How do transistors work, anyway?

No background in electrical engineering here.

The article began well, I hadn't previously heard the gas wave/pressure analogy. It helped me see current/voltage in a presumably useful way.

However, after introducing this analogy, the remainder of the article seemed to abandon it and instead load on more and more new terms without referencing them back to the analogy. This lost me, and toward the end I gave up.

I was disappointed with the article.

This doesn't explain anything new beyond a standard textbook. Same stuff - differently regurgitated. (If you want a swing on this topic MIT 6.004/nand2tetris does a better job & even talks about trade-offs such as doping, channel contaminations, propagation delays due to FET designs, which electrical engineers routinely deal with in production. There are few videos by Carver Mead as well who intuitively explains why transistor work the way they do.)

And for delving into F/R bias, junction effects, leakage current - there are no illustrations showing polarity or flow of charges. Sorry but this is just another low SNR article for today.

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.

(Minor edits for clarity.)

As someone who is not an electrical engineer, I found Ben Eater's videos on how semiconductors work and how transistors work interesting and enlightening.

How semiconductors work: https://www.youtube.com/watch?v=33vbFFFn04k

How a transistor works: https://www.youtube.com/watch?v=DXvAlwMAxiA

For me engineering mindset did a really nice job of helping me understand how they work https://youtu.be/J4oO7PT_nzQ

I think this article does a pretty poor job of explaining the most interesting and exciting idea that I think is at the core of semi-conductors. Which is that you can go from the energy levels of electrons in an atom - using the electron shell concept that you already know from school, and translate that into how current flows in a semi-conductor lattice - and that not only do we get current flow when electrons move into higher energy levels, but that you see an opposite effect where holes flow the other direction. To me, that seems like something extremely well suited for visualization and this explanation does none of that. The reason I think this is so important is because between understanding that, and understanding how doping works, the logic of everything else is very intuitive. Give me some energy band diagrams!

As a layman, I always thought it was just simpler to just think of how conduction works in practice. By that I mean, for electricity to flow between two pieces of conducting wire, they need to be touching. If there is an air gap between two pieces of the wire, no electricity flows and the circuit is considered to be open. If you fill that gap with something conductive, then the circuit is closed and electricity flows.

So how does a transistor do this?

Simple, by placing a semiconductor in for the gap. Now if you add electricity to that semiconductor (and change it's conductance), it's like you're filling in that gap using electricity itself. If you don't, then it's like the gap remains filled with air...

Ergo, it's like connecting a gap between two pieces of wire, by using electricity itself to bridge the gap (or not depending on which state it is you want).

Bias makes transistors work. Learned that in Navy BE/E school. Nothing about them made sense until then. The article doesn't mention bias until it starts talking about FET's which is odd because bias is the reason transistors work. The voltage can be negative at all three leads. That blew my mind. Bias.

Current cutting-edge theoretical descriptions of solid-state devices (solar PV cells and their partners, light-emitting diodes, field-effect transistors, diode lasers, etc. all seem to involve applications of the complicated density functional theory, a replacement for earlier many-nuclei, many-electron wave function approaches (which were computationally intractable).

It's a very complex linear-algebra-heavy theory, but anyone interested in this field will eventually end up having to grasp it. One could philosophize as to whether the resulting theoretical picture is closer to reality than the previous approach, but ultimately these are just models, and comparison of predictions with experimental results is the real test. For an introduction:

(2014) "Density Functional Theory in the Solid State", Hasnip et al.

https://sci-hub.se/10.1098/rsta.2013.0270

See section 4, "Semiconductor physics/device technologies":

> "DFT simulations are increasingly important in the field of device technologies, and used to study everything from conventional CMOS devices to future devices based on half-metals, thermoelectrics or multiferroics. Obtaining the correct band-gap for the constituent (doped/defect) materials is essential to describe the functionality of these devices."

Since transistor sizes have now reached the so-called quantum limit, the electron tunneling phenomenon is now a limiting issue, requiring new materials and DFT-based electronic structure prediction.

P.S. Wikipedia has a good article on field-effect transistors:

https://en.wikipedia.org/wiki/Field-effect_transistor

> How do transistors work, anyway?

Yes, how do they work? This article totally reads like decades old pre-academic level textbook explanations found in schools.

> Hey, there's voltage and current. Oh, and there's this P/N magic you all heard about already? Anyway, that's how they work.

I can infer what the "hydraulic analogy" is, but I can't say it has ever been present in my life. It certainly was never touched in high school electrical engineering. Anyone have differing experiences?

Those who want to know can also try this one, for example.

https://www.youtube.com/watch?v=hrpPKCDLRN0

as well as others by the same author.

There are much better explanations.

See https://www.youtube.com/watch?v=IcrBqCFLHIY for example.

Start with studying particle in a box.

I did a deep dive in this topic as an EE undergrad and I'm here to report that trying to actually understand these things will end in disappoint, not withstanding all of the cargo cult mumbo jumbo that gets past off as education in this area. My personal experience was that after badgering the prof week after week for a full term his final answer was "I don't know. Go and ask Dr. Soandso in the physics department"

It takes a lot of effort to track down the essential contradiction. In short the problem is "holes" and "electrons" stuff. In particular the "holes". In the beginning because of doping and band gaps and such the doping atom releases and electron into the conduction band which leaves behind a positively charged "hole". Ie the atom now has one more proton than electrons. So far so good. But a few weeks later we move on to the Hall Effect and now these "holes" are moving and this allows the hall effect to distinguish the N-type from the P-type (qv X h, and all that). But that can't be right because the positive charge is fixed with the doping atom and not moveable so the v should be forever zero.

Well of course the actual technology does work but the theory it's based on is far too difficult for mortal EE's (and their profs) to understand so instead they (the physicists) cooked up a bogus theory with a fatal flaw (conservation of charge - where does this new moving proton come from?) and everyone is happy cuz they never thought it through.

My recommendation is to save yourself a lot of time and worthless effort and work on the water flowing through a pipe analogy. This can take younquite a long way with DC circuits and extends with a few tweaks to ac circuits as well.

And with respect to transistors. I read through all the comments here and as far as I can tell no one distinguishes between bipolar and FET transistors. These are very different beasts with very different operating characteristics but, cutting to the chase, both behave very much like a water tap type.

With bipolar transistors the "gate" current allows a proportionally much large current to flow through the other two leads so it's and amplifier of current. Simple as that. Rotate the tap a bit and get a much bigger resonse in the water flowing.

With FET transistors the "gate" is controlled by voltage rather than current (there is a little leakage current but it's tiny) but the result of a larger current though the other two leads is pretty much the same.

Capacitors and inductors are readily added to your repertoire. When you hit phase locked loops though you're gonna have to crack some books.

In theory, practice and theory are the same. In practice they're not.