An two separate SOAs are used in conjunction with BJT i.e Forward Bias Safe Operating Area (FBSOA) as shown in fig.1 and Reverse Bias Safe Operating Area (RBSOA) as shown in fig.2

The terms forward bias and reverse bias refer to whether the base current bias source forward biases the B-E junction or reverse biases it.

Several different physical mechanisms are active in determining the boundaries of the FBSOA shown in fig.1 The current $I_{C M}$ is the maximum collector current even as a pulse that should be applied to the transistor. Exceeding this current may cause bonding wires or metalizations on the wafer to vaporize or otherwise fail. The thermal limit is a power dissipation limit set by the thermal resistance of the transistor and the maximum allowable junction temperature. The second breakdown boundary represents the maximum permissible combinations of voltage and current without getting into the region of the $i_{C}-v_{C E}$ plane where second breakdown may occur. The final portion of the boundary of the FBSOA is breakdown voltage limit $\mathrm{BV}_{C E O}$.

If the transistor is operated as a switch, then the boundaries of the FBSOA expand, as is indicated in the figure. The expansion of the SOA occurs for switch-mode operation because the silicon wafer and its packaging have a thermal capacitance and, hence, an ability to absorb a finite amount of energy without the junction temperature rising to excessive levels. If the transistor turns on in a few microseconds or less, the amount of energy that is absorbed is too small to cause any appreciable rise in the junction temperature, and as a result, the FBSOA is essentially square, being limited only by $I_{CM}$ and $BV_{ CEO}$

In a similar fashion. the RBSOA shown in fig.2 is constructed. The area encompassed by the RBSOA, which is a pulsed SOA, is somewhat larger than the FBSOA because of the extension of the area of higher voltages than $BV_{CEO}$ up to $BV_{CBO}$ at low collector currents. The operation of the transistor up to the higher voltage is possible because the combination of low collector current and reverse base current has made the beta small so that the breakdown voltage rises toward $BV_{CBO}$.