Reliability Testing of 4H-SiC Bipolar Junction Transistors in Continuous Switching Applications

Continued improvement in silicon carbide (SiC) material processing has allowed development of efficient high temperature devices which are uniquely suited to power electronics circuit designs. The 4H-SiC structure has several intrinsic characteristics that facilitate optimal speed and power handling during high temperature device operation. These characteristics include wide bandgap (3.2 eV), high dielectric breakdown (3.5 MV/cm), and high thermal conductivity (4.9 W/cm-K)[1,2]. By combining these properties, SiC bipolar junction transistors (BJTs) can achieve fast, low impedance switching at high voltages (1.2 kV). New generation devices are being developed with increased current handling capability, as well as improved forward voltage characteristics. The device considered here, along with its on-state DC characteristic, is shown in figure 1. The BJTs are approximately 5mm by 5mm, and are nominally rated for a maximum Ice of 50A. Measurements on the Tektronix 371B curve-tracer indicate current gains over 60 at 25 oC and roughly 40 at 150 oC. These results were obtained at collector currents up to 20A. The base current for BJTs is typically 300 to 800 mA, depending on device temperature and the maximum device current required. In order to meet current handling requirements of up to 80A, as required for power conversion in modern military systems such as the hybrid-electric vehicle (HEV), it is necessary to configure these devices in parallel with minimal external cooling. The resulting switching circuits must therefore be validated for operation at high temperatures (package temperatures of 90 oC, and junction temperatures to 150 oC). Validation includes characterization of the devices in clamped inductive circuits with devices configured both alone and in parallel over time. Figure 4 shows measurement waveforms obtained during continuous clamped inductive switching. The primary focus of this work is to establish the overall performance and reliability of these newer generation SiC BJTs in power conversion circuits. Failure analysis and critical performance issues, such as current sharing, energy loss, and total reverse recovery charge are addressed. The initial results of the experiments indicate that these SiC switches have the potential to perform reliably in high temperature power conversion.