7 ms·

How do transistors work, anyway?

175 points by weinzierl 4y ago

taf2 4y ago
For me engineering mindset did a really nice job of helping me understand how they work https://youtu.be/J4oO7PT_nzQ https://youtu.be/J4oO7PT_nzQ
srvmshr 4y ago
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.
- mhh__ 4y ago
  Bob Widlar also wrote an intro to semiconductors
  - abfan1127 4y ago
    my undergraduate analog circuits professor worked with Bob Widlar. He had some great stories. Widlar was a pioneer!
  - gaze 4y ago
    It's an extremely underappreciated text
  - ce4 4y ago
    That guy's career is a story unto itself: https://en.m.wikipedia.org/wiki/Bob_Widlar https://en.m.wikipedia.org/wiki/Bob_Widlar Edit - some nice reads: https://hackaday.com/2014/04/08/heroes-of-hardware-revolution-bob-widlar/ https://hackaday.com/2014/04/08/heroes-of-hardware-revolutio... https://web.archive.org/web/20190527014243/https://www.autodesk.com/products/eagle/blog/bob-widlar-life-engineering-legend/ https://web.archive.org/web/20190527014243/https://www.autod...
    - srvmshr 4y ago
      If I could think of anyone being Rick Sanchez (of Rick & Morty) in real life, it had to be Bob Widlar: Brash, extremely genius & mostly drunk.
    - weinzierl 4y ago
      I like how he had his retirement period in the middle of his life and not at the end. This quote in the section about his retirement in the Wikipedia article made me chuckle: His proud statement: "I don't work" caused him frequent troubles when crossing the Mexican border and eventually, Widlar created a set of fake business cards presenting him as a "road agent" for "Morgan Associates".
- weinzierl 4y ago
  Not disappointed, it's a good introductory text with beautiful illustrations. It is called "How do transistors work, anyway?" and not "How do transistors really work?" or "The big transistor deep dive!". I think the article matches the expectations set by the title pretty well. That is not to say an accessible transistor deep dive would be interesting. I enjoyed my fair share of band gap calculations at university and not to forget Shockley's Garage. That's what an in depth transistor coverage would be for me. You raised very different points and I would love to read that transistor article anytime.
  - srvmshr 4y ago
    Normally I avoid sounding critical but since you vociferously defend the article's merit, here is what I honestly feel: As people deeply involved or passionate about tech, we hope HN to be a place where * Read something we know from a different perspective (a.k.a the hacker way) * Read something deeper which explains the (usually simpler) stuff in due course (i.e. deepdive) I get it, you love the article and you want it on HN. But it does nothing more than regurgitating what we learned in university already. And the treatment of the topic is pedestrian (Please see why I critiqued it in terms of absence F/R bias & charge flow etc. Just having a sketch of the BJTs & FETs is extremely low bar)
    - sheusndudn 4y ago
      I didn't learn about this in university and was interested by the article. Maybe consider that not everyone took electric engineering and move on with your day?
      - srvmshr 4y ago
        Absolutely. But would you like to base your understanding on something well written, intuitively illustrated (not necessarily math or physics heavy) OR something which is a refined copypasta from any garden variety electronic textbook? If I could show you where to find a better place to learn, would you still consider me to "mind my own business" I am not HN's knight crusader. But I would be happy if people got a better appreciation of things others love. And that needs some constructive critique at times. At HN discussions I try to do that whenever I can.
- matkoniecz 4y ago
  Can you recommend a better article?
  - Koshkin 4y ago
    https://en.wikipedia.org/wiki/Transistor https://en.wikipedia.org/wiki/Transistor
- amirhirsch 4y ago
  You might be interested in this substantially more in depth collection of article about semiconductor physics: http://britneyspears.ac/lasers.htm http://britneyspears.ac/lasers.htm
- anikom15 4y ago
  https://youtu.be/KqIOGGjPFQU https://youtu.be/KqIOGGjPFQU I find this to be the best concise explanation of the PN junction, which is enough to figure out how a transistor works on one’s own.
