Semiconductor Electronics: Materials, Devices and Simple CircuitsIntroduction

Every electronic circuit is built from devices that control a flow of electrons. Before the transistor was invented in 1948, those devices were mostly vacuum tubes, also called valves: the vacuum diode with two electrodes, the anode (or plate) and the cathode; the triode with three, cathode, plate and grid; the tetrode with four and the pentode with five. A heated cathode supplies the electrons, and the voltages between the electrodes control their flow through the vacuum. The vacuum is needed because the electrons would lose their energy colliding with air molecules. Electrons can only go from the hot cathode to the anode, in one direction, which is why these tubes are called valves.

Both versions of the one-way device pass current only when their anode, the plate or the p-side, is connected to the + terminal. But the valve needs a hot cathode, a vacuum and, in practice, voltages of about 100 V. Valves are bulky, use a lot of power, and have a limited life and low reliability.

Valves Against Chips

Modern solid-state electronics grew from a discovery of the 1930s: some solid semiconductors, and the junctions between them, let us control both the number and the direction of the charge carriers flowing through them. Light, heat or a small applied voltage can change the number of mobile charges in a semiconductor. The carriers are supplied, and they flow, inside the solid itself, so no heating and no vacuum are needed. Semiconductor devices are small, use little power, work at low voltages, and have a long life and high reliability. Even the cathode ray tubes of old televisions and computer monitors, which work like vacuum tubes, have given way to liquid crystal displays (LCDs) with solid-state electronics. And long before anyone understood why, a natural crystal of galena (lead sulphide, PbS) touched by a fine metal point was used to detect radio waves.

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