High-voltage gallium nitride HEMTs with low on-resistance for switching applications

Power electronics as a means to control electrical energy is showing increasing importance in refining and innovating the social infrastructure in the new century. To break through the material limits of silicon and to realize the drastic performance improvement needed to meet the severe requirements in the future, wide bandgap semiconductors such as SiC and GaN have attracted much attention because of their superior physical properties. This dissertation focuses on high voltage AlGaN/GaN HEMTs for high speed, low loss switching applications.; AlGaN/GaN HEMTs were projected to have lower on-resistance and higher switching speed than SiC devices due to the high electron mobility of the 2-DEG. Theoretical analysis was conducted to quantitatively predict that the on-resistance of AlGaN HEMTs is 50 to 200 times lower than that of SiC FETs, depending on the aluminum composition. Device simulation was also carried on in ATLAS to probe into the operation mechanism of GaN HEMTs and assist the high breakdown voltage devices design. In device fabrication, highly resistive GaN buffer is the least requirement for high voltage operation. For this reason, the leakage of the GaN buffer was reduced to 0.2 mA/mm from 50 mA/mm at the beginning of the work by applying AP/LP growth on SiC substrates. With reduced electric field peak at the gate edge, a VBR higher than 500 V was obtained on field-plated GaN HEMTs for the first time. Further improvement was brought by low gate leakage from insulated-gate structure. 1300 V breakdown voltage together with 1.65 mΩ·cm 2 on-resistance was achieved on a device with 16 μm gate-drain distance and 100 Å SiO₂ underneath the gate.; To improve the switching speed, GaN MIS structures were investigated by photo-CV technique to search for the proper dielectrics for GaN HEMT passivation. Sputtered SiN was found to eliminate the current dispersion in GaN HEMTs and improved the CW output power density from 3.3 W/mm to 6.6 W/mm for microwave HEMTs at 6 GHz. Thus, SiN was applied to the gate dielectric of high voltage GaN HEMTs and reduced the dispersion, but it also increased the gate leakage and hence resulted in low breakdown voltage. To combine the advantage of passivation from SiN and low leakage from SiO₂, double-layer gate-dielectric structure was implemented in insulated-gate GaN HEMTs and displayed both low dispersion and over 1000 V breakdown voltage. Subsequent switching measurement revealed very short turn-on time of 3.5 ns and turn-off time of 7 ns for a large-scale device. Yield was the highest priority in the design and fabrication of large periphery GaN HEMTs. V_BR of 600 V and total R_on of 0.4 Ω were realized on a 38.4 mm device consisted of 64 small device units, which indicated a bright future of GaN HEMTs in the switching applications.