Overview
A diode is a device that conducts electricity much more readily in one direction than in the other. That apparently simple one-way behavior makes diodes useful for converting AC to DC, protecting circuits, controlling voltage, detecting radio signals, producing light, sensing light, switching high-frequency signals, and steering currents through electronic systems.
That vacuum-tube diode and the later crystal detector both performed rectification: they allowed one polarity of current more readily than the other. Modern semiconductor diodes perform the same basic circuit job with a tiny junction inside a solid material.
Why diodes matter before transistors
The semiconductor diode is one of the best introductions to solid-state electronics. It introduces several ideas that appear again in transistors:
- Semiconductors conduct differently from ordinary metals.
- Impurities can deliberately change the number and type of mobile charge carriers.
- P-type and N-type regions can be formed in one crystal.
- A boundary between those regions creates a depletion region and internal electric field.
- Applied voltage changes the behavior of that junction.
- Electrons and holes both participate in semiconductor current.
Once those ideas are comfortable, a bipolar junction transistor is much easier to approach. It contains two semiconductor junctions arranged so that a small input can control a much larger current. A transistor is not just two independent diodes wired together, but understanding a diode is the right first step toward understanding why a transistor works.
The two terminals
A semiconductor diode's terminals are called the anode and cathode. For an ordinary rectifier diode operating forward, conventional current flows from anode to cathode. The band printed around many axial diodes identifies the cathode end.
Ideal diode versus real diode
An ideal diode would conduct perfectly in one direction with zero voltage drop and block perfectly in the other direction. Real diodes have a forward voltage, leakage current, a maximum current, a maximum reverse voltage, finite switching speed, capacitance, resistance, and temperature-dependent behavior.
Common diode families
| Type | Main job | Important characteristic |
|---|---|---|
| Rectifier diode | Convert or steer power current. | Current and reverse-voltage capability. |
| Small-signal diode | Fast switching, detection, signal steering. | Low capacitance and switching behavior. |
| Schottky diode | Fast switching and low forward drop. | Metal-semiconductor junction; usually higher leakage. |
| Zener diode | Voltage reference or clamp in reverse breakdown. | Specified breakdown voltage. |
| LED | Convert current into light. | Forward voltage, wavelength/color, current. |
| Photodiode | Convert light into electrical signal. | Responsivity, dark current, speed. |
| Varactor | Voltage-controlled capacitance. | Junction capacitance versus reverse bias. |
| TVS diode | Clamp electrical transients. | Surge energy and clamping voltage. |
Continue through these pages
The Basics page explains polarity, forward and reverse bias, simple tests, and everyday circuits. Vacuum Diodes follows the light-bulb origin and thermionic valve. PN Junction introduces semiconductor doping, holes, electrons, and the depletion region. Diode Types surveys important special-purpose diodes, while Rectifiers & Uses puts them into practical circuits. The History page connects the lamp, radio detector, crystal rectifier, PN junction, LED, and transistor. The Advanced page adds semiconductor equations and switching effects, and To Transistors uses the diode as the bridge into the next solid-state device.