Hitachi Ltd. says it has developed a new kind of silicon-carbide-based MOSFET that has dramatically lower on resistance than comparable devices. The new MOSFET uses what Hitachi calls a TED-MOS structure, for Trench-etched DMOS-FET. Compared to conventional DMOS-FET (Double-diffused MOSFET) devices, the new structure is said to reduce electric field strength, an index of durability by 40% and resistance by 25%. Hitachi expects to the apply the new power devices in motor drives of electric vehicles.
The problem that the new power devices address is that the resistance of a SiC power semiconductor varies greatly depending on the crystal plane. Trench SiC MOSFET structures have been proposed as a way to let current flow on the crystal plane at a lower resistance than in conventional DMOS-FETs. But it has proven to be difficult to come up with a trench structure that is sufficiently durable because electric fields concentrate at the edges of the trench on the base plane, Hitachi says.
In contrast, the new fin-structure DMOS-FET, dubbed TED-MOS, employs a smaller trench pitch to reduce resistance and sees lower electric fields when operating at 3.3 kV. Hitachi uses a field relaxation layer (FRL) to reduce the electric field strength, and a current spreading layer (CSL) to reduce resistance in the n-JFET region. This region serves to form the electric current path connecting the sides fo the fin-like trenches as low-resistance crystal planes and the n-JFET region. Consequently, the device realizes both lower on resistance and electric field strength.
Hitachi prototyped the concept on a device that experienced a electric field strength 40% lower than that of a conventional DMOS-FET and a resistance 25% less. The prototype also had a rated operating voltage of 1.2 kV. Hitachi also reports the prototype saw faster switching speeds which resulted in a 50% improvement in energy loss.
Researchers presented these results at the International Symposium on Power Semiconductor Devices and ICs (ISPSD) in Chicago, and also at the European Conference on Silicon Carbide and Related Materials (ECSCRM) in the UK.