Diode

A diode is a two-terminal electronic component that lets electricity flow in just one direction. It works like a one-way valve for electric current, making sure the flow goes only one way and not the other.

The most common type today is a semiconductor diode. This is made from a special piece of material, usually silicon, that has a special connection called a p–n junction between two points where you can connect wires. These diodes were the first devices made from semiconductor materials. The ability to control the flow of electricity this way was first discovered in 1874 by a scientist named Ferdinand Braun.

Diodes are used in many everyday things. They help change alternating current, the kind of electricity that comes from a wall outlet, into direct current, which is used in cars and many electronic devices. They are also used in radios to pick out the signals we want to hear, and some special diodes can even light up, like the tiny lights on your computer or phone that show it is on.

Main functions

Unidirectional current flow

A diode lets electricity flow in just one direction. This is called the forward direction. In the opposite direction, called the reverse direction, the diode blocks the electricity. This one-way flow can change alternating current (AC) into direct current (DC). This process is known as rectification. Diodes used this way can help get signals from radio waves in radios.

Threshold voltage

A diode needs a certain amount of voltage to let electricity flow easily. This is called the forward threshold voltage. Below this voltage, very little electricity flows. Above it, electricity flows more freely. The voltage needed can change a little with temperature, so diodes can sometimes be used to measure temperature.

Reverse breakdown

If too much voltage is put on a diode in the reverse direction, it suddenly lets electricity flow. This can be used to keep circuits safe from big voltage spikes.

Other functions

By choosing different materials and adding certain impurities, diodes can be made to do special jobs. Some diodes help tune radios and TVs, while others can create radio waves or even produce light. Some diodes can also be used in special electronic circuits.

History

Thermionic (vacuum-tube) diodes and solid-state (semiconductor) diodes were developed separately around the same time in the early 1900s for use in radio receivers as detectors. Until the 1950s, vacuum diodes were more commonly used in radios because early semiconductor diodes were less stable. Many radios also used vacuum tubes for boosting signals, which could easily include these diodes. Vacuum diodes were better at handling high-voltage tasks compared to early semiconductor diodes.

In 1873, Frederick Guthrie noticed that a hot metal ball could affect electrical charges. In 1880, Thomas Edison observed one-way current flow in a bulb, later called the Edison effect. In 1904, John Ambrose Fleming created the first true thermionic diode, called the Fleming valve, which was used in many electronic devices like radios and televisions. Though they are less common today, vacuum diodes are still used in some special applications.

In 1874, Karl Ferdinand Braun found that electricity could flow in one direction between a metal and a mineral. Later, scientists used crystals to detect radio waves. During World War II, researchers developed these crystal diodes for use in radar. After the war, these diodes were used in microwave towers and many radar systems. In the early 1950s, new types of semiconductor diodes were developed.

Thermionic

Further information: Vacuum tube § History and development

Semiconductor

Etymology

Main article: Rectifier

Early devices that let electricity flow in only one direction were called rectifiers because they changed alternating current into direct current. In 1919, a scientist named William Henry Eccles created the word diode using two Greek words: di meaning "two" and hodos meaning "path". He used it to describe a special vacuum tube with two electrodes.

Today, the word diode means any device with two connections that lets electricity flow more easily in one direction than the other. The word rectifier is often used for bigger devices that change power, while diode is used for smaller parts in circuits.

Vacuum tube diodes

Main article: Vacuum tube characteristics

A thermionic diode is a special type of glass or metal container with no air inside. It has two parts inside called a cathode and a plate. The cathode gets very hot, around the temperature of red-hot metal, which makes tiny particles called electrons come out of it.

When the plate is positively charged compared to the cathode, it pulls in these electrons, and a flow of electricity happens from the cathode to the plate. But if the plate is negatively charged, it pushes the electrons away, so no electricity flows. This means the diode only lets electricity flow in one direction.

Other types of diodes include the mercury vapor diode, the xenon gas diode, the Cold-cathode rectifier, and the magnetron.

Semiconductor diodes

A semiconductor diode is a common electronic part that lets electricity flow in just one direction. It’s made from a special material called a semiconductor, usually silicon. Inside the diode, there are two parts: one with extra electrons (called n-type) and one with fewer electrons (called p-type). When these two parts are joined together, they create a diode that can control the flow of electricity.

Diodes are used in many everyday devices. For example, they help make sure electricity flows the right way in circuits, which is important for things like lights, computers, and radios. They can also be used to turn alternating current (which changes direction) into direct current (which flows only one way), a process called rectification.

Related devices

Some devices work in similar ways to diodes. These include rectifiers, which change alternating current to direct current, transistors that control current flow, and thyristors (also called silicon controlled rectifiers or SCRs). Other related devices are TRIACs, DIACs, and varistors.

In the world of light, a device similar to a diode is the optical isolator, sometimes called an optical diode. This device lets light travel in only one direction, using a special part called a Faraday rotator.

Applications

Radio demodulation

The first use for a diode was to get audio from AM radio broadcasts. An AM signal has waves that go up and down, and the diode only lets the upward parts through. This makes it easier to get the audio from the radio waves.

In microwave and millimeter wave technology, starting in the 1930s, researchers improved small crystal detectors. Point contact diodes (crystal diodes) and Schottky diodes are used in radar, microwave and millimeter wave detectors.

Power conversion

Main article: Rectifier

Rectifiers are made from diodes to change alternating current (AC) electricity into direct current (DC). Car alternators use diodes to change AC to DC, which works better than older methods.

Reverse-voltage protection

Most electronic circuits can break if the power is connected backward. A diode can help protect them by only allowing power to flow one way.

Over-voltage protection

Diodes are often used to protect sensitive electronic parts from too much voltage. They usually don’t let current flow, but if the voltage gets too high, they let the extra voltage away from the parts they protect.

Logic gates

Diode–resistor logic can make AND and OR logic gates. Adding another part can also make an inversion.

Ionizing radiation detectors

Besides light, semiconductor diodes can also sense very energetic radiation. This is used in special tools to detect radiation. These tools can measure how much energy the radiation has by how much charge it creates in the diode.

Temperature measurements

A diode can also help measure temperature because the voltage across it changes with temperature, like in a silicon bandgap temperature sensor.

Current steering

Diodes stop current from flowing the wrong way. For example, they can help make sure a battery only gives power when needed, like in an uninterruptible power supply. They are also used in musical keyboards and pinball machines to stop unwanted notes or actions.

Waveform clipper

Main article: Clipper (electronics)

Diodes can be used to limit how high or low a signal can go.

Clamper

Main article: Clamper (electronics)

A diode clamp circuit can move a signal up or down so that its highest or lowest points are at a certain level, without changing how much the signal goes up and down overall.

Computing exponentials and logarithms

The way a diode’s current changes with voltage can be used to calculate exponentials and logarithms with analog voltage signals.

Abbreviations

Diodes are often called D for diode on PCBs. Sometimes, people use the abbreviation CR which stands for crystal rectifier.