Last week, I unveiled the secrets of the physics behind the “Joule thief,” an electrical circuit that extracts more energy from seemingly depleted batteries. The secret lay in the clever combination of a transformer and a transistor. However, the transistor deserves its own spotlight, as it may well be the most significant invention of the twentieth century.
Life would be drastically different without transistors. For starters, personal computers and mobile phones would not exist, which in turn means that platforms like Amazon, video games, dating apps, messaging services, streaming, social media, Apple Pay, Google Maps, and even artificial intelligence would be absent. Cars contain billions of transistors, and they permeate every aspect of our lives. So, what exactly is a transistor?
The Evolution of Transistors
The best way to grasp this fundamental technology is to trace its evolutionary path back to the electric relays used in nineteenth-century telegraphs. Telegraph lines essentially functioned as long electrical circuits. You might not have considered this, but they operated on batteries—primitive low-voltage cells that occupied an entire cabinet at the telegraph station—due to the absence of an electric grid at that time.
The electric relay, invented in 1835, played a pivotal role in these systems. It acted as a switch that turned an electric current on and off, akin to a wall switch used to illuminate a room. However, lacking fingers, the relay employed a second electric current to operate the switch. Why use one current to activate another? That is an intriguing question.
Imagine wanting to turn on a light in a city 80 kilometres away. You would need to lay very long wires to the streetlights. This set-up would allow you to switch the light on and off, and even use a code, like Morse, to send text messages. The challenge, however, was that the longer the wire, the greater its resistance, which meant that insufficient current reached the other end to convey a clear signal.
The solution involved splitting the circuit into two segments of 40 kilometres (25 miles) and connecting them with a relay. When the switch on the first circuit was closed, it activated the relay, sending the same current pattern from a second battery through the second circuit and lighting up the lamp. While the telegraph employed a buzzer instead of a light, the fundamental principle remained the same. Relays are still widely used today, and the concept of using one current to activate another has opened up a multitude of possibilities. For instance, in automobiles, it enables low-power controls to activate high-power circuits that operate components such as the starter motor, headlights, or air conditioning.
Understanding Relays and Their Functionality
How does a relay function? Essentially, it is an electromagnet. Inside, there is a coil of wire wound around an iron core. When an electric current flows through this control wire, it generates a magnetic field. This causes the metal switch to move downwards, making contact with the output wire. You may have heard of relays; when your oven’s thermostat turns the heating element on and off, that audible click is produced by a relay.
While relays are fantastic and useful, they are merely on/off switches. However, there is a more advanced component that resembles a variable relay: the valve or vacuum tube, invented around 1905, which enabled the radios of yesteryear.
The Functionality of Vacuum Tubes
One can envision a vacuum tube as a modified light bulb. An incandescent bulb consists of a thin filament that heats up significantly when electric current passes through it, emitting light. This filament is encased in a glass container with the air removed to prevent burning (the bulb’s primary function is to keep oxygen out).
Yet, something unseen happens: when the filament heats up sufficiently, it emits electrons. Since a flow of electrons constitutes an electric current, these thermal electrons can be used similarly to a relay. Here is a simple diagram of a rudimentary vacuum tube:
The electrons travel through the bulb to a collector plate, generating an output current. It may seem like a peculiar method, but if we add another wire (a grid) between the filament and the collector, we can control this current. A negative voltage on the control grid will push the electrons away from the collector, reducing the output current. Conversely, applying a positive voltage to the grid will increase the flow of output current.
This is, once again, a current switch, and like the relay, it is controlled by a different wire. However, there are two significant differences: first, there is no mechanical contact, allowing for much faster variations in output current. Second, the output current is not limited to simply on or off; it can vary according to the intensity of the control voltage.
From Vacuum Tubes to Early Computers
This capability made the first audio amplifiers possible. If a weak signal was received from a distant radio station, it would not generate enough current to power a speaker. Yet, if that signal were applied to the control voltage of a vacuum tube, a much more powerful output could be achieved while preserving the original signal’s pattern (such as music).
But wait, there’s more that could be accomplished with vacuum tubes: constructing a computer. Indeed, early computers were essentially a collection of vacuum tubes controlled by other vacuum tubes, creating logic gates. You would use an input signal of 1 volt or 0 volts. An AND gate had two inputs and one output. If both inputs were at 1 volt, the output would also be 1 volt. Otherwise, it would yield 0 volts. An OR gate would output 1 volt if any of the inputs were at 1 volt.
In reality, it was possible to build a computer using electric relays. However, relays are considerably slower than vacuum tubes, and the accompanying noise would have been maddening. Vacuum tubes were silent, purely electronic components with no moving parts, marking a radical shift.
The Arrival of the Transistor
Nevertheless, vacuum tubes had three significant drawbacks. They consumed a great deal of power, causing early computers to generate excessive heat, necessitating large cooling systems, and making their operation prohibitively expensive. Secondly, vacuum tubes were fragile and prone to burning out, requiring constant maintenance (literally teams dedicated to locating and replacing faulty tubes). Lastly, vacuum tubes were large; early computers such as the ENIAC from 1945 occupied entire rooms.
The transistor, invented at Bell Labs in 1947, addressed all these issues by utilising semiconductors. This term is frequently heard, but what exactly is a semiconductor? Certain materials (like copper) conduct electricity while others (like rubber) act as insulators. A semiconductor (such as silicon) can alternate between being one or the other.
There are two types of semiconductors: if electrons are added to silicon, it becomes an n-type semiconductor; if electrons are removed, it becomes a p-type semiconductor. Electrons, of course, carry a negative charge, so the missing electrons act as positive charges, and we refer to these as “electron holes.”
A transistor is created by combining different types of semiconductors. Here’s an example known as an NPN transistor: two n-type semiconductors are separated by a p-type semiconductor. This configuration prevents the electrons from moving from the input region (the “source”) to the output region (the “drain”). However, if a voltage is applied to the control or gate, the electrons can flow to the output. In essence, it’s still a switch, where one current activates another, but now with much greater precision.
The greatest advantage of transistors is that they can be miniaturised, and they certainly have been. Floor radios were replaced by transistor radios, which teenagers carried everywhere during the 1950s and 1960s. These contained between six and ten transistors. Today, the iPhone 17 Pro boasts up to 30 billion transistors. It’s astounding, yet this is what drives our digital world.