Datasheets and other documents for bipolar transistors contain a safe operating area (SOA) graph. What is it?

The safe operating area (SOA) is defined as the current and voltage conditions over which a bipolar transistor can be expected to operate without suffering damage or degradation. The bipolar transistor must not be exposed to conditions outside the safe operating area even for an instant. Separate SOA curves are plotted for DC and different pulse width conditions. In the case of continuous-pulse operation, it is necessary to ensure that the bipolar transistor will not exceed not only the SOA for single-pulse operation but also the rated junction temperature. For an example of junction temperature calculation, see Bipolar Transistor Application Note: Thermal Stability and Thermal Design.

The borders of the SOA are straight lines bound by: 1) collector current limitation, 2) thermal breakdown limitation, 3) secondary breakdown limitation, and 4) collector-emitter voltage limitation.