Paul Potyraj

A 1680-V (at 1

A high-voltage normally ON 4H-SiC vertical junction field-effect transistor (VJFET) of 0.143- cm2 active area was manufactured in seven photolithographic levels with no epitaxial regrowth and with a single masked ion-implantation event. The VJFET exhibits low gate-to-source p-n-junction leakage current with relatively sharp onset of breakdown. At a drain-current density of 1 mA/cm2, the VJFET blocks 1680 V at a gate bias of -24 V. A self-aligned floating guard-ring structure provides edge termination that blocks 77% of the 11.8-mum SiC drift layers limit. At a gate bias of 2.5 V and a corresponding gate current of 2 mA, the VJFET outputs 53.6 A (375 A/cm2) at a forward drain voltage drop of 2.08 V (780 W/cm2). The transistor current gain is ID / IG = 26 800, and the specific on-state resistance is 5.5 mOmegamiddotcm2. To our best knowledge, this is the largest area SiC vertical-channel JFET reported to date and outputs more drain current than any 1200-V class vertical-channel JFET under identical heat-load and gate biasing conditions.

\hbox{mA/cm}^{2}

A normally on 4H-SiC vertical-junction field-effect transistor (VJFET) of 6.8-mm2 active area was manufactured in seven photolithographic levels with no epitaxial regrowth and a single masked ion-implantation event. The VJFET exhibits low leakage currents with very sharp onsets of voltage breakdowns. At a forward gate bias of 2.5 V, the VJFET outputs 24 A (353 A/cm2) at a forward drain-voltage drop of 2 V (706 W/cm2), with a current gain of ID/IG = 21818, and a specific ON-state resistance of 5.7 mOmegaldrcm2. Self-aligned floating guard rings provide edge termination that blocks 2055 V at a gate bias of -37 V and a drain-current density of 0.7 mA/cm2. This blocking voltage corresponds to 94.4% of the VJFETs 11.7-mum/3.46 times 1015-cm3 SiC drift layer limit and is the highest reported blocking-voltage efficiency of any SiC power device under similar drain-current-density conditions.

) 54-A (at 780

SiC VJFETs are excellent candidates for reliable high-power/temperature switching as they only use pn junctions in the active device area where the high-electric fields occur. VJFETs do not suffer from forward voltage degradation, exhibit excellent short-circuit performance, and operate at 300°C. 0.19 cm2 1200 V normally-on and 0.15 cm2 low-voltage normally-off VJFETs were fabricated. The 1200-V VJFET outputs 53 A with a forward drain voltage drop of 2V and a specific onstate resistance of 5.4mΩcm2. The low-voltage VJFET outputs 28 A with a forward drain voltage drop of 3.3 V and a specific onstate resistance of 15mΩcm2. The 1200-V SiC VJFET was connected in the cascode configuration with two Si MOSFETs and with a low-voltage SiC VJFET to form normally-off power switches. At a forward drain voltage drop of 2.2V, the SiC/MOSFETs cascode switch outputs 33 A. The all-SiC cascode switch outputs 24 A at a voltage drop of 4.7 V.

\hbox{W/cm}^{2}

High-voltage normally-on VJFETs of 0.19 cm2 and 0.096 cm2 areas were manufactured in seven photolithographic levels with no epitaxial regrowth and a single ion implantation event. A self aligned guard ring structure provided edge termination. At a gate bias of -36 V the 0.096 cm2 VJFET blocks 1980 V, which corresponds to 91% of the 12 μm drift layer’s avalanche breakdown voltage limit. It outputs 25 A at a forward drain voltage drop of 2 V (368 A/cm2, 735 W/cm2) and a gate current of 4 mA. The specific on-resistance is 5.4 mΩ cm2. The 0.19 cm2 VJFET blocks 1200 V at a gate bias of -26 V. It outputs 54 A at a forward drain voltage drop of 2 V (378 A/cm2, 755 W/cm2) and a gate current of 12 mA, with a specific on-resistance of 5.6 mΩ cm2. The VJFETs demonstrated low gate-to-source leakage currents with sharp onsets of avalanche breakdown.

) Normally ON 4H-SiC JFET With 0.143-

Electron-hole recombination-induced stacking faults have been shown to degrade the I-V characteristics of SiC power p-n diodes and DMOSFETs with thick drift epitaxial layers. In this paper, we investigate the effect of bipolar gate-to-drain current on vertical-channel JFETs. The devices have n- drift epitaxial layers of 12-μm and 100-μm thicknesses, and were stressed at a fixed gate-to-drain current density of 100 A/cm2 for 500 hrs and 5 hrs, respectively. Significant gate-to-drain and on-state conduction current degradations were observed after stressing the 100-μm drift VJFET. Annealing at 350°C reverses the stress induced degradations. After 500 hours of stressing, the gate-to-source, gate-to-drain, and blocking voltage characteristics of the 12-μm VJFET remain unaffected. However, the on-state drain current was 79% of its pre-stress value. Annealing at 350°C has no impact on the post-stress on-state drain current of the 12-μm VJFET. This leads us to attribute the degradation to a “burn-in” effect.

\hbox{cm}^{2}

High-voltage vertical-junction-field-effect-transistors (VJFETs) are typically designed normally-on to ensure low-resistance voltage-control operation at high current-gain. To exploit the high-voltage/temperature capabilities of VJFETs in a normally-off voltage-controlled switch, high-voltage normally-on and low-voltage normally-off VJFETs were connected in the cascode configuration. The cascode gate’s threshold voltage decreases from 2.5 V to 2 V as the temperature increases from 25°C to 225°C, while its breakdown voltage increases from -23 V to -19 V. At 300°C, the drain current of the cascode switch is 21.4% of its 25°C value, which agrees well with the reduction of the 4H-SiC electron mobility with temperature. The VJFET based all-SiC cascode switch is normally-off at 300°C, with its threshold voltage shifting from 1.6 V to 0.9 V as the temperature increases from 25°C to 300°C. This agrees well with the measured reduction in VJFET built-in potential. Finally, the reduction in cascode transconductance with temperature follows that of the theoretical 4H-SiC electron mobility. Overall, the measured thermally-induced cascode parameter shifts are in excellent agreement with theory, which signifies fabrication of robust SiC VJFETs for power switching applications.

Active Area

SiC VJFETs are excellent candidates for reliable high power/temperature switching as they only use PN junctions in the active device area where the high electric fields occur. VJFETs do not suffer from forward voltage degradation, exhibit excellent short circuit performance, and operate at 300degC. 0.19 cm2 1200 V normally-on and 0.15 cm2 low-voltage normally- off VJFETs were fabricated. The 1200 V VJFET outputs 53 A with a forward drain voltage drop of 2 V and a low specific on-state resistance of 5.6 mOmega cm2. The low-voltage VJFET outputs 38 A with a forward drain voltage drop of 3 V and a specific on- state resistance of 10 mOmega cm2. The 1200 V SiC VJFET was connected in the cascode configuration with a Si MOSFET and with a low-voltage SiC VJFET to form normally-off power switches. At a forward drain voltage drop of 2 V, the SiC/MOSFET cascode switch outputs 31 A and the all-SiC cascode switch outputs 19 A.