How Computers Work, Part 2: Transistors

From my previous post, we understand what a diode is, and how it works. In this post, we’re going to move on to the transistor. Once we’ve covered that, then we’ll start to move up a layer, and stop looking at the behavior of individual subcomponents, and focus on the more complicated (and interesting) components that are built using the basic building blocks of transistors.

By a fortuitous coincidence, today is actually the anniversary of the first transistor, which was first successfully tested on December 16th 1947, at Bell Labs.

Transistors are amazing devices. It’s hard to really grasp just how important they are. Virtually everything you interact with in 21st century live depends on transistors. Just to give you a sense, here’s a sampling of my day so far.

  • Get woken up by the alarm on my phone. The phone has billions of transistors. But even if I wasn’t using my phone, any bedside clock that I’ve had in my lifetime has been built on transistors.
  • Brush my teeth and shave. Both my electric razor and toothbrush use transistors for a variety of purposes.
  • Put on my glasses. These days, I wear lenses with a prismatic correction. The prism in the lens is actually a microscoping set of grids, carved into the lens by an ultraprecise CNC machine. Aka, a computer – a ton of transistors, controlling a robotic cutting tool by using transistors as switches.
  • Come downstairs and have breakfast. The refrigerator where I get my milk uses transistors for managing the temperature, turning the compressor for the cooling on and off.
  • Grind some coffee. Again, transistor based electronics controlling the grinder.
  • Boil water, in a digitally controlled gooseneck kettle. Again, transistors.

There’s barely a step of my day that doesn’t involve something with transistors. But most of us have next to no idea what they actually do, much less how. That’s what we’re going to look at in this post. This very much builds on the discussion of diodes in last weeks post, so if you haven’t read that, now would be a good time to jump back.

What is a transistor? At the simplest, it’s an electronic component that can be used in two main ways. It’s works like an on-off switch, or like a volume control – but without any moving parts. In a computer, it’s almost always used as a switch – but, crucially, a switch that doesn’t need to move to turn on or off. The way that it works is a bit tricky, but if we don’t really get too deep and do the math, it’s not too difficult to understand.

We’ll start with the one my father explained to me first, because he thought it was the simplest kind of transistor to understand. It’s called a junction transistor. A junction transistor is, effectively, two diodes stack back to back. There’s two kinds – the PNP and the NPN. We’ll look an the NPN type, which acts like a switch that’s off by default.

An NPN field effect transistor

Each diode consists of a piece of N type silicon connected to a piece of P type silicon. By joining them back to back, we get a piece of P type silicon in the middle, with N type silicon on either side. That means that on the left, we’ve got an NP boundary – a diode that wants to flow right-to-left, and block current left-to-right; and on the right, we’ve got a PN boundary – a diode that wants to let current flow from right to left, and block left to right.

If we only have the two outer contacts, we’ve got something that simply won’t conduct electricity. But if we add a third contact to the P region in the middle, then suddenly things change!

Let’s give things some names – it’ll help with making the explanation easier to follow. We’ll call the left-hand contact of the transistor the emitter, and the right hand the collector The contact that we added to the middle, we’ll call the base.

(Quick aside: these names are very misleading, but sadly they’re so standardized that we’re stuck with them. Electrical engineering got started before we really knew which charges were moving in a circuit. By convention, circuits were computed as if it was the positive charges that move. So the names of the transistor come from that “conventional” current flow tradition. The “collector” recieves positive charges, and the emitter emits them. Yech.)

If we don’t do anything with the base, but we attach the emitter to the negative side of a battery, and the collector to the positive, what happens is nothing. The two diodes that make up the transistor block any current from flowing. It’s still exactly the same as in the diodes – there’s a depletion region around the NP and PN boundaries. So while current could flow from the emitter to the base, it can’t flow into the collector and out of the circuit; and the base isn’t connected to anything. So all that it can do is increase the size of the depletion zone around the PN boundary on the right.

What if we apply some voltage to the base?

Then we’ve got a negative charge coming into the P type silicon from the base contact, filling holes and creating a negative charge in the P-type silicon away from the NP boundary. This creates an electric field that pushes electrons out of the holes along the NP-boundary, essentially breaking down the depletion zone. By applying a voltage to the base, we’ve opened a connection between the emitter and the collector, and current will flow through the transistor.

Another way of thinking about this is in terms of electrons and holes. If you have a solid field of free electrons, and you apply a voltage, then current will flow. But if you have electron holes, then the voltage will push some electrons into holes, creating a negatively charged region without free electrons that effectively blocks any current from flowing. By adding a voltage at the base, we’re attracting holes to the base, which means that they’re not blocking current from flowing from the emitter to the collector!

The transistor is acting like a switch controlled by the base. If there’s a voltage at the base, the switch is on, and current can flow through the transistor; if there’s no voltage at the base, then the switch is off, and current won’t flow.

I said before that it can also act like a volume control, or an amplifier. That’s because it’s not strictly binary. The amount of voltage at the base matters. If you apply a small voltage, it will allow a small current to flow from the emitter to the collector. As you increase the voltage at the base, the amount of current that flows through the transistor also increases. You can amplify things by putting a high voltage at the emitter, and then the signal you want to amplify at the base. When the signal is high, the amount of voltage passing will be high. If the voltage at the emitter is significantly higher than the voltage of the signal, then what comes out of the collector is the same signal, but at a higher voltage. So it’s an amplifier!

