Demonstration of a 700 volt asymmetrical, 4H-SiC gate-turn-off thyristor (GTO)

The potential of SiC-based power switching devices has been well recognized due to its superior materials properties relative to silicon. These properties (which include approximately 10x higher breakdown voltage, 3x higher band gap, 3x higher thermal conductivity and 2x higher electron saturated velocity) have the potential to yield significant size, weight and cost reductions as well as efficiency improvements at the systems level for commercial/military power utility and motor control applications. Power MOSFETs with blocking voltages of 760 V and on-resistance of 66mW-cm(2) have been reported(1). However, the low conductance and oxide reliability concerns of MOSFETs2 combined with the low forward drop, high current handling and high temperature capabilities of thyristors(3) make SiC thyristors more attractive for high power applications. 900 Volt/2 Amp npnp symmetrical thyristors have previously been reported using 4H-SiC.(3) These devices were shown to operate up to 1000 A/cm(2) at room temperature and at temperatures of up to 500 degrees C. However, simple thyristors rely on the presence of a negative AC cycle to turn off the device. Thus, they cannot be used in DC power switching applications without an independent gate turn off capability possessed by a Gate Turn-Off thyristor (GTO). We report here progress on asymmetrical GTOs fabricated in 4H-SiC. The asymmetrical structure is easier to turn-off and has improved breakdown voltage capability compared to the symmetrical structure. The devices fabricated in this study have forward blocking voltage of 700 V in air even though the structures did not have any advanced edge termination. A forward voltage drop of 4.8 V was measured at 350 degrees C for a current density of 500 A/cm(2). Four GTO cells were combined to switch 1.4 amps. Storage times at turn-off were measured to be <100 ns at a current density of 500 A/cm(2), corresponding to a turn-off current gain of 1. In addition, 8 devices were combined in a package yielding 5 amps, indicating that current hogging is not a problem in these devices.