When a P-type semiconductor (rich in holes) is joined to an N-type semiconductor (rich in electrons), electrons and holes meet at the boundary and recombine.

The region at this junction that contains neither electrons nor holes is called the depletion layer. This depletion layer functions as a “wall that controls the pathway for electricity.”

Immediately after the junction is formed, electrons move from the N side toward the P side, and holes move from the P side toward the N side. As they recombine, the charge distribution becomes uneven.

As a result, fixed negative charge remains on the P side and fixed positive charge remains on the N side, creating an internal electric field — a voltage barrier — at the boundary.

This internal electric field then works to prevent the free movement of further electrons and holes.

The depletion layer is the region where electrons and holes have recombined, leaving it with no carriers present. Because this region does not conduct electricity, it forms the fundamental basis for a semiconductor’s ability to allow current to flow in only one direction.

In other words, it functions much like a railroad crossing gate: it opens if the current tries to flow in the correct direction, but stays closed if the current tries to flow the opposite way.

(1) Forward Bias

・A positive voltage is applied to the P-type side, and a negative voltage to the N-type side.

・The internal electric field barrier is lowered, and electrons and holes begin moving again.

・As a result, current flows.

(2) Reverse Bias

・A negative voltage is applied to the P-type side, and a positive voltage to the N-type side.

・The internal electric field barrier is raised, preventing carriers from moving.

・Almost no current flows.

In this way, a PN junction only allows current to flow in one direction. This property is known as its rectifying action.

A diode is a device built around a single PN junction. Because it allows current to flow in only one direction, it functions in a circuit much like a check valve that prevents backflow.

Applications include:

・Power supply circuits (rectifying AC to DC)

・Preventing reverse current

・Voltage stabilization (Zener diodes)

・Generating light (LEDs)

When a forward voltage is applied, almost no current flows at first. However, once the voltage exceeds a certain level — around 0.7V for silicon — current suddenly begins to flow.

This is known as the threshold voltage, and its value differs depending on the material (for example, it is around 0.3V for germanium).

Light-emitting diodes (LEDs) are also a type of PN junction. Under forward bias, as electrons and holes recombine, the excess energy is released in the form of light (photons).

The color of light emitted is determined by the size of the material’s band gap (for example, a smaller gap produces red light, while a larger gap produces blue light).

Normally, no current flows under reverse bias. However, once the voltage exceeds a certain level, dielectric breakdown occurs and current begins to flow. Devices called Zener diodes make use of this phenomenon to hold voltage at a constant level.

In this way, the PN junction serves as the starting point for control — the foundation for deliberately allowing or blocking the flow of current by design.

・Controlling the depletion layer becomes more difficult as devices are scaled down to the nanometer level.

・Countermeasures are needed against leakage current (a small residual current under reverse bias).

・A shift toward materials suited to high-temperature and high-voltage operation, such as SiC and GaN.

・Research into new switching methods based on quantum tunneling effects.

Overcoming these challenges will make possible more efficient power conversion and more energy-saving devices.

・A PN junction creates an “electrical wall” in the region where P-type and N-type materials meet.

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

・This property is the basic operating principle behind diodes, LEDs, and similar devices.

・As miniaturization and performance improve, new junction structures — such as gate-all-around (GAA) and 3D stacked structures — are also emerging.

1.What is the depletion layer?

2.Why does a diode allow current to flow in only one direction?

3.Why does an LED emit light?

 

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.