Tech Tips

How diodes work and what they are used for! What kind of situations are they used in? | Matsusada Precision

How Diodes Work

A diode is an electronic component that directs the flow of electricity in one direction. They are called "active components," the same as transistors and ICs. It is a basic component made of semiconductors. It can regulate the flow of electricity, keep the voltage constant, and detect waves.

First, let's review the properties of the "semiconductor" used in diodes. "Can this material conduct electricity?" It is classified into "conductor", "semiconductor", and "insulator" based on the question. A "semiconductor" is a material with properties in between a conductor that conducts electricity well and an insulator that does not.

In general, metals conduct electricity well because the electrons of each atom become free electrons when metallic elements bond with each other. When a voltage is applied, the free electrons in the metal crystal move around and carry an electric charge, which is how electricity flows.

Semiconductors can behave as conductors or insulators depending on the state of the electricity flowing through them. Semiconductors do not have an abundance of free electrons like metals do. When a voltage is applied, electrons move in turn to fill in the missing holes, or they carry electricity with fewer free electrons than metal bonds.

Semiconductors are divided into P-type semiconductors and N-type semiconductors according to the difference in the mechanism of electricity flow; P-type semiconductors are those in which the electrons of the former move in sequence to fill in the missing holes. A tetravalent element such as silicon mixed with a trivalent additive such as boron or boron becomes a P-type semiconductor. Since it lacks one electron, it is considered to be positively charged.

N-type semiconductors are those that carry electricity with fewer free electrons than the latter metal bonds. A tetravalent element such as silicon mixed with a monovalent additive such as phosphorus becomes an N-type semiconductor. Since it has one extra electron, it is considered to be negatively charged.

In a PN diode, the electrode connected to the P-type semiconductor is called the anode (A), and the electrode connected to the N-type semiconductor is called the cathode (K). (Figure 1)

Anode Cathode Battery Figure PN | Matsusada Precision

When "-" is connected to the anode side and "+" is connected to the cathode side of a PN diode, the electricity in the semiconductor is attracted to the electrode side, and a blank zone of electricity is generated at the PN junction. As a result, no electricity flows. (Figure 2)

Conversely, if "+" is connected to the anode side and "-" to the cathode side, the "+" and "-" electricity in the semiconductor will stick together at the P and N junctions and cancel each other out, but the next electricity will be sent from the electrode, so electricity will flow. (Figure 3)

In this way, diodes have the property of conducting electricity only in a fixed direction. The light-emitting diodes (LEDs) that we often see in our daily lives are designed to emit light when electricity flows through the PN junction. Diodes are also used in various places where we cannot see them, supporting our daily lives.

The Role of Diodes

Diodes have the following four main roles.

(1) Rectification

The direction of the current is always changing because of the AC current in general power supplies. Diodes have the property of only allowing electricity to flow in a certain direction, so only the forward current can be extracted from the AC current. This is called the rectifying action of the diode.

(2) Radio wave detection

Diodes play a role in extracting audio signals from radio waves. This is called wave detection. Radio waves are created by combining high frequency signals used for communication with low frequency signals such as voice.

(3) Voltage control

In general, diodes only carry current in a certain direction, but when the voltage in the opposite direction exceeds a certain value, voltage begins to flow. However, when the voltage in the reverse direction exceeds a certain value, the voltage starts to flow, and even if the current increases, the voltage does not change. This is called the breakdown phenomenon, and the voltage at which the breakdown phenomenon occurs is called the "breakdown voltage" or "Zener voltage.
The yield phenomenon is used in diode voltage control, and diodes used in this way are called Zener diodes.

(4)Current conversion

When light strikes the PN junction, electrons on the N side near the junction move. As a result, electricity will continue to flow as long as the light is shining. This is what a solar cell is made of.
When no voltage is applied from the outside, it acts as a battery, but when voltage is applied, it acts as a diode. Some diodes respond to visible light, while those that respond to invisible light are used in applications such as the light receiving part of infrared remote controls.

Types of Diodes

There are various types of diodes. The following is a list of some of the most common types.

Silicon diodes

The most common type of PN diode. Most often refers to rectification diodes.

Germanium diodes

Like silicon diodes, these are diodes that combine PNs. They are often used for detecting waves because of their low forward falling voltage, especially in the region where the current flowing is as small as 0.1mA. However, due to the high cost of germanium, Schottky barrier diodes are now widely used.

Schottky diode

This is a diode that is made by bonding a metal and a semiconductor. These diodes have superior switching characteristics compared to silicon diodes, and are therefore used in high-speed circuits.

Switching Diode

A diode used to open and close a power circuit like a switch. It turns ON when voltage is applied in the direction of power flow, and turns OFF when voltage is applied in the direction where power does not flow.

Esaki Diode

A diode that utilizes the tunneling effect discovered by Nobel laureate Leona Esaki. The tunneling effect is a property of PN junction diodes with high impurity concentration that allows current to flow even when it should not due to quantum mechanical effects. Because of their extremely fast response time, they are used to generate microwaves.

Light-emitting diode(LED)

A diode in which the junction emits light when current flows through the PN junction. When electricity flows through the semiconductor, the holes and electrons in the P-type semiconductor combine, and the energy is emitted as light. It is sometimes used as both a power lamp and a rectifier.

Zener diode

A diode used to apply voltage in the opposite direction to that in which current normally flows. It is used to obtain a constant voltage and also to protect the circuit from overvoltage.