFET. The AlN epilayer was composed of columnar crystals due to a full strain relaxation.; Patterned growth technique with maskless and single-step overgrowth was investigated, to demonstrate a simpler means in improving device performance. Using this method, AlGaN/GaN HEMTs grown on grooved sapphire substrates were compared with those grown on unpatterned substrates. The reverse gate leakage current was over three orders of magnitude lower. The on-wafer output power, linear gain and power-added efficiency of an unpassivated 1x100 mum device measured at 4 GHz were 3.41 W/mm, 25.7 dB and 56% respectively, which is very high performance of III-nitride HEMTs on sapphire substrates currently. Stronger CL and EL from the trench regions of the grooves in InGaN/GaN based blue LEDs grown on grooved sapphire substrates were observed, confirming its better crystalline quality over the trench regions, further supported by the EL mapping results.; Isoelectronic indium-surfactant-doping in the 2DEG channel, the AlGaN spacer and the AlGaN cap layer of AlGaN/GaN HEMTs grown on sapphire substrates was designed and developed. The In-doped sample showed a high 2DEG mobility resulting from reduced defects, alloy disorder and interface scattering on the 2DEG. The In-doped HEMTs also showed negligible current collapse and improved DC and RF performance compared to conventional HEMTs. The high performance of In-doped AlGaN/GaN HEMTs is attributed to the reduced mixed-dislocations, low surface states and increased 2DEG mobility. |
