Medium Voltage Hybrid Si IGBT/SiC JBS Diode Module Development

Abstract

Wide bandgap semiconductors are an obvious choice for high voltage applications due to their large critical electric field. While both SiC and GaN will support large electric fields, approximately 10x that of the dominant power semiconductor Si, the wide availability of SiC substrates up to 150 mm diameter provides a cost effective platform over GaN for which even 50 mm substrates are in high demand. Besides substrate technology, high growth rate epitaxy, ion implantation and anneal technologies are well established for SiC. The typical insertion path begins with majority carrier devices such as Schottky diodes which are robust and relatively simple to manufacture. Junction barrier Schottky diodes have been shown to be stable at 10kV [1] and are commercially available up to 1.7 kV. The obvious application for JBS diodes is as a substitute for Si PiN diode freewheeling diodes. The key advantage is the transition from the minority carrier Si bipolar device to the majority carrier unipolar SiC device. This transition to the SiC Schottky virtually eliminates the reverse recovery effects typical of Si PiN diodes. Further, the adoption of the junction barrier Schottky structure results in low reverse leakage current and stable blocking voltages. In this work medium voltage, 4.5 kV SiC JBS diodes matched to commercially available 4.5 kV IGBTs are joined to form efficient hybrid modules. Dynamic and static characteristics as well as reliability phenomena are evaluated. Compared to the all-Si module, the hybrid module shows up to a factor of six reduced turn-on loss as is demonstrated in Fig. 1 where 4.5 kV Si PiN diode compared to a SiC JBS diode. The recovery current is shown on the left and the impact on the IGBT turn-on characteristics is shown on the right. Through simulations and hardware testing the modules have been optimized for the IGBT/JBS die are ratio in terms of total power dissipation. Other considerations include surge capability which is limited by the JBS diode and the reverse blocking voltage of the IGBT. An external circuit that clamps the voltage across the diode is developed and demonstrated greatly increasing the surge rating. Reliability considerations such as high temperature dV/dt and HTRB are assessed and will be presented.Overall, the hybrid modules are extremely efficient and robust. Total power losses are reduced by one-half by replacing only the freewheeling Si PiN diode with a SiC JBS diode. The optimized solution reduces the needed SiC die area and thus module costs. Reliability assessments show that the modules are robust.