Lesson 1 of 12 · 6 min

From valves to semiconductors

NCERT §14.1

Rohan unscrews an old wall adapter that once charged his grandfather's radio. Inside: a transformer, four tiny black cylinders with a silver band, and one fat capacitor. No glowing tubes anywhere.

The story this chapter follows: Rohan opens an old adapter

Rohan unscrews an old wall adapter and finds a transformer, four small diodes and one large capacitor. Following the current through it takes him from valves and energy bands to doped silicon, the p-n junction and the rectifier: a silicon diode that switches on near 0.7 V, a forward resistance of 10 Ω against a reverse resistance of 1.0 × 10⁷ Ω, and 50 Hz mains turned into 100 one-way pulses a second, then smoothed by a capacitor.

The lesson in notes

In short

Every electronic circuit is built from devices that control a flow of electrons. Until the transistor arrived in 1948, those devices were mostly vacuum tubes.

Vacuum tubes are named by their electrode count: the diode has two (anode, also called plate, and cathode), the triode three (cathode, plate, grid), the tetrode four and the pentode five.

A heated cathode supplies the electrons, and the voltages on the electrodes steer them. The space between electrodes must be evacuated, or the electrons would lose energy colliding with air molecules.

Electrons can travel only from cathode to anode, one way, which is why these tubes were also called valves.

Valves are bulky, draw a lot of power, usually run at high voltages (around 100 V), and have short lives and poor reliability.

From the 1930s it became clear that some solid semiconductors and their junctions let us control both how many charge carriers there are and which way they move. Light, heat or a small voltage can change the number of mobile charges.

In a semiconductor device the carriers are produced and flow inside the solid itself, so there is no heated filament and no vacuum. The devices are small, use little power, work at low voltages and last long.

Even cathode ray tube screens, which work like vacuum tubes, have given way to LCD monitors run by solid-state circuits. Long before the theory was understood, a galena crystal (lead sulphide, PbS) touched by a metal point served as a radio-wave detector.

This chapter covers the p-n junction diode, a two-electrode device, and its use in rectifier circuits.