From Physics to Logic
Every digital logic gate is built from transistors, microscopic electronic switches made from semiconductors. Materials fall into three electrical classes:
| Class | Behavior | Example |
|---|---|---|
| Conductors | Charge flows freely | Metals |
| Insulators | Charge barely moves | Glass |
| Semiconductors | Conductivity is controllable | Silicon |
Pure silicon conducts poorly, but its conductivity is tuned via doping (adding impurity atoms):
- n-type: dopants adding extra mobile electrons (negative carriers).
- p-type: dopants creating holes, missing electrons that act as mobile positive carriers.
Joining p-type and n-type silicon creates a p-n junction (diode), which conducts electricity in only one direction.
The Power of the Switch
A transistor combines these junctions. Its central trick: a small voltage on a control terminal (gate) switches a much larger current between the other two terminals on or off.
This voltage-controlled switching maps directly to binary logic: 1 is on, 0 is off. Billions of these tiny silicon switches form the gates, adders, and processors of modern computers, scaling via Moore's law.
Common pitfall: Thinking a transistor is a passive resistor or wire. Its defining feature is that a small signal on its gate controls a large current, acting as an active switch/amplifier. A plain resistor cannot do this.