Ultra-high voltage 4H-SiC bi-directional insulated gate bipolar transistors

Abstract

4H- Silicon Carbide (4H-SiC) is an attractive material for power semiconductor devices due to its large bandgap, high critical electric field and high thermal conductivity compared to Silicon (Si). For ultra-high voltage applications (BV >10 kV), 4H-SiC Insulated Gate Bipolar Transistors (IGBTs) are favored over unipolar transistors due to lower conduction losses. With improvements in SiC materials and processing technology, promising results have been demonstrated in the area of conventional unidirectional 4H-SiC IGBTs, with breakdown voltage ratings up to 27 kV.

Finally, the experimental results on fabricated SiC BD-IGBTs are presented. Prototype transistors were fabricated on novel, lightly doped n-type free-standing substrates. On Si-face, the BD- IGBTs showed good conductivity modulation, with a forward voltage drop (VF) of 9.7 V at 50 A/cm2 at room temperature, increasing to 11.5 V at 150 oC. On-state performance in third quadrant operation was limited by high threshold voltage on C-face. We have also demonstrated control over minority carrier injection by using a backgate bias, which can be used to drastically reduce switching losses.

Second, the process technology necessary for the fabrication of high voltage SiC BD-IGBTs is optimized. The effect of different process steps on parameters such as breakdown voltage, carrier lifetime, gate oxide reliability, SiO2-SiC interface charge density is quantified. A carrier lifetime enhancement process has been optimized for lightly doped 4H-SiC free-standing substrates (FSS), with long carrier lifetimes up to 10 µs at room temperature. The FSS wafer technology and double sided, ion implanted process used in this research has wide applicability beyond that of BD-IGBTs. As compared to previous reports using epitaxial P+ collectors, this process can be easily adapted for conventional IGBTs, as well as reverse blocking and reverse conducing IGBTs.

First, the performance limits of SiC IGBTs are calculated by using analytical methods. The performance benefits of SiC IGBTs over SiC unipolar devices and Si IGBTs are quantified. Numerical simulations are used to optimize the unit cell and edge termination structures for a 15 kV SiC BD-IGBT. The effect of different device parameters on BD-IGBT static and switching performance are quantified.

This research presents the experimental demonstration of the world’s first high voltage bi-directional power transistors in 4H-SiC. Traditionally, four (two IGBTs and two diodes) or two (two reverse blocking IGBTs) semiconductor devices are necessary to yield a bidirectional switch. With a monolithically integrated bidirectional switch as presented here, the number of semiconductor devices is reduced to only one, which results in increased reliability and reduced cost of the overall system. Additionally, by using the unique dual gate operation of BD-IGBTs, switching losses can be reduced to a small fraction of that in conventional IGBTs, resulting in increased efficiency.