Transistor Data Sheet -

Use: ( T_J = T_ambient + (P \times R_thJA) ). If ( T_J ) exceeds max, you need a heatsink or lower power.

For high-speed applications (like PWM or Radio Frequency), you need to look at: Cibocap C sub i b o end-sub Cobocap C sub o b o end-sub transistor data sheet

Typical graphs include:

Pro tip: The SOA graph is the single most ignored yet most critical graph for power transistors. Use: ( T_J = T_ambient + (P \times R_thJA) )

(Thermal Resistance, Junction-to-Ambient): How well the device sheds heat into the air without a heatsink. RθJCcap R sub theta cap J cap C end-sub transistor data sheet