Power devices operated in integrated circuits typically operate with a voltage in the range 20V to 1.2 kV and typically higher than 30V or 50V or so. Power devices typically operate with a current in the range 10 mA to 50 A and typically higher than 0.1 A and smaller than 5 A. Such devices may also be referred to as “high voltage/power devices”. These devices are typically capable of delivering from a few mWatts to 1 Watt or even a few tens of Watts of power. Their application may range from domestic appliances, electric cars, motor control, and power supplies to RF and microwave circuits and telecommunication systems.
Lateral devices in integrated circuits have the high voltage/low voltage main terminals (variously called the anode/cathode, drain/source and emitter/collector) and the control terminal (termed the gate or base) placed at the top surface of the device in order to be easily accessible. In power ICs, such devices are often monolithically integrated with CMOS-type or BiCMOS-type low voltage and/or low power circuits and therefore it is desirable that the lateral high voltage devices are CMOS compatible. It is also possible that several high voltage, power devices are integrated within the same chip.
(It will be appreciated that terms such as “top” and “bottom”, “above” and “below”, “lateral” and “vertical”, and “under” and “over”, “underlying”, etc. may be used in this specification by convention and that no particular physical orientation of the device as a whole is implied).
MOS bipolar power devices, such as the LIGBT shown in FIG. 1, are based on MOS control with bipolar current conduction in the lowly-doped drift layer or region of the device. Such devices are based on the conductivity modulation concept. At high levels of charge injection, when the current in the device increases, a mobile charge of electrons and of holes is built up in the drift layer, leading to a desirably sharp increase in the conductivity of the drift layer. The mobile charge accumulated, known as plasma, in the on-state dictates the on-state/switching performance of the device given that the plasma must be removed in order to switch the device to the off-state.
The field of LIGBTs therefore continues to provide a need for an LIGBT having improved characteristics, for example increased switching speed and/or a wider range of operating conditions (e.g., any combination of one or more predetermined range of continuous and/or switching current between main terminals, voltage between main terminals, junction and/or ambient temperature, etc.).
For use in understanding the present invention, the following disclosures are referred to:                U.S. Pat. No. 7,381,606 (corresponding to application U.S. Ser. No. 11/783,966, which is related to application U.S. Ser. No. 11/133,455 (U.S. Pat. No. 7,301,220)), F. Udrea, Cambridge Semiconductor Ltd., published Mar. 20, 2008;        WO-A-02/25700, Udrea, Cambridge Semiconductor Ltd., published 2006-03-02;        U.S. Pat. No. 6,703,684, Udrea, Cambridge Semiconductor Ltd., published Apr. 11, 2002;        US-A-2004-0084752, Udrea, Cambridge Semiconductor Ltd., published May 6, 2004;        US-A-2004-0087065, Udrea, Cambridge Semiconductor Ltd., published May 6, 2004; and        Microelectronics Reliability Vol. 39, Issues 6-7, June-July 1999, Pages 863-868, J. A. van der Pol et al.        