The combination of wide band-gap and built-in electrical polarization renders III-nitride HEMTs ideal for high-power, hgh-frequency, and high-breakdown mi-crowave power amplifiers. However, III-nitrides heteroepitaxially grown on sapphire substrates contain high-density dislocations, which adversely affect device perform-ance. 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 A1GaN/GaN heterostructures....[ Read more ]

The combination of wide band-gap and built-in electrical polarization renders III-nitride HEMTs ideal for high-power, hgh-frequency, and high-breakdown mi-crowave power amplifiers. However, III-nitrides heteroepitaxially grown on sapphire substrates contain high-density dislocations, which adversely affect device perform-ance. 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 A1GaN/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 prin-ciple, 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 A1GaN layer became partially relaxed. High frequency PECVD Si₃N₄ passivationcan pro-vided superior long-term thermal stability due to the strain solidification of the A1-GaN epilayer induced by a denser Si₃N₄. 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 Si₃N₄ layer.

Some novel III-nitride HEMTs were grown and fabricated to improve the short-comings of conventional III-nitride HEMTs. A1GaN/GaN MIS-HFET was demon-strated by incorporating a 4 nm re-grown A1N 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 A1GaN/GaN HFET. The A1N epilayer was composed of columnar crystals due to a fulI strain relaxation.

Patterned growth technique with maskless and single-step overgrowth was in-vestigated, to demonstrate a simpler means in improving device performance. Using this method, A1GaN/GaN HEMTs grown on grooved sapphire substrates were com-pared 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 μm 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 fiom 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 A1GaN spacer and the A1GaN cap layer of A1GaN/GaN HEMTs grown on sapphire substrates was designed and developed. The In-doped sample showed a high 2DEG mobility result-ing 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 A1GaN/GaN HEMTs is attributed to the reduced mixed-dislocations, low surface states and increased 2DEG mobility.