Reliability of High Voltage 4H-SiC MOSFET Devices
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0911-B13-01
Reliability of High Voltage 4H-SiC MOSFET Devices Sumi Krishnaswami1, Sei-Hyung Ryu1, Bradley Heath1, Anant Agarwal1, John Palmour1, Aivars Lelis2, Charles Scozzie2, and James Scofield3 1 SiC Power Device, Cree, Inc., 4600 Silicon Drive, Durham, NC, 27703 2 Army Research Labs, 2800 Powder Mill Road, Adelphi, MD, 20783 3 Air Force Research Labs, 1950 Fifth St, Wright-Patterson AFB, OH, 45433 ABSTRACT The commercialization of 4H-SiC MOSFETs will greatly depend on the reliability of gate oxide. Long-term gate oxide reliability and device stability of 1200 V 4H-SiC MOSFETs are being studied, both under the on- and off-states. Device reliability is studied by stressing the device under three conditions: (a) Gate stress - a constant gate voltage of +15 V is applied to the gate at a temperature of 175°C. The forward I-V characteristics and threshold voltage are monitored for device stability, (b) Forward current stress – devices are stressed under a constant drain current of Id = 4 A and Vg = 20 V. The devices were allowed to self-heat to a temperature of Tsink = 125°C and the I-V curves are monitored with time, and (c) High temperature reverse bias testing at 1200 V and 175°C to study the reliability of the devices in the off-state. Our very first measurements on (a) and (b) show very little variation between the pre-stress and post-stress I-V characteristics and threshold voltage up to 1000 hrs of operation at 175ºC indicating excellent stability of the MOSFETs in the on-state. In addition, high temperature reverse bias stress test looks very promising with the devices showing very little variation in the reverse leakage current with time. INTRODUCTION The development of SiC power MOSFETs in fast switching applications has come a long way. At Cree, MOSFETs in 4H-SiC with a record low value for the specific on-resistance (8 mΩ-cm2) and superior switching performance have been recently demonstrated [1]. Device reliability and stability are currently being studied. The concern about the gate oxide reliability arises because of the smaller barrier height between the conduction band of SiC and the conduction band of the oxide, due to the wider bandgap of 4H-SiC, as shown in Figure 1. From the band diagrams, the barrier height from the 4H-SiC conduction band to the oxide conduction band is only 2.7 eV as compared to 3.1 eV in a silicon MOS system. Under low gate voltages, corresponding typically to oxide electric fields of Eox ≤ 3 MV/cm, the gate current flowing in the oxide is negligible. As the gate voltage increases, the electric field in the oxide also increases, resulting in large current flow due to Fowler-Nordheim tunneling of the carriers into the oxide from the conduction of the SiC. Large gate currents in the oxide can result in premature breakdown, leading to failure of the device. The reduction of the barrier generally occurs under high electric fields and elevated temperature [2]. In a 4H-SiC vertical power MOSFET structure, there is concern about MOS reliability under the following two conditions: (1
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