There are two modes of safe operating area (SOA).
(1) Forward Bias SOA (F.B. SOA): Usable area of current and voltage at ON status.
(2) Reverse Bias SOA (R.B. SOA): Usable area of current and voltage at turn-off operation.
Applied pulse width is very narrow because of use under switching operation.
Each mode can be defined as shown in Fig. 3-12(a).