Invisible wrote:
> What *is* the difference between a BJT and a FET?

I think I can describe the difference (a little) with an analogy.

Think of a BJT like two pools of water, with a low spot between, hoses 
connected. To make the water flow, you pump enough water into the low 
spot that the water from the hoses can flow across. Of course, this 
washes away the water you pumped in, so you have to pump in more.

A FET is like a balloon wrapped around a hose. To make the water stop 
running, you pump water into the balloon, which squeezes off the hose. 
But since it's in the balloon, it really doesn't leak as such, so the 
efficiency is much better.

Of course, these are *way* simplified descriptions. If you really care 
how the semiconductors work at the quantum level, I can go into that, 
but it didn't sound like the kind of question you were asking.

Look at the wiki pages for each. In the BJT, the current flows from the 
blue, thru the green, and into the other blue. That's the "bipolar 
junctions", the two boundaries.

In the FET, there's the "gate" and the "oxide" that keeps the gate from 
actually conducting the electricity you put on it. But when you charge 
up the gate, the "static electricity" pulls/pushes away the charges that 
actually conduct between the two "N" pads there. So, FETs are used in 
things like wrist watches, which are expected to run for a year on a 
tiny little battery, because they work on voltage rather than current, 
and batteries are better at supplying voltage than current.

> Are they different designs for doing the same thing, or do they actually 
> "do" something different?

The do "the same thing" in different ways. The BJT is less efficient but 
it can handle much larger currents (i.e., the "hose" can be bigger 
because you don't need to wrap a balloon around it).

> How do you use them anyway?

They're basically amplifiers of electricity. If you want to make binary 
circuits, you over-drive them so they're either completely off or 
completely on.

-- 
Darren New / San Diego, CA, USA (PST)

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