I am glad that this article emphasizes the messiness of the bipolar junction transistor, but also disappointed that it doesn't engage with the "obvious" question I had back in the day, when I was taking high school electronics, and also doing hobby electronics (this would be early 1980s....)
If you've worked with them at all, you know that bipolar junction transistors are not symmetric -- if you put them in your circuit with the collector and the emitter reversed, then they don't behave the same as if you put them in the right way. Hobbyists know that the schematic-diagram symbol always has the arrow on the base-emitter leg, and the arrow points from P to N.
But the BJT diagram in this article and in elementary textbooks is symmetric -- there's no physical structural difference between the collector and the emitter in the picture.
This isn't necessarily bad, simplifying the problem to illustrate the fundamentals is a legitimate pedagogical technique.
The "obvious" question is, what else is going on in real devices?
I think the answer is that actual commercially-available discrete BJTs are fabricated on a planar substrate, the same way they're done for integrated circuits, and then cut out of the wafer and packaged for individual use. In this process, the collector (I think?) is generally the lowest layer on the wafer, and is much larger than the emitter, which is on top, and is generally much smaller. In the planar case, there's an obvious geometric asymmetry that explains the device's asymmetric behavior.
I'm not 100% solid on that explanation, and I don't recall where I saw it, but this is the kind of next-level detail that I would have liked to see from the article.
IIRC one side is more heavily doped than the other, but I cannot recall anything beyond that...
Update: The lack of symmetry [with respect to electrical rather than structural properties] is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter.
Second to last paragraph of the article covers exactly this, does it not?
> The idealized drawing of a BJT might imply that it should be a bidirectional device, working equally well if you reverse the polarity of the collector and the emitter. In practice, the emitter region is usually doped more heavily to facilitate the transport of electrons in one direction, so reversing the component leads to compromised performance.
You're right that the collector and emitter are not identical in size nor geometry nor surface area bordering the base. There's a nice cross-section about three-quarters of the way down this page[1] that shows the basic layout for a BJT. As to why we fabricate them as nested wells instead of side-by-side rectangles, I believe it is to maximize junction area relative to die area. These layers are exceedingly thin, so almost all of the junction is top-to-bottom, not side-to-side.
If you've worked with them at all, you know that bipolar junction transistors are not symmetric -- if you put them in your circuit with the collector and the emitter reversed, then they don't behave the same as if you put them in the right way. Hobbyists know that the schematic-diagram symbol always has the arrow on the base-emitter leg, and the arrow points from P to N.
But the BJT diagram in this article and in elementary textbooks is symmetric -- there's no physical structural difference between the collector and the emitter in the picture.
This isn't necessarily bad, simplifying the problem to illustrate the fundamentals is a legitimate pedagogical technique.
The "obvious" question is, what else is going on in real devices?
I think the answer is that actual commercially-available discrete BJTs are fabricated on a planar substrate, the same way they're done for integrated circuits, and then cut out of the wafer and packaged for individual use. In this process, the collector (I think?) is generally the lowest layer on the wafer, and is much larger than the emitter, which is on top, and is generally much smaller. In the planar case, there's an obvious geometric asymmetry that explains the device's asymmetric behavior.
I'm not 100% solid on that explanation, and I don't recall where I saw it, but this is the kind of next-level detail that I would have liked to see from the article.
(Minor edits for clarity.)