The diode is one of the most fundamental components among semiconductor devices. Its role is simple: to let electricity flow in only one direction — in other words, to allow current to travel one way while blocking it in the reverse direction.

This simple principle forms the foundation for a wide range of applications, including rectification, signal detection, voltage control, and light emission.

A diode is formed by joining a P-type semiconductor with an N-type semiconductor.

・P-type semiconductor: A region rich in holes (positive charge).

・N-type semiconductor: A region rich in electrons (negative charge).

When these two are joined, electrons and holes recombine at the junction, forming an electrically neutral region known as the depletion layer. This layer acts as a “wall” that controls the flow of current through the diode.

When a positive voltage is applied to the P side and a negative voltage to the N side, the following occurs:

・The external voltage pushes down the barrier formed by the depletion layer.

・Electrons move from the N side to the P side, and holes move from the P side to the N side.

・As a result, current begins to flow.

This condition is known as forward bias. In practice, for a silicon diode, current typically begins to flow once the applied voltage exceeds around 0.7V.

Conversely, when a negative voltage is applied to the P side and a positive voltage to the N side, the following occurs:

・The external voltage widens the depletion layer.

・The movement of electrons and holes is impeded.

・Almost no current flows.

This condition is known as reverse bias. In theory, the current under reverse bias would be zero, but in practice a small amount of leakage current does flow.

While a standard diode blocks current under reverse bias, the Zener diode is an important exception.

Once the reverse voltage exceeds a certain threshold — known as the Zener voltage — a stable current begins to flow even in the reverse direction. This property is used to create a stable reference voltage source, keeping voltage constant.

The most common application of diodes is rectification — converting alternating current (AC) into direct current (DC), which flows in only one direction.

・A single diode produces half-wave rectification.

・Four diodes connected in a bridge configuration produce full-wave rectification.

This mechanism is widely used in power supply circuits that convert household AC power into DC.

When a forward voltage is applied to a PN junction, electrons and holes recombine. LEDs (Light Emitting Diodes) take advantage of the fact that some of the energy released during this recombination is emitted as light.

The color of the light emitted depends on the semiconductor material used:

・GaAs → Infrared light

・GaP → Green light

・GaN → Blue light

The advent of the blue LED made it possible to produce white light, bringing about a revolution in lighting and display technology.

A Schottky diode is formed by a junction between a metal and a semiconductor, known as a Schottky junction. It has a smaller voltage drop than a standard PN-junction diode and is capable of high-speed switching. Schottky diodes are widely used in power supply circuits and high-frequency communication equipment.

・As temperature rises, reverse leakage current increases.

・The forward voltage (Vf) decreases as temperature rises.

For these reasons, thermal management and temperature compensation are important considerations in circuits that use diodes.

・A diode is a semiconductor device that allows current to flow in only one direction.

・The depletion layer formed by the PN junction is what controls the flow of current.

・Current flows under forward bias but not under reverse bias.

・Diodes come in many forms — LEDs, Zener diodes, Schottky diodes, and others — each suited to a different purpose.

・The diode is a foundational component underlying every transistor and IC.

1.What structure allows a diode to achieve “one-way conduction”?

2.At roughly what voltage does current begin to flow under forward bias in a silicon diode?

3.What is the main application of a Zener diode?

Column Supervisor: Koji Kakumoto (Otis Group Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined Otis Group Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as Representative Director, working to drive business growth and strengthen the organization.

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.