Design and fabrication of 4H silicon carbide MOSFETS

The 4H-SiC power MOSFET is an excellent candidate for power applications. Major technical difficulties in the development of 4H-SiC power MOSFET have been low MOS channel mobility and gate oxide reliability. In this dissertation, a novel 4H-SiC power MOSFET structure has been presented with the aim of solving these problems. The research started from the study and improvement of the channel mobility of lateral trench-gate MOSFET that features an accumulation channel for high channel mobility. The design, fabrication and characterization of lateral trench-gate MOSFET are presented. The fabricated lateral trench-gate MOSFET with an accumulation channel of 0.15μm exhibited a high peak channel mobility of 95 cm2/Vs at room temperature and 255 cm2/Vs at 200oC with stable normally-off operation. Based on the successful demonstration of high channel mobility, a vertical trench-gate power MOSFET structure has been designed and developed. This structure also features an epitaxial N-type accumulation channel to take advantage of high channel mobility. Moreover, this structure introduces a submicron N-type vertical channel by counter-doping the P base region via a low-dose nitrogen ion implantation. The implanted vertical channel provides effective shielding for gate oxide from high electric field. A process using the oxidation of polysilicon was developed to achieve self-alignment between the submicron vertical channel and the gate trench. A "sandwich" process, including nitric oxide growth, dry oxygen growth and nitric oxide annealing, was incorporated to grow high-quality gate oxide. The fabricated single-gate vertical MOSFET can block up to 890 V at zero gate bias. The device exhibited a low specific on-resistance of 9.3 mΩcm2 at VGS=70 V, resulting in an improved FOM ( ) of 85 MW/cm2. A large-area MOSFET with an active area of 4.26x10-2 cm2 can block up to 810V with a low leakage current of 21μA and conducted a high on-current of 1 A at VDS=3 V and VGS=50 V. The fabricated devices all exhibited the stable normally-off operation with threshold voltages of 5~6 V. Their subthreshold characteristics with high on/off ratios of 3~5 indicates that the MOSFETs are capable of operating stably as switching devices.Ph.D.Includes bibliographical references (p. 151-156)by Jian W