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Jai Ganesh

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Registered: 2005-06-28

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Diode

Diode

Gist

A diode is a two-terminal semiconductor device that acts as a one-way valve for electricity, allowing current to flow easily in one direction (forward-biased) while restricting it in the opposite direction (reverse-biased). Primarily used to convert alternating current (AC) to direct current (DC), they are essential for circuit protection, rectification, and voltage regulation.

A diode is a two-terminal semiconductor device acting as a one-way electrical valve, allowing current to flow easily in one direction (forward-biased) but blocking it in the reverse direction, making it crucial for power conversion (AC to DC), overvoltage protection (Zener diodes), signal demodulation (radios), light emission (LEDs), and building digital logic gates in computers. They are fundamental in electronics, found in everything from chargers to solar panels, by controlling current flow and protecting circuits. 

Summary

A diode is a two-terminal electronic component that conducts electric current primarily in one direction (asymmetric conductance). It has low (ideally zero) resistance in one direction and high (ideally infinite) resistance in the other.

A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. It has an exponential current–voltage characteristic. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other semiconducting materials such as gallium math and germanium are also used.

The obsolete thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from the cathode to the plate.

Among many uses, diodes are found in rectifiers to convert alternating current (AC) power to direct current (DC), demodulation in radio receivers, and can even be used for logic or as temperature sensors. A common variant of a diode is a light-emitting diode, which is used as electric lighting and status indicators on electronic devices.

Details

A diode is an electrical component that allows the flow of current in only one direction. In circuit diagrams, a diode is represented by a triangle with a line across one vertex.

The most common type of diode uses a p-n junction. In this type of diode, one material (n) in which electrons are charge carriers abuts a second material (p) in which holes (places depleted of electrons that act as positively charged particles) act as charge carriers. At their interface, a depletion region is formed across which electrons diffuse to fill holes in the p-side. This stops the further flow of electrons. When this junction is forward biased (that is, a positive voltage is applied to the p-side), electrons can easily move across the junction to fill the holes, and a current flows through the diode. When the junction is reverse biased (that is, a negative voltage is applied to the p-side), the depletion region widens and electrons cannot easily move across. The current remains very small until a certain voltage (the breakdown voltage) is reached and the current suddenly increases.

Light-emitting diodes (LEDs) are p-n junctions that emit light when a current flows through them. Several p-n junction diodes can be connected in series to make a rectifier (an electrical component that converts alternating current to direct current). Zener diodes have a well-defined breakdown voltage, so that current flows in the reverse direction at that voltage and a constant voltage can be maintained despite fluctuations in voltage or current. In varactor (or varicap) diodes, varying the bias voltage causes a variation in the diode’s capacitance; these diodes have many applications for signal transmission and are used throughout the radio and television industries. (For more detail about these and other types of diodes, see semiconductor device.)

Early diodes were vacuum tubes, an evacuated glass or metal electron tube containing two electrodes—a negatively charged cathode and a positively charged anode. These were used as rectifiers and as detectors in electronic circuits such as radio and television receivers. When a positive voltage is applied to the anode (or plate), electrons emitted from the heated cathode flow to the plate and return to the cathode through an external power supply. If a negative voltage is applied to the plate, electrons cannot escape from the cathode, and no plate current flows. Thus, a diode permits electrons to flow from cathode to plate but not from plate to cathode. If an alternating voltage is applied to the plate, current flows only during the time when the plate is positive. The alternating voltage is said to be rectified, or converted to direct current.

Additional Information

A diode is a semiconductor device, typically made of silicon, that essentially acts as a one-way switch for current. It allows current to flow easily in one direction but severely restricts current from flowing in the opposite direction.

Diodes are also known as rectifiers because they change alternating current (AC) into pulsating direct current (DC). Diodes are rated according to their type, voltage, and current capacity.

What Does a Diode Do?

Diodes have polarity, determined by an anode (positive lead) and cathode (negative lead). Most diodes allow current to flow only when positive voltage is applied to the anode.

When a diode allows current flow, it is forward-biased. When a diode is reverse-biased, it acts as an insulator and does not permit current to flow.

Strange but true: The diode symbol's arrow points against the direction of electron flow. The reason is that engineers conceived the symbol, and their schematics show current flowing from the positive (+) side of the voltage source to the negative (-). It's the same convention used for semiconductor symbols that include arrows — the arrow points in the permitted direction of "conventional" flow, and against the permitted direction of electron flow.

Types of Diodes

Different diode types all perform key functions in an electrical system. Here are some of the most important types of diodes.

* Rectifier Diodes: A rectifier diode converts AC into DC. Direct current flows in one consistent direction, making it easier to control.
* Zener Diodes: Zener diodes are designed to protect electrical systems from overvoltage. They conduct current in reverse whenever the cathode reaches a predetermined threshold voltage.
* Light Emitting Diodes: A light-emitting diode (LED) emits light when forward-biased current flows through it. LEDs are widely used in displays and indicators.
* Schottky Diodes: Schottky diodes, also known as barrier diodes or hot-carrier diodes, are often used in high-speed applications because of their fast-switching capabilities and their low forward voltage drop.
* Photodiodes: Photodiodes produce electrical currents when they absorb photons. They are widely used in sensors and in solar cells.

Diode Ratings and Testing

Every diode has a current and voltage rating, determined by factors like materials and design. Those ratings represent the maximum levels of voltage and currency the diode can tolerate.

Exceeding currency and voltage levels can cause permanent damage to the diode or to the whole circuit. High voltage levels may also cause the diode to short circuit and either allow current to flow in both directions or halt current from flowing in either direction.

Proper diode function can be tested with a digital multimeter (DMM).

Testing Diodes with a Digital Multimeter

There are two methods of testing diodes using a digital multimeter: diode test mode and resistance mode. Diode test mode is by far the more effective means of testing. Resistance testing is less reliable and should only be used if the multimeter does not have a diode test mode.

A digital multimeter's diode test produces a small voltage between the test leads, enough to forward-bias a diode junction. A good forward-bias diode displays a voltage drop from 0.5 to 0.8 volts (for most silicon diodes). The meter will display ‘OL’ when a good diode is reverse-biased. OL will indicate the diode is functioning as an open switch.

The forward-biased resistance of a good diode should range from 1000 ohms to 10 Mohms. Thereverse-biased resistance on a good diode, will read OL The diode is bad if readings are the same in both directions.

Applications of Diodes

* Power Conversion: Rectifiers convert AC signal into pulsing DC signal, making it easier to control the flow of power.
* Signal Demodulation: Demodulation diodes are a critical part of radio receivers. The diodes retrieve the original message sent through the airwaves for transmission.
* Overvoltage Protection: Zener diodes are used to protect circuits from unsafe voltage levels. They are also used to protect supply lines and power supply control lines.
* Logic Gates: Diodes play a crucial role in modern computing and digital technology. They make it possible to reinforce binary systems through gates that perform simple logic functions, like and/or/not.

Final Thoughts

Though small and simple, the diode is an essential part of modern electrical systems. By controlling the flow and direction of current, diodes enable countless applications of modern electronics.

Fortunately, tools like digital multimeters make it easy to measure diode health and determine when it’s time to replace diodes. This allows technicians to keep circuits in good running order, powering the electrical systems we all rely on.

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