Improvement in breakdown characteristics of 4H-SiC MESFET with a gate-drain surface epi-layer and optimization of the structure

A novel SiC MESFET structure with a p-type surface epi-layer is proposed and 4H-SiC MESFET models are presented which precisely describe the working mechanism of the device. Considering carrier velocity saturation, impact ionization and electric field modulation, the effect on distribution of electric field is analyzed. Also, the output current (I-ds) and breakdown voltage (V-B) dependences on the dimensions of the p-type epi-layer are studied based on abrupt junction approximation. The high electric field peak at the gate edge is suppressed by introducing a new electric field peak at the drain side, and the built-in field of p-n junction formed along channel surface further weakens the electric field peaks, leading to smoother distribution of electric field. By comparison with the conventional and the fieldplated MESFETs, it is shown that the proposed structure greatly improves the characteristic of SiC MESFET. In addition, the optimized dimensions are obtained and the results show that V-B is greatly increased by 33% with I-ds unchanged (less than 3%) when the thickness and the doping concentration of the surface epi-layer are chosen as 0.12 mu m and 5 x 10(15) cm(-3), respectively.