I know one 9v battery wont run the 12V solenoid, I think that is why they used 2. Even so, Im sure two 9v batteries might run it for just a little while.
I doubt it, you have been told, you will learn the hard way I suspect.
So back to the mosfet and bjt.
I understand one difference is that bjt's are current driven whereas mosfets are voltage driven.
- Voltage driven, thus doesnt waste current at the Gate, which means less overheating?
The main reasons are that the Vsat can be a low lower, and switching losses are much less.
- Doesnt require a resistor in series with the gate, why?
Not that simple - the gate is a large value capacitor, effectively, so if the chip driving it
can't handle large current pulses a resistor may be needed to prevent overload. If driving
several MOSFET gates in parallel resistors may be needed to prevent/damp an oscillation mode.
Sometimes resistors are added to slow down the switching somewhat to reduce EMI generation.
- Does require a pulldown resistor so the 'gate doesnt float on reboot'. Why?
Because the gate is a capacitor, it can retain its charge from last time and turn on unexpectedly,
or be half-on-half-off and burn up.
- Better for high power applications because switching is faster. Again, why?
Because its a majority carrier device, unlike a BJT which uses minority carriers and is
dependent on carrier-recombination to turn off quickly (but fast recombination means a
lower gain device)
- Are better for low power applications like LEDs?
No, but they may be cheaper in through-hole package.
I guess I want the underlying explanation of why mosfets are faster and waste less current and why that makes them better for high power applications?
They are more efficient, much faster at high power, don't suffer from secondary breakdown,
need less drive power, however they are substantially less rugged - overvoltage on the gate
will damage/destroy the device very easily. Typically you use a MOSFET driver chip to
provide protection from this as well as allowing the fastest switching.