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Design Space of Vertical GaN Superjunction Power Transistors for 1.2–10 kV Applications

Schematic structures
Fig. 1. Schematic structures of the (a) GaN SJ-CAVET and (b) GaN SJ-FinFET proposed in this work.
Vertical superjunction devices could break the theoretical limits of on-resistance (Ron) and breakdown voltage (BV) in unipolar power devices and allow a linear dependence of Ron on BV. While no experimental demonstration of vertical gallium nitride (GaN) superjunction devices have been reported so far, Si superjunction devices have achieved huge commercial success. Our work explores the design space of vertical GaN superjunction field effect transistors (SJ-FETs) for 1.2 kV to 10 kV voltage classes, based on theoretical analysis and TCAD simulation. To obtain the best trade-off between Ron and BV, we identified and optimized key device designs, including the pillar width, doping concentration, superjunction thickness, etc. Based on the analysis, two novel vertical GaN SJ-FET with 2DEG and fin channels, i.e. a GaN superjunction current-aperture vertical electron transistor (SJ-CAVET) and a GaN superjunction fin field-effect-transistor (SJ-FinFET), were proposed and simulated. Fig. 1 shows the device structure schematics of two kinds of GaN superjunction devices. In addition, a well-calibrated physics-based TCAD simulation was used to design 1.7 kV, 50 A normally-off GaN SJ-CAVETs and SJ-FinFETs. Following, a device-circuit mixed-mode simulation was utilized for their switching performance. The device on-resistance (0.35 mΩ?cm2) and chip area (0.2 mm2) are at least 25-fold smaller than today’s best 1.7 kV, 50 A power transistors. Due to the smaller device area, the device junction capacitances and switching charges are also significantly smaller. A practical switching frequency of ~MHz was determined from the trade-offs between switching and conduction losses. The simulation of higher-voltage GaN SJ-CAVETs and SJ-FinFETs up to 10 kV reveal consistent advantages over their commercial counterparts. Fig. 2 shows Ron,sp∼VB trade-offs for GaN SJ-CAVETs and SJ-FinFETs designed for 1.2 kV∼10 kV power applications. Our simulation revealed at least 10-to-100-fold smaller Ron in 1.2 kV-10 kV vertical GaN SJ-FETs, compared to unipolar vertical GaN power transistors with the same BV classes. These results show the great potential of GaN SJ-CAVETs and SJ-FinFETs for future medium-to-high-voltage high-frequency power applications.
Specific resistance vs. brakedown voltage
Fig. 2. R(on,sp)~VB trade-offs for GaN SJ-CAVETs and SJ-FinFETs designed for 1.2 kV~10 kV power applications. Theoretical limits for SiC and GaN unipolar devices as well GaN superjunction devices, the experimental data for state-the-art GaN vertical transistors with unipolar drift regions and SiC MOSFETs are included.
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