Now you know what a semiconductor is — a material that can switch between conducting and blocking electricity. The next question: what do we BUILD with semiconductors? The answer is the most important invention of the 20th century: the transistor.

Key Concept

Transistor

A tiny electronic switch made from semiconductor material. It has three parts: the Source (where current enters), the Drain (where current exits), and the Gate (the control that turns current flow on or off). Every digital device — phones, computers, cars, appliances — is built from billions of transistors.

That's it. A transistor is just a switch. ON or OFF. 1 or 0. But when you put billions of these switches together and flip them billions of times per second, you get everything from web browsers to artificial intelligence.

Like a water faucet

A transistor is exactly like a water faucet. The pipe coming in is the Source — that's where water (electricity) enters. The pipe going out is the Drain — that's where water exits. The handle you turn is the Gate — it controls whether water flows or not. Your computer has billions of these tiny faucets turning on and off up to 5 billion times per second.

The Three Parts

Key Concept

Source

The input terminal of a transistor where current enters. Think of it as the entry point.

Key Concept

Drain

The output terminal of a transistor where current exits. Think of it as the exit point.

Key Concept

Gate

The control terminal of a transistor. Applying voltage to the gate creates an electric field that either allows or blocks electrons from flowing through the channel between source and drain. The gate is what makes a transistor a switch.

Between the source and drain is the channel — a thin strip of semiconductor material where electrons actually travel. The gate sits above (or around) the channel. When you apply a voltage to the gate, it creates an electric field that either pulls electrons into the channel (ON — current flows) or pushes them away (OFF — current stops).

Diagram · Transistor cross-section

interactive

Click the gate to toggle the transistor

Click the gate to toggle the transistor ON and OFF. Watch the electrons move through the channel.

Checkpoint · +5 XP

The three parts of a transistor are:

Threshold Voltage

Key Concept

Threshold Voltage (Vth)

The minimum voltage that must be applied to the gate to turn the transistor ON. Below this voltage, the transistor is OFF and no current flows. Above it, the transistor switches ON and current flows from source to drain. Modern transistors typically have a threshold voltage between 0.3V and 0.7V.

Like a stiff door handle

Threshold voltage is like the minimum amount of force needed to turn a stiff door handle. Push lightly (below threshold) — nothing happens, the door stays shut. Push hard enough (above threshold) — the handle turns, the door opens, and people (electrons) can walk through.

Interactive · Gate Voltage Switch

Gate Voltage (Vgs)

0.70 V

● ON

0VVth = 0.5V1.5V

Current (log)40.0 µA

ON/OFF Ratio4.0e+4

ON/OFF Ratio

Key Concept

ON/OFF Ratio

The ratio between the current flowing when the transistor is ON versus when it's OFF. A ratio of 10⁶ (one million) means the ON current is a million times larger than the OFF current. Higher is better — it means the transistor is a cleaner, more decisive switch.

Why does ON/OFF ratio matter? Because in computing, every transistor represents a 0 or a 1. If the difference between ON and OFF isn't dramatic enough, the computer can't reliably tell them apart, and you get errors. A good modern transistor has an ON/OFF ratio of at least 10⁶.

Gate Length — Size Matters

Key Concept

Gate Length

The physical size of the gate, measured in nanometers (nm). One nanometer is one billionth of a meter. Smaller gates mean smaller transistors, which means you can fit more transistors on a chip. Modern chips use gate lengths of 3nm. A human hair is about 80,000nm wide — so a modern transistor gate is roughly 27,000 times thinner than a hair.

For 60 years, the strategy was simple: make gates smaller → fit more transistors → more computing power. But at 3nm, a gate is only about 15 atoms wide. Below that, quantum physics starts breaking things. Electrons can 'tunnel' through barriers they shouldn't be able to cross. That's the wall we're hitting, and it's the topic of the next lesson.

Try It Yourself

Try shrinking a transistor

Open the Transistor Sim. Select the 'Planar FET' architecture. Set the gate length slider to 100nm and note the switching frequency. Then move it to 3nm. See how much faster it gets? Now try 1nm — notice the leakage current skyrocketing. That's quantum tunneling.

Open Transistor Sim

Lesson Summary

A transistor is a tiny voltage-controlled switch with three parts: Source, Drain, and Gate.
Like a water faucet — Source is the inlet, Drain is the outlet, Gate is the handle.
Threshold voltage (Vth) is the minimum gate voltage needed to turn the transistor ON.
ON/OFF ratio measures how cleanly the switch separates 1 from 0 (good ≥ 10⁶).
Smaller gate length = faster, denser chips — but quantum tunneling breaks things below ~3nm.

Test Your Knowledge · +50 XP

What are the three parts of a transistor?

What does the gate do?

What is threshold voltage?

A gate length of 3nm means the gate is approximately how many atoms wide?

What is a good ON/OFF ratio for a transistor?