On Wed, 14 Dec 2005 17:22:59 -0800, Big Mouth Billy Bass wrote:
> On Tue, 13 Dec 2005 19:46:55 -0600, via <dnntk3$c9$1@xxxxxxxxxxxxx>,
> "Pete Holland Jr." <peterg@xxxxxxx> spake thusly:
>>when it gets to transistors, it says
>>the electrical path is emitter to collector or emitter to base.
> Missed this bit the first time. It's wrong if stated as you say.
> Here's kinda how it works: A potential is developed between collector
> and emitter, but there is no current because of a phenomenon called
> the "depletion region," which is a transistor's way of resisting
> current. The emitter flow (either "electron" or "hole" flow) wants to
> move to the collector, but is impeded by the depletion region.
> A potential is applied at the base in order to influence the depletion
> region to shrink, until finally it is small enough to allow energy to
> pass from emitter to collector. However, the energy injected at the
> base also flows to the collector. Again, based on type, NPN or PNP,
> the transistor changes the type of flow from "electron" to "hole."
> Well, you asked for it, so there it was! Check this:
This corroborates that the charge injected into the base flows to the
emitter. The emitter current is the sum of base and collector currents,
coming out instead of going in (or the opposite). In the NPN example, the
conventional currents are flowing into the collector, with a little
control current flowing into the base, and they're both coming out the
emitter. (well, that's "conventional current": in a PNP, the flow is the
opposite direction, and with electron flow, it's all reversed. That's not
confusing at all, is it? ;-) )
There are those who proclaim that a transistor is "voltage controlled",
but to model it as a voltage controlled device, you have to do some
nasty exponential arithmetic. I (and probably lots of other techs)
prefer to think of it as, "once the base-emitter junction is forward
biased enough to actually conduct, the collector current is equal to
beta times the base current (with a few correction factors that we
don't need to worry about if we're not designing TVs or cell phones
from scratch, but only flashing an LED.)".