4H-SiC Vertical Tri-gate Power MOSFETs Technology Development

Advances in power electronic systems, especially those in hybrid and electric automobiles and renewable power generation systems, demand high blocking voltage, fast switching performance and low thermal budget from power semiconductor devices. State-of-the--art, silicon based power semiconductor devices are limited by material properties in meeting these demands. Due to the relatively low critical electric field, the on-resistance of the devices is high, and increases significantly with blocking voltage. As a result, current silicon (Si) power MOSFETs rated at above 600 V suffer from unacceptably high conduction losses. Innovative designs, such as the insulated gate bipolar transistor (IGBT), have been developed which use conductivity modulation through the injection of minority carriers to reduce on-resistance. But the involvement of minority carriers gives rise to stored charge and a turn-off delay, dramatically increasing switching losses compared to unipolar devices. Silicon carbide (SiC), a wide band gap semiconductor provides an alternative to Si, and offers a 7x higher electric field strength, 2x higher saturation velocity, and 3x higher thermal conductivity. A thinner, more heavily doped drift region is required for a SiC power device for a particular voltage, which reduces on-resistance and power consumption. However, the channel resistance of SiC metal oxide semiconductor field effect transistors (MOSFETs) is high due to the poor quality of the dielectric-semiconductor interface. Thus the SiC MOSFET fails to live up to the full promise of the material. Minimization of the channel resistance is essential, especially for applications requiring blocking voltages under 1 kV, where this component dominates others. In this work, a novel tri-gate SiC MOSFET is proposed to address this issue. This new structure utilizes both the conventional horizontal surface as well as the sidewalls of a trench to increase the effective width of the channel without increasing the device area. With proper optimization, it should be possible to achieve 3x lower specific on-resistance compared to current SiC unipolar power devices.