There’s a bunch of other types of transistors – I’m not going to go through all of them. But I will look at one more, because it’s just so important. It’s called a MOSFET – metal oxide semiconductor field effect transistor. Pretty much all of our computers are built on an advanced version of MOSFET called CMOS.

Just to be annoying, the terminology changes for the names of the contacts on a MOSFET. In a MOSFET, the negative terminal is called the source, and the positive terminal is the drain. The control terminal is still called the gate. In theory, there’s a fourth terminal called the body, but in practice, that’s usually connected to the source.

The way a field effect transistor works is similar to a junction transistor – but the big difference is, no current ever flows through the base, because it’s not actually electrically connected to the P-type silicon of the body. It’s shielded by a metal oxide layer (thus the “metal oxide” part of MOSFET).

An NPN MOSFET transistor in the off state, with depletion regions

In a MOSFET, we’ve got a bulky section of P-type silicon, called the substrate. On top of it, we’ve got two small N-type regions for the source and the drain. Between the source and the drain, on the surface of the MOSFET, there’s a thin layer of non-conductive metal oxide (or, sometimes, silicon dioxide – aka glass), and then on top of that metal oxide shield is the base terminal. Underneath the gate is P-type silicon, in an area called the channel region.

Normally, if there’s a voltage at the drain, but no voltage on the gate, it behaves like a junction transistor. Along the boundaries of the N-type terminals, You get electrons moving from the N-type terminals to the P-type holes, creating a depletion region. The current can’t flow – the negatively charged P-side of the depletion region blocks electrons from flowing in to fill more holes, and the open holes in the rest of the P-type region prevent electrons from flowing through.

An NPN MOSFET in the on state, with a positive voltage at the base producing an inversion zone.

If we apply a positive voltage (that is, a positive charge) at the gate, then you start to build up a (relative) positive charge near the gate and a negative charge near the body terminal. The resulting field pushes the positively charged holes away from the gate, and pulls free electrons towards the gate. If the voltage is large enough, it eventually creates what’s called an inversion region – a region which has effectively become N-type silicon because the holes have been pushed away, and free electrons have been pulled in. Now there’s a path of free electrons from source to drain, and current can flow across the transistor.

That’s what we call an N-type MOSFET transistor, because the source and drain are N-type silicon. There’s also a version where the source and drain are P-type, and the body is N type, called a P-type transistor. A P-type MOSFET transistor conducts current when there is no voltage on the base, and stops doing so when voltage is applied.

There’s an advanced variant of MOSFET called CMOS – complementary metal oxide semiconductor. It’s an amazing idea that pairs P-type and N-type transistors together to produce a circuit that doesn’t draw power when it isn’t being switched. I’m not going to go into depth about it here – I may write something about it later. You can see an article about it at the computer history museum.

On a personal note, in that article, you’ll see how “RCA Research Laboratories and the Somerville manufacturing operation pioneered the production of CMOS technology (under the trade name COS/MOS) for very low-power integrated circuits, first in aerospace and later in commercial applications.” One of the team at RCA Somerville semiconductor manufacturing center who worked on the original CMOS manufacturing process was my father. He was a semiconductor physicist who worked on manufacturing processes for aerospace systems.

While doing that, my father met William Shockley. Shockley was the lead of the team at Bell Labs that developed the first transistor. He was, without doubt, one of the most brilliant physisists of the 20th century. He was also a total asshole of absolutely epic proportions. Based on his interactions with Shockley, my dad developed his own theory of human intelligence: “Roughly speaking, everyone is equally intelligent. If you’re a genius in one field, that means that you must be an idiot in all others”. I think of all the people he met in his life, my dad thought Shockley was, by far, the worst.

If you don’t know about Shockley, well… Like I said, the guy was a stunningly brilliant physisist and a stunningly awful person. He was a coinventor of the transistor, and pretty much created silicon valley. But he also regularly told anyone who’d listen about how his children were “genetic regressions” on his intellectual quality (due of course, he would happily explain, to the genetic inferiority of his first wife). After collecting his Nobel prize for the invention of the transistor, he dedicated the rest of his life to promoting eugenics and the idea that non-white people are genetically inferior to whites. You can read more about his turn to pseudo-scientific racism and eugenics in this article by the SPLC.)


A few of the sources I looked at while writing this. (As usual, I’m a bit scattered, so I’m sure there were other places I looked, but these are the links I remembered.)

1 thought on “How Computers Work, Part 2: Transistors

  1. Steve Ruble

    Thanks for starting this series, it’s very interesting. It makes me want to go back and play “KOHCTPYKTOP: ENGINEER OF THE PEOPLE”, a very early (and very hard) Zachtronics puzzle game where (as I recall, it’s been a decade or so) you need to lay down paths of n and p silicon to make circuits. I think I got as far as making a half adder before I was overwhelmed. After reading this series I might be better equipped to tackle it again.


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