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A diode circuit is any of a variety of electrical circuits that take advantage of the distinguishing characteristics of diodes. A class of crystalline semiconductors with two terminals, diodes exhibit a strong bias toward carrying an electrical charge "forward" in one direction while all but completely inhibiting it in the other. Diode circuits are commonly used in power supply applications to convert alternating current (AC) to direct current (DC) and to tune TV and radio receivers. They are also used as analog and digital logic switches, as capacitors to temporarily store and increase electrical charge, in surge protectors to prevent voltage spikes from damaging equipment, and as sensors to detect light and to produce light. Besides rectifier diodes, other common types include light emitting diodes (LEDs), varicap diodes, and Zener diodes.
Diodes were the first semiconductor electronic devices to be invented. Used widely in the electronics industry, they are usually made of silicon, though germanium is used as well. The electrical resistance of a diode circuit is minimal in the forward direction, from the anode to the cathode, hence the term "forward bias." Silicon diodes, for instance, have a 0.6-0.7 volt voltage drop, the threshold point, when carrying current in the forward direction. A relatively high minimum voltage must be reached for current to flow through a diode in the reverse direction. It is these properties that make diode circuits very useful in a wide variety of electronic devices.
In a diode circuit, a diode may be connected to any of a wide variety of other electrical or electronic devices — capacitors, resistors, transformers, power supplies, etc. — depending on the application. Diodes in circuits may be arranged in series or in parallel. An initial application of a diode circuit, one still in widespread use today, is the switching of analog signals. In the early days of digital computing, diode circuits were used to perform the digital logic operations AND and OR.
Of the many different types of diodes used in circuits, LEDs produce light of visible and non-visible frequencies when current passes between the electrodes. Varicap, or varactor, diodes are used to tune radio and TV receivers. Another type, the photodiode, detects light. They typically operate in reverse bias and are used to generate electricity and in solar photovoltaic cells. Zener diodes also operate in reverse bias and are used widely in power supplies to regulate voltage by producing a stable reference voltage.