Electronics

Pure silicon is the bridge between a conductor and an insulator. Every atom shares four electrons in covalent bonds, so at absolute zero there are no free charges at all — it acts like an insulator.

Warm it up and a little energy snaps a bond. The freed electron can now wander (a negative carrier), and the empty spot it leaves — a hole — behaves like a positive carrier, because a neighbouring electron can hop in to fill it, and so the hole "moves" the other way.

• Electron — negative carrier freed from a broken bond.
• Hole — the vacancy left behind; acts like a positive charge that drifts.

Pure silicon barely conducts. We deliberately add a tiny pinch of impurity — about one atom in a million — to flood it with one kind of carrier. This is doping.

• n-type — add a pentavalent atom (phosphorus, arsenic). It has five outer electrons; four bond, the fifth is left over as a free electron. Majority carriers are negative.
• p-type — add a trivalent atom (boron, gallium). It has only three outer electrons, leaving a hole. Majority carriers are positive.

Form one crystal that is p-type on one side and n-type on the other. Right at the join, free electrons from the n-side rush across to fill holes on the p-side — they recombine and both disappear.

This leaves a thin zone with no free carriers, but with exposed fixed ions — a depletion region. The trapped charges set up a small potential barrier (about 0.7 V in silicon) that stops any more carriers crossing.

Connect the battery's positive terminal to the p-side and negative to the n-side. The applied voltage pushes holes and electrons toward the junction, shrinking the depletion region.

Once the voltage exceeds the barrier (~0.7 V), the gate opens: carriers pour across and a large current flows. The diode is like a one-way valve propped wide open.

Now reverse the battery: positive to the n-side, negative to the p-side. The voltage drags carriers away from the junction, so the depletion region grows wider and the barrier gets taller.

Almost no current flows — only a tiny leakage from thermally generated carriers. The one-way valve is firmly shut.

Wall sockets give alternating current (AC) that swings positive and negative. Our gadgets need steady direct current (DC). A diode's one-way action converts one to the other — this is rectification.

• Half-wave — a single diode passes only the positive humps and blocks the negatives. Simple, but half the wave is wasted.
• Full-wave bridge — four diodes in a bridge flip the negative humps up to positive. Every part of the wave is used.

A transistor is two junctions in one crystal — three layers (n–p–n or p–n–p) with three leads: emitter, base, collector. A small current into the thin base controls a much larger current from emitter to collector.

Think of a water tap: a gentle twist of the handle (the base current) releases a powerful gush from the pipe (the collector current). That is amplification — and switched fully on or off, it is the on/off bit at the heart of every chip.

Group transistor switches together and you get logic gates — the building blocks of every digital decision. Inputs and outputs are just 1 (high) or 0 (low).

• AND — output is 1 only when both inputs are 1. Two switches in series.
• OR — output is 1 when either input is 1. Two switches in parallel.
• NOT — a single input inverter: 1 becomes 0, 0 becomes 1.

You have climbed the whole ladder of electronics — from how charge moves in a single silicon crystal to the logic that powers a computer.

1. Pure silicon has electron–hole pairs; doping makes n-type (extra electrons) or p-type (extra holes).
2. Joining them makes a p–n junction with a depletion region and a ~0.7 V barrier — a diode.
3. Forward bias conducts; reverse bias blocks — the one-way valve.
4. Diodes rectify AC into DC: half-wave, or full-wave with a bridge.
5. A transistor lets a small base current control a big current — amplifier and switch.
6. Transistor switches build AND, OR, NOT logic gates.
7. Millions of transistors on one chip make the microprocessors in phones and computers.