Among existing power semiconductor devices, thyristors are one of the oldest types, which have superior on-state losses and the highest power-handling capability. However, thyristors have many drawbacks, such as slow switching speed, complicated current gate control and di/dt and dv/dt limitations. It would be beneficial to be able to connect thyristors in parallel, but this results in current crowding, and generally the hottest device takes substantially all the current, resulting in thermal runaway.
In recent years there have been considerable improvements in the manufacture of IGBTs, which become the device of choice for the applications from 600V to 4000V. Advanced IGBTs enable fully controllable capability, simple MOS gate drive, fast switching performance and paralleling capability. Such devices do, however, have problems associated with them, as when the blocking voltage increases, the on-state voltage drop increases dramatically preventing their extending into the high voltage applications.
MOS-gated thyristors have attracted considerable attention for high power applications in the past two decades, as they may combine advantages of IGBTs and thyristors. The devices can potentially have good conduction properties, the high input impedance of a MOSFET, fast switching speed, improved di/dt and dv/dt ratings, and also potentially good serial and parallel connection capability. Moreover, the adoption of IGBT fabrication processes can lead to easier manufacturing and lower cost. However, it has proved difficult in practice to produce a device which reliably meets the conflicting requirements mentioned above.
Many device structures have been proposed, such as MOS-controlled thyristor and emitter-switched thyristor. Those proposed devices either have a high on-state voltage drop due to the addition of a MOSFET in series with the thyristor structure, or have a complex fabrication process and drive circuitry due to dual gates or additional silicon layers. Moreover it has proved difficult to adapt an IGBT process for the fabrication of thyristor devices. The modern IGBT fabrication adopts the processes of an IC foundry and the area of an IGBT chip is limited within 2 cm2 due to the demand for high yield. Therefore, thousands of cells are connected in parallel in a chip and several chips in a module. However, the paralleling of thyristors has many difficulties including both turn-on and turn-off failures, as well as uneven current sharing. Because of these and other problems, no MOS-gated thyristor is yet commercially available. Some recently developed thyristor-type devices, such as IGCT and ETO, also have respective drawbacks.
Background thyristor prior art can be found in U.S. Pat. No. 5,616,938 and U.S. Pat. No. 5,324,670. Further background prior art can be found in: U.S. Pat. No. 6,952,335; U.S. Pat. No. 6,933,541; U.S. Pat. No. 6,710,639; “Parallel operation of the emitter turn-off (ETO) thyristor”, Industry Applications Conf., 2002, Record of the 37th IAS Annual Meeting, Vol 4, pages 2592-2596, Xigen Zhou et al, 13-18th Oct. 2002; and “Series and parallel operation of the emitter turn-off (ETO) thyristor” IEEE Transactions on Industry applications, Vol 38, No 3, May/June 2002, pp 706-712, Yuxin Li et al.
The present invention seeks to provide an improved structure for power semiconductor devices, which addresses the aforementioned problems.