A signal diode is one of many types of diodes, which are small components of electrical circuits, manufactured from semiconductors, that force electricity to flow in only one direction. Signal diodes are designed to pass very small currents, and have several applications in signal processing.
"Semiconductor" is the name given to the class of material—including silicon and germanium—that neither allows electricity to flow as freely as conductors like silver or gold, nor resists it as strongly as insulators like glass or quartz. The atomic structure of semiconductors contains neither the preponderance of floating "free electrons" found in conductors nor the almost total absence of such electrons found in insulators, but is arranged in a pattern called a "crystal lattice" that can be artificially modified (by the introduction of various types of atoms from other materials, called "impurities") to control the lattice's conductivity.
Most semiconductors are made of silicon. Single atoms of materials like arsenic, aluminium, phosphorus or boron are introduced into the silicon's crystal lattice to modify its conductivity. The simplest and most common use of such modified semiconductors is as diodes, to control the flow of electricity in circuitry.
Diodes can best be thought of as electrical valves. (Indeed, during the earliest days of circuitry manufacture, they were actually called valves.) They are semiconductors that allow electricity to flow in one direction but not the other. Passing through a diode requires a small expenditure of energy, and results in a forward voltage drop of about 0.7 volts for the current crossing the diode. Diodes have several uses—one very common variety, the LED, emits light when current flows through it, and is considerably cheaper, less power-hungry and less fragile than a standard filament-based lightbulb.
Signal diodes are so called because they are commonly found in circuits like those in radios or televisions, to pass very low-power but high-frequency currents. They pass currents of up to 100 milliamps, and are often used to process information found in electrical signals. Germanium diodes, for example, which have a lower-than-average forward voltage drop (about 0.2 volts), are used as detectors in radio circuits. By allowing the current to flow only in one direction, they block half the oscillations of the radio wave, converting the alternating-current wave to direct-current and allowing its varying strength to be read as an audio signal and converted to sound. In circuits that do not require the precision of a germanium diode, silicon signal diodes are usually employed for their lower resistance and vulnerability to heat.
Early radios did not use crystal diodes to detect signals but rather much larger vacuum tubes. In 1906 the mineral galena was found to work as a semiconductor, and the first "crystal set" radios were developed. The basic crystal set was eventually replaced by the transistor radio in the 1950s.