> As a side note, how come BJT's aren't used for this kind of application?

        BJTs are, and have been, used for this sort of thing for years. The are
actually some applications where a BJT would be a superior device. That
said, BJTs aren't used as often these days mainly because of base current.
With a MOSFET you don't need to supply any DC current to the device's gate
terminal (you do need current to discharge and charge the gate capacitance
but that isn't of much concern when switching very slowly). With a power BJT
you do. How much depends on how much current you are supplying to the load
and what the Beta of the BJT is. For higher currents it is possible to have
Betas as low as 10 (even lower). That means to control 1A, you need ~100mA
of current flowing into the base. As you can see that can be a big problem,
often you will need another transistor driving the power transistor!

        As an aside, there are some devices that take advantage of both worlds. For
example, IGBTs (insulated gate bipolar transistors) have a MOSFET
controlling a BJT. You get the benefit of no gate current and the benefits a
BJT gives you.

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