The Evolution of the Transistor: The Most Important Invention of the 20th Century
Last week, I broke down the clever hidden physics behind the “joule thief” — the humble little circuit that lets you wring extra usable energy out of batteries most people write off as dead. We now know the trick relies on a perfectly paired transformer and transistor. But the transistor alone deserves its own spotlight: it is arguably the single most impactful invention of the 20th century, and modern life as we know it simply would not exist without it.
For starters, no transistors means no personal computers or smartphones. That means no online shopping, no console or mobile gaming, no dating apps, no instant messaging, no streaming services, no social media, no mobile payments or GPS navigation, and no modern AI. Even modern cars are packed with billions of transistors. They are literally everywhere in daily life.
So what exactly is a transistor? The clearest way to understand this game-changing technology is to trace its evolutionary origins all the way back to 19th-century telegraph systems, and the electric relays that made long-distance communication possible.
Electric Relays
Early telegraph lines were essentially just extremely long electrical circuits. What you might never have stopped to consider is that these systems ran entirely on batteries — crude, low-voltage units that took up entire closets in telegraph stations, because there was no public power grid back then.
Invented in 1835, the electric relay was a core component of these early networks. At its most basic, it is a switch that toggles an electric current on and off, similar to the wall switch you use to turn on a room light. But instead of a human flipping it by hand, the relay uses a second small electric current to change its state.
Why would we use one current to turn another current on and off? That’s a great question. Imagine you want to send a signal to a town 50 miles away. You’d string long wires along utility poles, flip the switch to send a dot-and-dash code, but there’s a big problem: the longer the wire, the more electrical resistance it has. By the time current reached its destination, it was too weak to deliver a readable, clear signal.
The fix? Split the full 50-mile line into two 25-mile segments, connected by a relay. When you close the switch on the first segment, it activates the relay, which pulls the same signal pattern from a second local battery to send the current through the next 25-mile segment to the end device. (Telegraphs used buzzers instead of lights, but the core principle is exactly the same.)
Relays are still widely used today, and the core idea — using one small current to control a second larger current — unlocked all kinds of new possibilities. In cars, for example, relays let you use low-power dashboard controls to toggle high-power circuits for things like starters, headlights, and air conditioning.
How does a relay work physically? It’s just an electromagnet at its core. Inside the relay, you’ll find a coil of wire wrapped around an iron core. When current flows through this control coil, it generates a magnetic field that pulls down a flexible metal armature (the switch) to connect the output circuit. You’ve probably heard a relay click before: that audible tick you hear when your oven’s thermostat turns the heating element on and off? That’s the relay moving.
Relays are useful, but they’re simple, binary on/off switches. The next evolution was something closer to a variable relay: the vacuum tube, invented around 1905, which made early commercial radios possible.
Vacuum Tube
Think of a vacuum tube as a modified incandescent light bulb. An incandescent bulb uses a thin wire filament that gets super hot when current runs through it, making it glow. That filament is sealed inside a glass bulb with all the air pumped out, to keep it from burning up instantly (keeping oxygen out is the whole point of the glass bulb).
But an invisible effect also happens when that filament gets hot: it ejects electrons off its surface. Since electron flow is what makes an electric current, these thermally ejected electrons work the same basic way as a relay to control current. In a basic vacuum tube, electrons fly from the hot filament across the empty vacuum inside the glass to a conductive collector plate, which creates an output current.
That might seem like a roundabout way to move current, but add one extra component — a thin wire grid between the filament and the collector plate — and you gain full control over that output current. A small negative voltage applied to the grid repels electrons heading to the collector, cutting down the output current. A small positive voltage on the grid lets more electrons flow, increasing the output current.
Like a relay, this is still a current switch controlled by a separate input signal — but there are two huge differences. First, there are no moving mechanical parts, so the output current can change far faster than any mechanical relay. Second, the output current isn’t just on or off: it can vary smoothly to match the strength of the input control voltage. This is what made the first audio amplifiers possible. A faint signal from a distant radio station was too weak to drive a speaker on its own, but feed that weak signal into the vacuum tube’s control grid, and you get a much stronger output that perfectly preserves the original pattern (say, a music broadcast).
Vacuum tubes also enabled another huge leap: they made the first electronic computers possible. Early computers were built from networks of vacuum tubes controlling other vacuum tubes, forming the core logic gates that run all computing. You input a signal that’s either 1 volt (representing a binary 1) or 0 volts (representing a binary 0). An AND logic gate has two inputs and one output: it only outputs 1 volt if both inputs are 1 volt, otherwise it outputs 0. An OR gate outputs 1 volt if either input is 1 volt.
You technically could build a computer out of relays, but relays are far slower than vacuum tubes, and the constant mechanical clicking would have been unbearable. Vacuum tubes were silent, fully electronic, and had no moving parts — this was a total revolution.
The Transistor
Even so, vacuum tubes had three big critical flaws. First, they used massive amounts of power, so early computers ran extremely hot, required huge dedicated cooling systems, and cost a fortune to operate. Second, the glass tubes were fragile and burned out regularly, meaning early computers needed constant maintenance — literally full teams of technicians whose entire job was hunting for dead tubes and replacing them all day. Third, they were just plain big: the 1945 ENIAC, one of the first general-purpose computers, took up an entire room.
The transistor, invented at Bell Labs in 1947, solved all these problems by using semiconductors. You’ve heard the term a million times, but what actually is a semiconductor? We all know that some materials, like copper, conduct electricity really well, while others, like rubber, block electricity and are insulators. A semiconductor, like silicon, can switch back and forth between acting like a conductor and an insulator.
There are two types of doped semiconductors: add a tiny number of extra electrons to silicon, and you get n-type semiconductor. Remove electrons, and you get p-type. Since electrons have a negative charge, those missing electrons act like positive mobile charges, which we call “electron holes.”
A transistor is made by layering these different types of semiconductors together. Take the common NPN transistor, for example: two layers of n-type semiconductor separated by a thin layer of p-type. This setup normally blocks electrons from moving from the input (called the source) to the output (called the drain). But apply a small voltage to the control terminal (called the gate), and the barrier drops, letting electrons flow through to the output.
Just like relays and vacuum tubes before it, a transistor is at its core a switch, where a small input voltage/current turns on a much larger output current. But this time, we get far finer control over the output than ever before. Best of all, transistors can be shrunk down to incredibly tiny sizes — far smaller than any vacuum tube or relay ever could.
By the 1950s and 60s, big floor-standing radio consoles were replaced by pocket-sized transistor radios that teens could carry anywhere, packed with just 6 to 10 transistors. Today, a top-end iPhone 17 Pro packs up to 30 billion transistors. That number sounds insane, but it’s exactly what powers our entire modern digital world.