- oumua_don17 4y ago
  Agreed. One of the best books to understand how transistors work is the GE transistor manual from 1964 :). [1] PS: pdf link [1] https://worldradiohistory.com/BOOKSHELF-ARH/GE-Books/GE-Transistor-Manual-1964.pdf https://worldradiohistory.com/BOOKSHELF-ARH/GE-Books/GE-Tran...
sphinxster 4y ago
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.
- ergonaught 4y ago
  Ditto. I wish folks who expend the effort to create things like this would spend just a bit more identifying the target audience and ensuring they're speaking to it. I've lost count of the things I've begun reading which lose the thread in this manner.
wankle 4y ago
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.
- Workaccount2 4y ago
  Bias is how you make transistors work in a circuit, but not how they work internally.
  - posterboy 4y ago
    It's not a closed system internally. Theorizing about the open circuit voltage is legitimate for an ideal voltage source but entirely useless since it is completely isolated. It is indeed a confusing abstraction, I have to concure.
- capn_duck 4y ago
  Bias is how all electric components work. All they know about is the difference in voltage between their electrodes, not any absolute voltage w.r.t the rest of the circuit.
  - itishappy 4y ago
    Voltages lack the concept of zero, so we need a comparison to be able to do certain math (like addition) nicely. Here's an article explaining why! https://math.ucr.edu/home/baez/torsors.html https://math.ucr.edu/home/baez/torsors.html Here's the post I pulled it from! https://news.ycombinator.com/item?id=33678771 https://news.ycombinator.com/item?id=33678771
Waterluvian 4y ago
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?
- Timwi 4y ago
  I think it's a recent thing. I first heard about this analogy some time in the 2010s.
  - hunter2_ 4y ago
    Voltage has been called "pressure" for a very long time, although I'm not sure if it's strictly a hydraulic analogy.
    - retrac 4y ago
      Yep. Traditionally, voltage was the "tension" applied to the system. Pulling rather than pushing there, but same idea. Still occasionally seen in "high tension lines" etc., and for some reason (worked on too many tube amps over the years?) I sometimes still write HT/LT instead of HV/LV.
      - posterboy 4y ago
        Where else would you get "voltage sink", "gate" (similarly German "Gatter" - gate, flip flop, switch).
  - dboreham 4y ago
    Pretty sure Maxwell used it. The analogy is just to try to convey the concept of a field and the associated vector calculus. An analogy isn't necessary for Newtonian mechanics and scalar calculus because humans intuitively understand thrown balls etc.
    - whatshisface 4y ago
numlock86 4y ago
> 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.
- posterboy 4y ago
  The important part is that they have asked the right question. Besides, the rule of headlines applies.
reidacdc 4y ago
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.)
- mannykannot 4y ago
  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. https://en.wikipedia.org/wiki/Bipolar_junction_transistor#Structure https://en.wikipedia.org/wiki/Bipolar_junction_transistor#St...
posterboy 4y ago
npilk 4y ago
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 https://www.youtube.com/watch?v=33vbFFFn04k How a transistor works: https://www.youtube.com/watch?v=DXvAlwMAxiA https://www.youtube.com/watch?v=DXvAlwMAxiA
photochemsyn 4y ago
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 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 https://en.wikipedia.org/wiki/Field-effect_transistor
osigurdson 4y ago
Start with studying particle in a box.
- SilverBirch 4y ago
  And how exactly do you plan to establish the particle is indeed in the box?
Koshkin 4y ago
Those who want to know can also try this one, for example. https://www.youtube.com/watch?v=hrpPKCDLRN0 https://www.youtube.com/watch?v=hrpPKCDLRN0 as well as others by the same author.
RustLove 4y ago
There are much better explanations. See https://www.youtube.com/watch?v=IcrBqCFLHIY https://www.youtube.com/watch?v=IcrBqCFLHIY for example.
Rury 4y ago
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).
SilverBirch 4y ago
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!
entropicgravity 4y ago
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.

