The reason is, that BJTs can show very high collector emitter saturation voltage at higher collector currents. Also, collector emitter saturation voltage of BJTs suffers from very high manufacturing tolerances, as a look into databook proves.

An example:
BC337 (or BC817 as SMD) can handle collector currents of up to 1000mA. But collector emitter saturation voltage can be more than 1V at this current! Even at much lower collector currents, from 100µA to 100mA namely, collector emitter saturation voltage can be higher than 0.15V. This would be a loss of supply voltage of 3%, which is much for a 5V powered application.
Of course, typical collector emitter saturation voltage is much lower, only about 0.025V at 10mA collector current, for instance, but you must check this first, which makes it more expensive than taking a MOSFET right from the beginning.


This is only the static current consumption!! Take care, the TA4427 is a 1.5A dual high-speed power MOSFET driver. So, for brief periods up to 3A can flow through the supply pins and through your turn-off transistor!! Especially if the 10µF decoupling cap is a bit older or is omitted by mistake.

Other advantages of MOSFETs are:

1. They don't need static power to be controlled, just in opposite to BJTs.

2. They often contain already a protection diode from drain to source, which can be very helpful in certain applications.

So, you see, using a MOSFET makes sense.

Kai