A transistor is a semiconductor device that allows controlling a current or voltage using another signal.
If we had to choose one of the most important inventions of the 20th century, the transistor would undoubtedly be on the list.
Before its invention (1947), computers occupied entire rooms and used vacuum tubes that heated up and burned out like light bulbs. Today, thanks to the transistor, you carry billions of them in your pocket inside your smartphone.
But what exactly is it, and why has it changed technology so much? In this post, we will understand its fundamental concept, setting aside formulas and types for a moment.
The Faucet Analogy
The best way to understand a transistor is to imagine it as a water faucet.
A faucet has three parts:
- The inlet pipe: where pressurized water arrives.
- The outlet pipe: where water comes out.
- The valve: the handle we turn to control the flow.
A transistor does exactly the same thing, but with electrons instead of water.
- We have a current input.
- We have a current output.
- And we have a control terminal.
The difference is that the “hand” turning the valve is an electrical signal.
The transistor allows us to control a large current using a very small electrical signal.
The Three Legs
In the most common discrete BJTs and MOSFETs, we find three terminals:
- Base, collector, and emitter in a BJT.
- Gate, drain, and source in a MOSFET.
Applying this to our analogy:
Power Input (collector / drain): where the main current we want to control enters (the water from the mains).
Power Output (emitter / source): where the current exits towards our load (motor, LED, etc.).
Control (base / gate): this is the “valve”. By applying a small voltage or current here, we open or close the path between the other two.
The Two Modes of Operation
Depending on how we handle that “valve,” the transistor can work in two very different ways. This difference separates its use as a switch from its use as an amplifier.
As a Switch
Here we use the transistor brutally: either all or nothing.
- Cut-off (
OFF): the transistor barely conducts. - Conduction (
ON): we aim for the smallest possible voltage drop for the load current.
In a BJT, we call saturation the region normally used for the ON state. In a power MOSFET used as a switch, we seek the ohmic region with a low
This is the mode we use in microcontrollers and Arduino. When you turn on a relay or an LED from a digital pin, you are using the transistor as a static switch. It is the basis of computers (binary).
As an Amplifier
Here we are more subtle. Instead of opening the valve all the way, we leave it half-open. If we move the valve a little, the water stream changes proportionally.
- If we apply a small signal (like the electrical signal from a microphone) to the control leg, the transistor will replicate that same waveform at the output, but with much more current.
- We have converted a weak signal into a strong one. We have amplified it.
This is the mode we use in audio and radio.
Why Are There So Many Types?
If you look at a catalog, you will see thousands of transistors. But they all fall into two major technologies that dominate the market and that we will see in upcoming articles:
- BJT (Bipolar Junction Transistor): the classics (NPN and PNP). They are controlled by current. They are robust and cheap, but they consume a bit of power to stay on.
- MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor): the modern ones. They are controlled by voltage. They are ultra-efficient and fast. These are used inside processors and for controlling high power.
Long ago, the BJT was the king. Today, the MOSFET dominates almost all power and digital electronics. However, the BJT remains very useful for simple tasks and small-signal amplification.
