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.
SiC VJFETs are excellent candidates for reliable 100 high power/temperature switching as they only use pn junctions NE 1000 in the active device area where the high electric fields occur. Si VJFETs do not suffer from forward voltage degradation, exhibit E excellent short circuit performance, and operate at 300°C. a 100 0.19 cm2 1200 V normally-on and 0.15 cm2 low-voltage normallyoff VJFETs were fabricated. The 1200 V VJFET outputs 53 A with a forward drain voltage drop of 2 V and a low specific on-4H-SiC state resistance of 5.6 mQ cm2. The low-voltage VJFET outputs s. 10 38 A with a forward drain voltage drop of 3 V and a specific ono state resistance of 10 mQ cm2. The 1200 V SiC VJFET was . l connected in the cascode configuration with a Si MOSFET and .
Silicon Carbide (SiC) power transistors offer large benefits for high power, low loss power switching applications. The 10× increase in critical breakdown field and 2.5× increase in thermal conductivity of SiC compared to silicon make SiC a far superior material for power switching. The Vertical Junction Field-Effect Transistor (VJFET) possesses many advantages compared to other SiC power devices, including the lack of a critical SiC/SiO 2 interface, the speed of majority carrier switching, and the ability to reliably parallel many devices to achieve high currents. The challenge that VJFETs present is making them normally-off with low on-state resistance. In this work, we report the design and fabrication of normally-off SiC VJFETs with very low specific on-resistance (0.87 m -cm 2 ) and very high current levels (200 A). These are some of the highest total current levels and lowest specific on-state resistances reported in SiC power transistors to date. The devices block voltages ranging from 50-250 V and are optimized for use as the normally-off device in a high-voltage all-SiC cascode switch. Fabrication: The ion-implanted VJFET structure is shown in Figure 1 [1]. Baseline devices had a 2.7 m, 5x10 15 cm -3 drift and 1.4 m, 2x10 16 cm -3 channel. Gates and sources were defined by etching source pillars and then using the same mask to implant self-aligned, recessed Al gates. For a normally-off VJFET, the p + implants must be close enough that the built-in depletions regions pinch-off the channel with no applied gate bias. The total widths used to mask the implant ranged from 0.8-1.2 m. Ohmic contacts were formed using nickel silicide and interconnection done with gold. Individual device cells ranged from 6.4x10 -4 to 4.5x10 -3 cm 2 in area. Device characteristics: A baseline device blocking 60 V normally-off (V GS =0V) is shown in Figure 2a. By adjusting the spacing of the p + layers, normally-off blocking voltages up to 250 V have been achieved. Figure 2b show the on-state characteristics of the 60 V device. The specific on-state resistance at V GS =2.5 V, V DS =0.5 V is 1.74 m -cm 2 . Device yields were very high for the normally-off VJFETs, with values as high as 82% for a 3" wafer. Using device simulation, 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 50 100 150 200 250 300 350 400 Voltage (V) Current (mA) V GS =0 V -1 -2 -3 -4 -5 a b 0 200 400 600 800 1000 0 1 2 3 Drain Voltage (V) Drain Current Density (A/cm 2 ) V G =2.5 V V G =2 V V G =1.5 V Figure 2: (a) Normally-off VJFET blocking >50 V, and (b) on-state characteristics for same device.Figure 1: Vertical Junction FieldEffect Transistor (VJFET) structure.
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