The Physics of the Switch
Every logic gate in Unit 6 is ultimately a transistor — a tiny electronic switch — and transistors are made possible by the physics of semiconductors. This lesson connects the physics unit back to the digital hardware at the heart of the course.
Materials fall into three electrical classes by how easily charge moves: conductors (metals, charge flows freely), insulators (charge barely moves), and semiconductors (in between — and controllable). Silicon is the workhorse semiconductor. On its own it conducts poorly, but its conductivity can be tuned by doping — adding tiny amounts of impurity atoms:
- n-type — dopants that add extra mobile electrons (negative carriers).
- p-type — dopants that create "holes," absences of electrons that act as mobile positive carriers.
Joining p-type and n-type silicon makes a p-n junction (diode), which conducts in only one direction — the basis of rectifiers and LEDs. Combining junctions makes a transistor, whose central trick is that a small voltage on a control terminal (gate) switches a much larger current between the other two terminals on or off. A transistor is thus a voltage-controlled switch — and a switch is exactly what you need to build a logic gate: 1 = on, 0 = off.
This is the bridge that makes computing physical. Billions of transistors, each a microscopic silicon switch, are etched onto a chip; wired together they form the gates, adders, registers, and processors of the earlier units. Moore's law — the decades-long doubling of transistors per chip — came from making these switches ever smaller. The abstract 0s and 1s of computation are, at the deepest level, transistors turning currents on and off.
Common pitfall: thinking a transistor is just a passive resistor or wire, rather than a switch/amplifier controlled by a third terminal. A transistor's defining feature is that a small signal on its gate controls a large current between its other terminals — turning it fully on or off. That controllability (not mere conduction) is what lets transistors compute; a plain resistor cannot switch or amplify.