MOSFETs Withstand Stress of Linear-Mode Operation For applications like electronic loads that require power MOSFETs to operate in their linear region , a novel transistor structure and process technol

P ower MOSFETs are most often used in switchedmode applications where they function as onoff switches. But in applications like electronic loads, linear regulators or Class A amplifi ers, power MOSFETs must operate in their linear region. In this operating mode, the MOSFETs are subjected to high thermal stress due to the simultaneous occurrence of high drain voltage and current, resulting in high power dissipation. When the thermo-electrical stress exceeds some critical limit, thermal hot spots occur in the silicon causing the devices to fail. To prevent such failure, MOSFETs operating in the linear region require high power dissipation capability and an extended forward-bias safe operating area (FBSOA). A series of linear power MOSFETs developed by IXYS achieves an extended FBSOA capability by suppressing the positive feedback of electro-thermal instability. The design of these new MOSFETs features a nonuniform distribution of transistor cells, as well as cells with different threshold voltages. Every transistor cell is designed with a ballast resistor at the source to limit its current. The parasitic bipolar junction transistor (BJT) of each cell is heavily bypassed so that it will not turn on under extreme electrical stress conditions. In addition, the thermal response of each power MOSFET is tested to assure no solder voids. The linear MOSFET’s effectiveness can be demonstrated in the design of an electronic load developed for power-supply testing.