The area where transistors and MOSFETs are not destroyed or prone to deterioration is called the safe operating area (SOA). It is important that the operating point is within this curve. In the case of a bipolar transistor, the curve in the SOA is divided into a thermal dissipation limit and the limit given by secondary breakdown. In the case of a MOSFET, the SOA is divided into a current limit, a thermal dissipation limit, a limit given by secondary breakdown, and a voltage limit. Recently, there are products for which an on-resistance limit area is specified in the data sheet.
Maximum Ratings: Power MOSFET Application Notes (PDF:1,075KB)