175 points by weinzierl 4y ago

taf2 4y ago

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

srvmshr 4y ago

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.

mhh__ 4y ago
Bob Widlar also wrote an intro to semiconductors
- abfan1127 4y ago
  my undergraduate analog circuits professor worked with Bob Widlar. He had some great stories. Widlar was a pioneer!
- gaze 4y ago
  It's an extremely underappreciated text
- ce4 4y ago
  That guy's career is a story unto itself: https://en.m.wikipedia.org/wiki/Bob_Widlar https://en.m.wikipedia.org/wiki/Bob_Widlar Edit - some nice reads: https://hackaday.com/2014/04/08/heroes-of-hardware-revolution-bob-widlar/ https://hackaday.com/2014/04/08/heroes-of-hardware-revolutio... https://web.archive.org/web/20190527014243/https://www.autodesk.com/products/eagle/blog/bob-widlar-life-engineering-legend/ https://web.archive.org/web/20190527014243/https://www.autod...
  - srvmshr 4y ago
    If I could think of anyone being Rick Sanchez (of Rick & Morty) in real life, it had to be Bob Widlar: Brash, extremely genius & mostly drunk.
  - weinzierl 4y ago
    I like how he had his retirement period in the middle of his life and not at the end. This quote in the section about his retirement in the Wikipedia article made me chuckle: His proud statement: "I don't work" caused him frequent troubles when crossing the Mexican border and eventually, Widlar created a set of fake business cards presenting him as a "road agent" for "Morgan Associates".
weinzierl 4y ago
Not disappointed, it's a good introductory text with beautiful illustrations. It is called "How do transistors work, anyway?" and not "How do transistors really work?" or "The big transistor deep dive!". I think the article matches the expectations set by the title pretty well. That is not to say an accessible transistor deep dive would be interesting. I enjoyed my fair share of band gap calculations at university and not to forget Shockley's Garage. That's what an in depth transistor coverage would be for me. You raised very different points and I would love to read that transistor article anytime.
- srvmshr 4y ago
  Normally I avoid sounding critical but since you vociferously defend the article's merit, here is what I honestly feel: As people deeply involved or passionate about tech, we hope HN to be a place where * Read something we know from a different perspective (a.k.a the hacker way) * Read something deeper which explains the (usually simpler) stuff in due course (i.e. deepdive) I get it, you love the article and you want it on HN. But it does nothing more than regurgitating what we learned in university already. And the treatment of the topic is pedestrian (Please see why I critiqued it in terms of absence F/R bias & charge flow etc. Just having a sketch of the BJTs & FETs is extremely low bar)
  - sheusndudn 4y ago
    I didn't learn about this in university and was interested by the article. Maybe consider that not everyone took electric engineering and move on with your day?
    - srvmshr 4y ago
      Absolutely. But would you like to base your understanding on something well written, intuitively illustrated (not necessarily math or physics heavy) OR something which is a refined copypasta from any garden variety electronic textbook? If I could show you where to find a better place to learn, would you still consider me to "mind my own business" I am not HN's knight crusader. But I would be happy if people got a better appreciation of things others love. And that needs some constructive critique at times. At HN discussions I try to do that whenever I can.
matkoniecz 4y ago
Can you recommend a better article?
- Koshkin 4y ago
  https://en.wikipedia.org/wiki/Transistor https://en.wikipedia.org/wiki/Transistor
amirhirsch 4y ago
You might be interested in this substantially more in depth collection of article about semiconductor physics: http://britneyspears.ac/lasers.htm http://britneyspears.ac/lasers.htm
anikom15 4y ago
https://youtu.be/KqIOGGjPFQU https://youtu.be/KqIOGGjPFQU I find this to be the best concise explanation of the PN junction, which is enough to figure out how a transistor works on one’s own.
oumua_don17 4y ago
Agreed. One of the best books to understand how transistors work is the GE transistor manual from 1964 :). [1] PS: pdf link [1] https://worldradiohistory.com/BOOKSHELF-ARH/GE-Books/GE-Transistor-Manual-1964.pdf https://worldradiohistory.com/BOOKSHELF-ARH/GE-Books/GE-Tran...

