How Diodes Work

A diode is an electronic component that directs the flow of electricity in a single direction. These are called "active components" and are basic components of semiconductors. They can regulate the flow of electricity, maintain a constant voltage, and extract signals from radio waves.

First, we review the properties of the "semiconductor" used in diodes. A material is classified as a "conductor," "semiconductor," and "insulator" based on whether it can conduct electricity. A "semiconductor," as the name implies, is a material with properties between those of a conductor that conducts electricity and an insulator that does not.

Metals are good conductors of electricity 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 lattice move around and carry an electric charge, allowing the electricity to flow.

Semiconductors can behave as conductors or insulators depending on the direction of electricity flowing through them. Metal semiconductors do not have an abundance of free electrons. When a voltage is applied, electrons move to fill the missing holes, or they carry electricity with fewer free electrons.

Semiconductors are divided into p-type semiconductors and n-type semiconductors based on the difference in the electricity flow mechanism: p-type semiconductors are those in which the electrons move in sequence to fill in the missing holes. Tetravalent elements, such as silicon mixed with a trivalent additive like boron or gallium, become p-type semiconductors. A p-type semiconductor has more holes than electrons, which allows the current to flow from hole to hole. Because it lacks one electron, it is considered positively charged.

N-type semiconductors carry electricity with fewer free electrons than metal bonds. Tetravalent elements, such as silicon mixed with a pentavalent additive like phosphorus, become n-type semiconductors as they provide more electrons to the structure. Because it has one extra electron, it is considered 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)

When an n-type (extra electron) and p-type (extra hole) semiconductor is attached, a momentary flow of electrons occurs from the n to the p side, resulting in a blank zone between the two. Therefore, when "-" is connected to the anode side and "+" is connected to the cathode side of a PN diode, the electrons in the semiconductor are attracted to the anode side, and a blank zone of electricity is generated at the PN junction. Consequently, no electricity flows (Figure 2) in the circuit.

Conversely, if "+" is connected to the anode side and "-" to the cathode side, the "+" and "-" charge in the semiconductor will stick together at the P and N junction and cancel each other out, but the electrons will be allowed to move from the cathode to the anode, letting electricity flow. (Figure 3)

Thus, diodes have the property of conducting electricity in a fixed direction only. Light-emitting diodes (LEDs), which we see around us, are designed to emit light when electricity flows through the PN junction. Diodes are extensively used in various applications that support our daily lives.

The Role of Diodes

There are four primary uses of diodes:

(1) Rectification

In a regular AC power supply, the direction of the current always changes. Diodes allow electricity to flow only in one direction; therefore, only the forward current is extracted from the AC current. This is known as the rectifying action of the diode.

(2) Radiowave Detection

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

(3) Voltage control

Normally, diodes carry current only in one direction. However, when the voltage in the opposite direction exceeds a certain value, the current starts flowing. However, when the voltage in the reverse direction exceeds a certain value, the current 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." This phenomenon is used in diode voltage control, and the diodes used for this purpose are called Zener diodes.

(4) Current conversion

When light strikes a PN junction, the electrons on the N side near the junction move. As a result, electricity continues to flow as long as the light is shining. This is the principle of a solar cell.
When no voltage is applied from the outside, it acts as a battery, but when a voltage is applied, it acts as a diode. Some diodes respond to visible light, whereas others respond to invisible light in applications such as the light-receiving part of infrared remote controls.

Types of Diodes

There are various types of diodes; the following are the most common types:

Silicon diodes

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

Germanium diodes

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

Schottky diode

This diode is fabricated by bonding metal and semiconductors. These diodes have superior switching characteristics compared with silicon diodes and are therefore used in high-speed circuits.

Switching diode

This diode opens and closes a power circuit, such as a switch. It turns ON when voltage is applied in the direction of power flow and turns OFF when voltage is applied in the reverse direction.

Esaki diode

This diode utilizes the tunneling effect Nobel laureate Leona Esaki discovered. The tunneling effect is a property of PN junction diodes that have high impurity concentrations, allowing current to flow due to quantum mechanical effects. Owing to their extremely fast response times, they are used to generate microwaves.

Light-emitting diode (LED）

In this diode, the junction emits light when the 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 both as a power source and a rectifier.

Zener diode

This diode is used to apply voltage in the direction opposite to that in which the current normally flows. It obtains a constant voltage and protects the circuit from overvoltage.