A visual, hands-on guide
They're the tiny switches behind every computer, phone, and chip. At heart, a transistor does one simple thing: it lets a small electrical signal control a much larger one. Master that, and the rest follows.
The mental model
A transistor is an electrically controlled valve. A small voltage on one terminal decides how much current flows between the other two.
Picture a tap on a water pipe. The water wants to flow from one end to the other, but a valve in the middle controls it. Turn the valve a little and a trickle gets through; turn it more and the flow surges. A transistor is that valve — except the "hand" turning it is itself electrical. That's the whole trick: electricity controlling electricity, with no moving parts.
Because the control signal can be tiny and the controlled flow large, one transistor can either act as a switch (fully on or fully off) or an amplifier (a small wiggle steering a big one). Those two behaviours build everything else.
Where the analogy leaks: nothing physically moves inside a transistor. The "valve" is an electric field reshaping where charge can travel. We'll open it up in Inside the silicon.
The valve, mapped
three terminals: gate · source · drain
01 — Play with it
Here's a single transistor. Drag the gate voltage up and down. Below a certain threshold, the channel stays shut and nothing flows. Push past it and current rushes from source to drain — and rises steeply the harder you push.
02 — Two behaviours
That single valve behaviour gets used in two very different ways. Almost all of modern electronics is one of these — or both at once.
digital · the logic of computers
Drive the gate hard — all the way off or all the way on. Off means "no current" (a 0); on means "current flows" (a 1). Wire millions of these switches together and you can compute anything. This is how every processor works.
analog · the heart of signals
Sit the gate in the middle of its range, where a small wiggle in voltage produces a large swing in current. A faint signal from a microphone or antenna becomes a strong one — without adding information, just power. This drives speakers, radios, and sensors.
03 — Build with it
Here's where computing is born. Wire two transistor-switches in series and current only gets through when both are on — that's an AND. Wire them in parallel and either one will do — an OR. Flip the inputs and watch the output light up.
| A | B | A AND B |
|---|
04 — Go deeper
So far, the valve. But why does a slab of silicon behave like one? The answer is the most-used transistor on Earth — the MOSFET — and a trick called the field effect. Open each layer when you're ready.
Metals always conduct; glass never does. Silicon sits in between — a semiconductor. On its own it barely conducts, but its conductivity can be precisely tuned. That tunability is what makes it useful: it can be coaxed between "blocking" and "conducting" on command.
Add a trace of certain impurities — "doping" — and you change what carries charge. Dope it one way and you get N-type, rich in free, negative electrons. Dope it the other way and you get P-type, full of positive "holes" (missing electrons that act like positive charges).
A transistor is built by placing these regions next to each other in a careful pattern. The boundaries between them are where the magic happens.
In a MOSFET, the gate is a metal plate sitting on a wafer-thin layer of insulating glass (oxide), just above the silicon. It doesn't touch the current path at all.
Put a positive voltage on the gate and its electric field reaches through the insulator and pulls negative electrons up toward the surface. Enough of them gather to form a thin conducting bridge — the channel — connecting source and drain. Now current can flow. Remove the voltage and the channel vanishes. No voltage, no channel, no current. That's the field effect — the "F" in MOSFET.
The threshold you felt in the first demo is exactly this: the gate voltage needed before enough electrons gather to form the channel.
Because the gate is insulated, holding a MOSFET on or off costs almost no power — current only flows during the instant of switching. Pairing two complementary types (the CMOS design) means a logic gate draws power mainly when it changes state, not while it sits still. That efficiency is why billions can share one chip without melting it.
Anatomy of a MOSFET
Common misconception
"A transistor is a tiny mechanical switch that physically flips." Nothing moves. There are no levers, gates that swing, or parts that wear out from flipping. Switching is purely an electric field appearing and disappearing — which is exactly why it can happen billions of times per second and survive for decades.
05 — Why it changed everything
A transistor is useful. Billions of them, each a few atoms wide and switching billions of times a second, are civilization-altering. The entire digital world rides on making them smaller and packing in more.
06 — Test yourself
Five quick questions, instant feedback, and the why behind each. No score pressure — retry as much as you like.
Sources & notes