| 4H-Silicon Carbide (4H-SiC) is a very promising semiconductor for high-power, high-temperature applications. Wider band-gap and higher value of critical electric field offer significant reduction in drift layer thickness and hence lower device on-resistance compared to silicon power devices of similar voltage ratings. High thermal conductivity will allow silicon carbide substrate to act as a natural heat sink, thus reducing weight and size of cooling systems in high-power circuits and modules.; Silicon Carbide technology made significant progress in the past 10 years, and many switching (three-terminal) devices (MOSFETs, BJTs, GTOs and JFETs) have been demonstrated. Voltage-controlled normally-off devices are particularly attractive for practical applications because of simplified gate-drive circuitry and fail-safe operation. Being free of inversion layer mobility and gate oxide reliability problems associated with MOSFETs, normally-off VJFETs are expected to fully utilize best properties of 4H-Silicon Carbide.; This dissertation presents design, fabrication and characterization of 1.2 kV and 3.0 kV normally-off planar VJFETs with implanted vertical channel. Device design incorporates a normally-off lateral JFET, which controls the current flowing through monolithically merged normally-on vertical JFET. Lateral JFET is formed by p-type implanted upper gate and epitaxial lower gate. No epitaxial re-growth is required for VJFET fabrication. Unit cell design, and detailed fabrication process are described, including use of thick-film photoresist for implantation masks, multi-step junction termination extension (MJTE) for device edge termination, activation annealing and others.; Static characteristics of the fabricated VJFETs are presented. Best devices demonstrated normally-off blocking voltage of 1535 V with specific on-resistance of 17 mO x cm2 at 3.5 V gate bias based on 15 mum drift layer, and 4340 V with specific on resistance of 40 mO x cm2 at 4.5 V gate bias based on 30 mum drift layer. Die- and wire-bonding of fabricated high-current VJFETs reduced device on-resistance substantially (10--50%) compared to on-chip testing.; Characteristics of the fabricated VJFETs compare very favorably with those of 4H-SiC MOSFETs of similar voltage ratings, and far surpass characteristics of silicon power switching devices. In fact, the VJFET of this thesis study achieved the best figure-of-merit among all normally-off SiC JFETs in the voltage range of about 4--5 kV. |