sphinxster 4y ago

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.

ergonaught 4y ago
Ditto. I wish folks who expend the effort to create things like this would spend just a bit more identifying the target audience and ensuring they're speaking to it. I've lost count of the things I've begun reading which lose the thread in this manner.

wankle 4y ago

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.

Workaccount2 4y ago
Bias is how you make transistors work in a circuit, but not how they work internally.
- posterboy 4y ago
  It's not a closed system internally. Theorizing about the open circuit voltage is legitimate for an ideal voltage source but entirely useless since it is completely isolated. It is indeed a confusing abstraction, I have to concure.
capn_duck 4y ago
Bias is how all electric components work. All they know about is the difference in voltage between their electrodes, not any absolute voltage w.r.t the rest of the circuit.
- itishappy 4y ago
  Voltages lack the concept of zero, so we need a comparison to be able to do certain math (like addition) nicely. Here's an article explaining why! https://math.ucr.edu/home/baez/torsors.html https://math.ucr.edu/home/baez/torsors.html Here's the post I pulled it from! https://news.ycombinator.com/item?id=33678771 https://news.ycombinator.com/item?id=33678771

Waterluvian 4y ago

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?

Timwi 4y ago
I think it's a recent thing. I first heard about this analogy some time in the 2010s.
- hunter2_ 4y ago
  Voltage has been called "pressure" for a very long time, although I'm not sure if it's strictly a hydraulic analogy.
  - retrac 4y ago
    Yep. Traditionally, voltage was the "tension" applied to the system. Pulling rather than pushing there, but same idea. Still occasionally seen in "high tension lines" etc., and for some reason (worked on too many tube amps over the years?) I sometimes still write HT/LT instead of HV/LV.
    - posterboy 4y ago
      Where else would you get "voltage sink", "gate" (similarly German "Gatter" - gate, flip flop, switch).
- dboreham 4y ago
  Pretty sure Maxwell used it. The analogy is just to try to convey the concept of a field and the associated vector calculus. An analogy isn't necessary for Newtonian mechanics and scalar calculus because humans intuitively understand thrown balls etc.
  - whatshisface 4y ago

numlock86 4y ago

> 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.

posterboy 4y ago
The important part is that they have asked the right question. Besides, the rule of headlines applies.

reidacdc 4y ago

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.)

mannykannot 4y ago
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. https://en.wikipedia.org/wiki/Bipolar_junction_transistor#Structure https://en.wikipedia.org/wiki/Bipolar_junction_transistor#St...

posterboy 4y ago

npilk 4y ago

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 https://www.youtube.com/watch?v=33vbFFFn04k How a transistor works: https://www.youtube.com/watch?v=DXvAlwMAxiA https://www.youtube.com/watch?v=DXvAlwMAxiA

photochemsyn 4y ago

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

osigurdson 4y ago

Start with studying particle in a box.

SilverBirch 4y ago
And how exactly do you plan to establish the particle is indeed in the box?

Koshkin 4y ago

Those who want to know can also try this one, for example. https://www.youtube.com/watch?v=hrpPKCDLRN0 https://www.youtube.com/watch?v=hrpPKCDLRN0 as well as others by the same author.

RustLove 4y ago

There are much better explanations. See https://www.youtube.com/watch?v=IcrBqCFLHIY https://www.youtube.com/watch?v=IcrBqCFLHIY for example.

Rury 4y ago

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).

SilverBirch 4y ago

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!

entropicgravity 4y ago

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.