# Layperson asking intuitive question about bipolar junction transistor

I am a layperson trying to understand how the bipolar junction transistor works from this excellent article: https://spectrum.ieee.org/transistor-history

The relevant paragraphs are as follows:

Consider an NPN device. The base is p-type, so it has excess holes. But it is very thin and lightly doped, so there are relatively few holes. A tiny fraction of the electrons flowing in combines with these holes and are removed from circulation, while the vast majority (more than 97 percent) of electrons keep flowing through the thin base and into the collector, setting up a strong current flow.

But those few electrons that do combine with holes must be drained from the base in order to maintain the p-type nature of the base and the strong flow of current through it. That removal of the “trapped” electrons is accomplished by a relatively small flow of current through the base. That trickle of current enables the much stronger flow of current into the collector, and then out of the collector and into the collector circuit. So, in effect, the small base current is controlling the larger collector circuit.

From these paragraphs, if I understand correctly, a more positive voltage at the base increases emitter-collector current flow. However, if one applied a sufficiently negative voltage at the base, all the holes in the base would be obliterated, thus turning the transistor into a giant slab of N type silicon and supporting a high emitter-collector current.

Could you help me reconcile the discrepancy in my understanding here?

• I'm not sure what you're drawing the conclusion from -- if the base is negative, no current flows and the holes stay put. Commented Dec 3, 2022 at 0:30
• A filled hole is not the same as an N-type dopant atom; does that answer your question?
– Frog
Commented Dec 3, 2022 at 0:36
• Yes, that answers my question, thank you @Frog. Obvious once stated, but I was too dumb to see. If you could make it an answer, I would gladly accept.
– Bob
Commented Dec 3, 2022 at 0:47
• It might be worth noting that there isn't really any such thing as the "voltage at the base." Voltage is always defined between two points, A and B, and it's a measure of how much energy is needed to move charge from one place to the other. Electrical engineers sometimes casually say "the voltage at the base" to mean "the voltage between something else and the base," under the assumption that we humans will be able to figure out from the context what that "something else" is. If you're speaking precisely, then there's absolutely no such thing as "the voltage at the base." Commented Dec 3, 2022 at 1:23
• Agreed @Tanner. I suppose the relevant quantity here is the relative voltage between base and emitter (or maybe voltage between base and the average of emitter and collector?)
– Bob
Commented Dec 3, 2022 at 1:32

## 2 Answers

Promoting my comment to an answer (although I confess an incomplete one) - A filled hole is not the same as an N-type dopant atom. The OP is content to quantify the difference themselves :-)

Bob - you are "trying to understand how the bipolar junction transistor works" - and you want to solve "the discrepancy in my understanding here?"

In this context, I like to point your direction to another discrepancy in the quoted text:

Sentence 1: A tiny fraction of the electrons flowing in combines with these holes and are removed from circulation, while the vast majority (more than 97 percent) of electrons keep flowing through the thin base and into the collector, setting up a strong current flow.

Sentence 2: That trickle of current enables the much stronger flow of current into the collector, and then out of the collector and into the collector circuit. So, in effect, the small base current is controlling the larger collector circuit.

Does this sound logical to you?

A "tiny fraction" forms the base current - and this tiny fraction (which is "removed" from the emitted electron flow) should "enable the much stronger flow of curent into the collector" ? And the conclusion is that "the small base current is controlling the larger collector circuit."

To me, this is a simple claim without any evidence. On the other hand, there are many design rules, observations and theoretical explanations which clearly show that (and why) the bipolar transistor is a voltage-controlled device (Ebers-Moll equations) - with the transconductance gm as the most important parameter. The base current is nothing else than an unavoidable side effect.

(I know that there are some books and other publications which state that the BJT would be current-controlled. However, the authors could not present a single proof for such a claim - except the wrong interpretation of the relation Ib=Ic/B. But such an expression must not be used to derive any statement about cause and effect.)

Remark

In the above context, the following article may be of interest: "Barrie Gilbert, Fellow IEEE: Introduction To The Transistor - A New Semiconductor Amplifier, (Proceedings of the IEEE 1999)"

In this paper, the late Barrie Gilbert explains why the „older beta-view Ic=B*Ib is still taught in many textbooks“ (historical reasons) and why „the BJT is ...being decidedly a transconductance device“.

• This is good explanation that the NPN or PNP transistor is a voltage controlled current device. Some authors on this site seem to have written that it is a current controlled device with current supplied to base. Commented Dec 3, 2022 at 9:57
• You are quite correct @LvW now you mention it, those two sentences are non sequitur, with sentence 2 representing an unsubstantiated claim. I have read about BJTs, and understand up to the depletion layers, but then texts typically jump to an exponential equation leaving my intuitive understanding lacking. Oddly, I find the more modern FETs, in which the voltage at the gate controls the width of the conducting channel between source and drain, easy to intuitively understand.
– Bob
Commented Dec 3, 2022 at 20:40
• @Bob Regarding your intuitive understanding: Remember the voltage-current relationship for the classical pn-diode. The pn junction within the BJT behaves exactly in the same manner. Both, Ic and Ib, are exponentially controlled by the voltage Vbe.
– LvW
Commented Dec 4, 2022 at 10:17
• Ah, that is very helpful @LvW! It is really quite simple when looked at from that perspective. So, would two diodes soldered back to back (i.e. np -> pn), act as a transistor if controlled by a voltage applied to the wire connecting them?
– Bob
Commented Dec 4, 2022 at 11:00
• @Bob No, this would not work because both "diodes" must share a common (and extremely small) p-region (npn case).
– LvW
Commented Dec 4, 2022 at 11:17