FROM EDN EUROPE: MEMS technology builds faster smart power chips
By Graham Prophet -- EDN, January 6, 2005
Cambridge Semiconductor is a UK-based start-up that plans to bring a new generation of "smart" power switch devices, for use in offline converters and power supplies, to market in the first half of 2005. The technology merges aspects of MEMs fabrication with power device technology to produce a significant incremental improvement in device characteristics, that will enable higher densities and efficiencies, through the use of higher switching speeds, than is possible with today's FETs and other switches.
PowerBrane devices will combine a lateral insulated gate bipolar transistor (L-IGBT) together with gate drivers and control and protection circuitry, on a single die. The LIGBT itself is expected to deliver today's current densities, at 30 A/cm2, and switching speeds of up to 10× today's values. LIGBTs offer a potential way to improve on the performance of the MOSFETs commonly used at present, but have been limited by issues of isolation and low breakdown voltage. In CamSemi's process, the silicon wafer is back-etched under the buried-oxide in the drift region of the LIGBT. The wafer is thinned until the (conductive) silicon is removed from the underside of the oxide leaving just a membrane - from which the device name is derived. Although the technique is derived from MEMS processing, there are no moving parts. LIGBTs suffer low breakdown voltages because the conducting silicon under the drift region causes compression of the electric field lines as they pass through the active region of the device, leading to high field strengths and punch-through. With no silicon, there is much reduced field strength and the breakdown voltage can be increased 20× to 650V, suitable for off-line switching. The device also switches off in 40-50 nsec (at 400V and 0.5A) due to reduced capacitances, which CamSemi's co-founder Dr. Florian Udrea says is faster than MOSFETs, and allows a switching speed of 500 kHz to be used. The process is completely CMOS-compatible, and can host high gate counts of logic alongside the power switch. This will enable complex devices such as lighting controllers to be built on one chip. CamSemi will introduce a range of switches for use in the range 1 - 1000W.
Cambridge Semiconductor, +44 1223 446450, www.camsemi.com.
