Transistor question -theory

Yes.... but the geometry of the emitter and the amount of dopant for the N material in it may differ from the collector's. Very old transistors were, I seem to recall, made from quite similar N materials for both the collector and emitter and were manually fused together. They were expensive, but they worked about like you suggest -- roughly equally either way -- if I got that much right. Perhaps someone with better knowledge about this can comment, though.

But modern BJTs aren't built that way. They can diffuse different concentrations of dopants for the emitter vs the collector and the physical shape/size is also different. Which emphasizes one orientation/use over the other. You can still wire them up the other way, but the beta will probably be a lot lower. Some other features may be better, though. I suppose some designers take advantage of that. Not so often that I've seen it done much, though.

Jon

The doping levels are different between the emitter and collector, and in many transistor processes the emitter is smaller than the collector, making it harder for the carriers to 'find' the emitter when it's operated in reverse.

Most bipolar transistors _can_ be operated in 'reverse', but with a severely degraded current gain -- I think a current gain less than unity is not uncommon.

Most FETs operate this way, at least to a first order. However, many power FETs have diodes that are 'built in' as a side effect of the manufacturing process, so you can't just freely turn any old FET around.

"Electronic switches" are commonly implemented as complementary small signal FETs in parallel, with appropriate gate control to turn them both on at the same time.

Now, if I'm lucky Jim Thompson hasn't answered by the time I press "send", so I can get credit for an incomplete explanation before someone who actually knows his stuff comes on line with the real deal.

Caveat (I probably know less than Jon about transistors)

But I=92ve always wanted to try that experiment. I think the biggest thing to look out for (besides the much reduced beta) will be the reverse bias Vbe voltage. Is this a zener breakdown?

So keep the supply voltage low and it might work..... Sounds like a project for a late Friday afternoon....

George H.

IIRC from my college days, TTL logic uses the reverse gain of the input stage to suck charge out of an intermediate stage's base, speeding up one of the transitions.

The forward gain of the multi-emitter transistor is intended to suck the current out of the base of the splitter transistor in the middle of the gate. The forward biased base-collector junction provides the current to turn the splitter on.

| | | .-. .-. | | | | | | | | | | | '-' '-' | | | |/ | o-------| | | |>

| | | | Splitter| | ----- |/ V A o------v v \--------| - | |>| o------------o O B o--------+ | | | |/ o-------| | |>

| | .-. | | | | | | | '-' | | | | | (created by AACircuit v1.28.6 beta 04/19/05

A Triac will allow current to flow in either direction, under most conditions. Some are better suited that others like alternistors, a variation of a triac.

Shaun

The doping is the same polarity, but not the same amount; the E doping is higher than B, with C doping usually inbetween those. So, reverse breakdown of BC junction is high (for PN2222, about 80V) and BE is low (about 7V), and 'emitter efficiency' of BE is high, of BC is low (this means lower reverse beta).

The only real advantage of the reverse configuration is the saturation voltage of a switched-ON reverse transistor, which can be in the 10 mV range; before MOS switch devices became common, this was how a chopper transistor was operated.

if you're only using low voltages you can, it it will still work, just not as well.

several.

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Just say a good thread about this on sci.electronics.design you might want to read it.

George h.