How semiconductors control current in diodes and transistors
In solids, electron energy levels form bands. Conductors have overlapping valence and conduction bands, insulators have a large energy gap, and semiconductors such as silicon and germanium have a small gap of about 1 eV. Pure semiconductors are intrinsic. Adding tiny amounts of a pentavalent impurity such as phosphorus makes an n-type semiconductor with extra electrons, while a trivalent impurity such as boron makes a p-type semiconductor with holes as majority carriers.
Where p-type and n-type regions meet, a p-n junction forms with a depletion layer and a barrier potential. In forward bias the barrier falls and current flows; in reverse bias only a tiny leakage current flows. This one-way behaviour lets diodes act as rectifiers, converting AC into DC. Zener diodes regulate voltage, LEDs emit light and photodiodes detect it. Logic gates such as AND, OR and NOT, and the universal NAND and NOR gates, follow truth tables and form the building blocks of digital circuits.
Why it matters
Every mobile phone, laptop, solar panel, LED bulb and television depends on semiconductor devices. Phone chargers rectify mains AC using diodes, and microprocessors contain billions of transistors working as switches. Understanding energy bands and junctions is the first step into electronics, computer hardware and renewable energy technology.
Key terms
- Energy gap
- The range of energies between the valence band and the conduction band where no electron states exist.
- Doping
- Adding a small amount of impurity to a pure semiconductor to change its conductivity.
- Depletion layer
- A region near a p-n junction that has no free charge carriers.
- Rectifier
- A circuit that converts alternating current into direct current.