Reliability and Performance Limitations in SiC Power Devices
Ranbir Singh, Allen R. Hefner Jr.
Despite Silicon Carbide's (SiC's) high breakdown electric field, high thermal conductivity and wide bandgap, it faces certain reliability challenges when used to make conventional power device structures like power MOS-based devices, bipolar-mode diodes and thyristors, and Schottky contact based devices operating at high temperatures. The performance and reliability issues unique to SiC discussed here include: (a) MOS channel conductance/gate dielectric reliability trade-off due to lower channel mobility as well as SiC-SiO2 barrier lowering due to interface traps; (b) reduction in breakdown field and increased leakage current due to material defects; and (c) increased leakage current in SiC Schottky devices at high temperatures. Although a natural oxide is considered a significant advantage for realizing power MOSFETs and IGBTs in SiC, devices to date have suffered from poor inversion channel mobility. Furthermore, the high interface state density presently found in the SiC-SiO2 system causes the barrier height between SiC and SiO2 to be reduced, resulting in increased carrier injection in the oxide. A survey of alternative dielectrics shows that most suffer from an even smaller conduction band offset at the SiC-dielectric interface than the corresponding Silicon-dielectric interface and have a lower breakdown field strength than SiO2. Thus, an attractive solution to reduce tunneling such as stacked dielectrics is required. In Schottky-based power devices, the reverse leakage currents are dominated by the Schottky barrier height, which is in the 0.7-1.2 eV range. Because the Schottky leakage current increases with temperature, the SiC Schottky devices have a reduction in performance at high temperature similar to that of Silcon PN junction-based devices, and they do not have the high temperature performance benefit associated with the wider bandgap of SiC. Defects in contemporary SiC wafers and epitaxial layers have also been shown to reduce critical breakdown electric field, result in higher leakage currents, and degrade the on-state performance of devices. These defects include micropipes, dislocations, grain boundaries and epitaxial defects. Optical observation of PN diodes undergoing on-state degradation shows a simultaneous formation of mobile and propagating crystal stacking faults. These faults nucleate at grain boundaries and permeate throughout the active area of the device, thus degrading device performance after extended operation.
alternative dielectrics, high voltage, IGBT, interface state density, MOSFET, PiN diode, power device, reliability, Schottky, Silicon Carbide, stacking faults., thyristors