The substances used as semiconductors are, for the most part, “crystals formed by covalent bonding.” The most representative of these is silicon (Si).

Silicon has a stable crystal structure, is highly workable, and is abundant on Earth, which has made it the mainstream material for semiconductors.

Other major materials include:

・Germanium (Ge)

・Gallium arsenide (GaAs)

・Silicon carbide (SiC)

・Gallium nitride (GaN)

Pure silicon (an intrinsic semiconductor) normally conducts almost no electricity. This is because all of its electrons are bonded, leaving none free to move.

However, by adding only a tiny amount of “impurity,” its conductivity can be dramatically altered. This technique is called “doping.”

Doping is a technique in which a very small number of foreign atoms (impurities) are mixed into a silicon crystal in order to either increase the number of electrons or create vacancies where electrons are missing.

Depending on the type of impurity added, two types of semiconductor result:

・N-type semiconductor (Negative type)

・P-type semiconductor (Positive type)

N-type semiconductors are made by adding elements such as phosphorus (P) or arsenic (As) to silicon.

These atoms each have five electrons. When they bond with silicon’s four, one electron is left over. Because this extra electron is free to move, current flows more easily.

In other words, an N-type semiconductor is one in which electrons act as the carriers.

P-type semiconductors are made by adding elements such as boron (B) or aluminum (Al) to silicon.

These atoms have only three electrons, so when they bond with silicon, a vacancy is created where an electron is missing — a “hole.”

This hole behaves as though it were moving in place of an electron, allowing current to flow.

In other words, a P-type semiconductor is one in which holes act as the carriers.

Doping is a delicate process in which even the addition of a minute amount — on the order of tens to hundreds of parts per million (ppm) — can dramatically change a material’s electrical properties.

In manufacturing, methods such as “ion implantation” and “thermal diffusion” are used to control this process with nanometer-level precision.

As a result, a device’s performance is determined by exactly where — and how much — impurity is introduced.

When a P-type region and an N-type region are placed next to each other, a “PN junction” is formed. This is the basic structure underlying diodes and transistors.

・Current flows in only one direction (a rectifying effect).

・Applying voltage enables ON/OFF control.

This mechanism is used inside virtually every electronic device, from smartphones to space rockets.

As temperature rises, the number of intrinsic carriers (electrons and holes naturally excited by thermal energy) increases. However, in a doped semiconductor, carriers originating from the added impurities vastly outnumber these intrinsic carriers, so around room temperature the material is relatively unaffected by temperature changes.

In other words, doping is what enables stable, predictable device behavior.

・As miniaturization progresses, the position of even a single atom becomes sensitive enough to affect performance.

・Controlling impurity diffusion within extremely thin layers becomes increasingly difficult.

・The influence of quantum tunneling effects grows more significant.

Looking ahead, research into technologies that control doping at the level of individual atoms, as well as doping techniques for new materials, will become increasingly important.

・Pure silicon conducts almost no electricity.

・Adding impurities (such as phosphorus or boron) can dramatically change its electrical properties.

・Electrons as carriers → N-type; holes as carriers → P-type.

・Combining P-type and N-type regions creates a PN junction, the foundation of all semiconductor devices.

1.What is doping?

2.What are the carriers in an N-type semiconductor?

3.What are the carriers in a P-type semiconductor?

 

 

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.