FETs are good switches. They don't depend on current through the gate, just voltage. There's no "gain" formula. Once you cross the threshold sufficiently and the transistor is conducting, its impedance (from drain to source) is fairly consistent. The way I see it, the only advantage bipolar transistors have is that they can be turned on with very low voltages at the base, provided the resistance is low enough to sink enough current.Since your reset pull-up can be tied to Vcc, all you need to do is diode-isolate the battery at the source. Then, Vcc (and thus reset) are both at either 3.7v (on batter), or 5v (with external power applied). As long as the external voltage is higher than the battery, the diode will be reverse biased, effectively disconnecting the battery. A FET switch here would be fun, but as long as you can deal with the voltage drop, and ensure the external supply is always higher (or won't be offended by back charge from the battery), a diode will do the trick.
At low currents the resistance of a FET causes a small voltage drop across it. At large currents that voltage drop can rise quite high. At that point a BJT becomes much more desirable, as the voltage drop will be the same regardless of the current through it.
Quote from: majenko on Jan 15, 2013, 11:16 pmAt low currents the resistance of a FET causes a small voltage drop across it. At large currents that voltage drop can rise quite high. At that point a BJT becomes much more desirable, as the voltage drop will be the same regardless of the current through it.That's all very well in theory, but in practice mosfets are better at large currents too. For example, if I want to switch 100A, then I can get a mosfet with 0.001 ohm Rds(on) such as http://www.farnell.com/datasheets/356273.pdf. The voltage drop will be 0.1V so power dissipation 10W. But the only BJTs I can find rated at 100A have Vce(sat) around 1V, so ten times higher voltage drop (and over 100W power dissipation) - and that's with 10A base drive.
Anyway, 100A is nothing I never said how much current... Try finding a MOSFET that can switch 10KA at 320KV...