By johnmcgee Updated Mar 24, 2022
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A diode is a two‑terminal semiconductor device that allows current to flow in only one direction when a sufficient voltage is applied. It is constructed from two differently doped semiconductor materials—commonly a p‑type (electron‑accepting) and an n‑type (electron‑donating). At the junction, electrons from the n‑type side fill vacancies in the p‑type side, creating a depletion region with an internal electric field. This field prevents reverse current until the applied voltage exceeds a small threshold, typically around 0.6–0.7 V for silicon diodes.
The first practical application of diodes was in AM radio receivers. Radio waves induce an alternating current (AC) in the antenna. To extract the audio signal, the receiver needed a rectifier that could convert this AC into a usable direct current (DC). The diode served this purpose by permitting only one half of the waveform to pass, effectively “half‑wave rectifying” the signal. Subsequent filtering removed the high‑frequency component, leaving a smooth DC waveform for audio amplification.
When a forward voltage is applied across a diode, electrons recombine with holes at the junction, releasing energy as photons. The wavelength—and therefore the color—of the emitted light depends on the semiconductor material and its bandgap. LEDs are now the dominant illumination source in displays, indicators, and general lighting due to their low power consumption, high reliability, and long lifespan.
Diodes are not only emitters but also detectors. Photodiodes absorb photons, generating electron–hole pairs that create a measurable current. This principle underlies infrared remote‑control systems: the remote’s LED emits infrared photons, which are captured by a photodiode in the device, converting the light into an electrical signal. The same concept powers photovoltaic panels, where sunlight creates a flow of charge across a junction, producing a DC current that powers homes and grid‑connected systems.
Diodes are essential safeguards in electronic circuits. Reverse‑polarity protection diodes allow only a minimal leakage current when a battery is inserted incorrectly, preventing damage to sensitive components. Surge‑suppressing “avalanche” diodes clamp excessive voltage spikes by conducting to ground once a critical voltage is exceeded, protecting downstream electronics from transient over‑voltage events.
