fan motor driver
This is not only when driving a fan, but in all applications where a transistor is working as a switch, it should be driven to saturation (ON), as the Vce is the lowest and therefore power lost in this transistor (Vce * Ic) is the lowest, and in this case power = heat, so the transistor will not overheat ..
To drive the transistor to saturation also means that most of the voltage appears across the load, and that in saturation it can conduct its maximum current ..
Regards,
IanP