Fabrication And Investigation Of Al₂O₃/AlGaN/GaN MOSHEMT

Because of the limited barrier height and surface defects in the AlGaN/GaN HEMT, the gate leakage current restrain the devices reliability in high temperature, high requency and high power.The metal-oxide-semiconductor HEMT structure solved the problem, which play a major role in the structure is the gate dielectric layer deposited on the semiconductor material. The stand or fall of gate dielectric layer play a crucial role on the performance of the device.The AlGaN/GaN structure used in this study was composed of a 40 nm low-temperature AlN nucleation layer, 1400 nm unintentionally doped GaN layer, 1.5nm AlN insert layer and 25 nm Al0.3Ga0.7N barrier layer on a 2-inch c-plane sapphire substrate. Hall measurements showed a sheet carrier density of 1.1?1013 cm-2 and an electron mobility of 1140 cm2/V?s.The deposition of gate oxide layer is a crucial step in the production of MOSHEMT device. In chapter IV, a AlGaN/GaN heterostructure material was divided into three portions and used the same methods to deposit different thickness Al2O3 gate dielectric layers(3.5nm, 7.5nm and 10nm). The three devices with different thickness of gate dielectric were tested, and C-V curves were drawn. Analysis shown that a thin Al2O3 layer device can have a larger gate capacitance. If the dielectric layer is less than 10 nm, a thick Al2O3 dielectric layer has a better passivation effect, while the thinner layer can not completely eliminate the influence of irregularities on AlGaN heterojunction surface caused by the MOCVD. In section II, through C-V measurements and simulations on the Al2O3/AlGaN/GaN MOSHEMTs of varying dielectric thicknesses, it showed that the donor-type interface fixed charge density(Qif) of 2.2×1013cm-2 existed at the Al2O3 /AlGaN interface, which induced the shift of the threshold voltage much more negative.In section III, for Al2O3/AlGaN/GaN MOSHEMT of different Al2O3 thickness, the interface trap density(Dit) at the Al2O3/AlGaN interface is(0.62~1.16)?1013 cm-2eV-1,(0.88~1.9) ?1013 cm-2eV-1 and(0.65~2.44)?1013 cm-2eV-1 for tox=3.5nm, 7.5nm and 10 nm respectively, measured by Terman method. The interface trap density increased with the increase of the thickness of the gate dielectric. With the increase of the thickness of Al2O3, the dielectric layer shows a polymorphic transition, which increased defect. While the role of dielectric layer’s body trap can not be ignored, so the overall test results should be slightly larger than the actual value. In section IV,we did simulation of devices with different energy levels and different capture cross-sections to study the impact of interface trap charge on the transient characteristics. Analysis found that the more superficial level and the larger the capture cross-section is,the greater impact of the interface trap charge on transient characteristics of the device. In chapter V, the same piece of AlGaN/GaN material is divided into two parts, one using ALD deposition of 10 nm Al2O3 oxide layer, another part first used plasma oxidation treatment with 10 min, and then using the ALD method deposited 8nm Al2O3 oxide layer to make comparative experiments. We found that the device used the plasma oxidation treatment has a positive threshold voltage drift tendency, which may be caused by the reductionof the fixed positive charge on Al2O3/AlGaN interface.