| AlGaN/GaN based heterostructure field effect transistors (HFETs) have emerged as the most promising candidates for the next generation microwave applications like wireless communications, satellite communications, military and commercial radar systems. With some innovative device designs like recessed gate, field-plate technique and surface passivation, power densities as high as 32 W/mm at 4GHz and 7W/mm at 40GHz are reported. For these devices to be viable for practical application, the reliability at these high power levels, which is affected mainly by the high gate leakage currents, has to be improved.; In the past it has been demonstrated that, the HFET structure with insulating SiO2 under the gate would tremendously reduce the gate leakage current and thereby improve the device reliability. However, the presence of SiO2 under the gate might bring in problems like threshold voltage dispersions if the quality of the oxide is not good. Apart from this, the MOSHFET devices suffer from huge current collapse even after silicon nitride passivation.; This dissertation reports on a novel digital oxide deposition (DOD) technique using PECVD for depositing high quality ultra-thin SiO2 insulating films with negligible oxide and interface charges. This technique, unlike the conventional method, gives a very precise thickness control at atomic level. The combination of high quality SiO2 and the improved fabrication process has resulted in RF powers as high as 14.5 W/mm at 2GHz with stability of >100 hours on field-plated MOSHFETs.; Low-threshold voltage MOSHFET design to further enhance the RF performance of these MOSHFET devices were also demonstrated using ultra-thin DOD SiO 2 and E-Beam deposited ZrO2 as the gate dielectrics. RF powers of 16W/mm and 14W/mm at 2GHz was achieved on the SiO2 and ZrO2 based low-threshold MOSHFETs respectively.; This research project will act as an important link between the dielectric properties and device performance of high power AlGaN/GaN MOSHFET devices. |
