@dodec bipolar junction transistor, the first type of transistor widely used. the other main type, FETs, are more commonly used now, but BJTs have their strengths
-F
@Felthry I'm curious now. What are they handy for?
On the relative merits of FETs and BJTs (long)
@terrana They're easier to use, in general.
- MOSFETs you need either a bipolar drive (i.e. capable of sourcing *and* sinking current) or an external pulldown resistor (which limits how quickly the FET can turn off), while BJTs you just need a current limit resistor.
- MOSFETs are in an undefined state while the gate isn't being driven, while BJTs are just off while the base isn't being driven.
- MOSFETs are very easily damaged by static electricity, while BJTs are largely immune to it
- MOSFETs, JFETs, and IGBTs have a threshold voltage that's fairly high, and have to be driven, in general, with signals on the order of 10~20 volts unless designed for logic-level input (and you can't design power FETs for logic-level input), while BJTs only need a diode drop, about 0.7 volts or so (double that for a darlington pair), to turn on fully
- MOSFETs and IGBTs (those with source tied to body anyway) inherently contain a diode between the source and drain, so they can only switch current in one direction and pass it in the other. BJTs can switch in either direction, but they're much less efficient in reverse (but they can still block reverse current)
That's not to say that BJTs are just better. They are nowhere near as efficient, since they require a current to turn on rather than a voltage, and the saturation voltage of a BJT is much higher than the drain-source voltage of a MOSFET at the same current (until you get to absurd currents, where the BJT's vce = log(current) + vcesat can technically be less than the MOSFET's vds = rdson*current, though IGBTs also have the BJT-like response), and they switch much slower due to the stored charge that accumulates in the base while they're turned on.
I just think BJTs are underrated, a lot of engineers think of them as obsolete but they're really not.
-F
@Felthry Oh, wow. So much I didn't know about transistors! As a lay person, I'm used to thinking of transistors as being things that behave a lot like BJTs as you describe them here.
@terrana Transistors are really neat!
-F
@Felthry Bacon, Jelly, and Tomato?