What Is Carrier Concentration?

Paul Reed

Carrier concentration is the number of electrons available to pass through a semiconductor. A semiconductor is an electronic device that will conduct electricity when an energy source is applied. Crystals or amorphous, or non-crystalline, materials are manufactured to form semiconductor material. Trace amounts of metal molecules, called dopants, can be added to provide extra electrons for transporting electrical current.

Solar panels have amorphous silicon cells connected to electrical circuits.
Solar panels have amorphous silicon cells connected to electrical circuits.

A molecule is made up of a central nucleus surrounded by rings or shells of electrons that are constantly in motion. Some materials, called donors, have an electron relatively far away from the nucleus that can be dislodged by electric currents or sunlight. Different molecules, called acceptors, lack an electron in the outer shell and can take free electrons that are present. A semiconductor uses donor and acceptor molecules placed in crystalline or amorphous material. Spaces for electrons in acceptor materials are often referred to as holes.

Silicon, both crystalline and amorphous, is commonly used for semiconductors. It can transmit some electrons as a pure material at different temperatures. This is known as the intrinsic carrier concentration. Pure silicon is rarely used as a semiconductor because the intrinsic concentration is quite low. Other materials, like germanium or silicon carbide, have a higher intrinsic carrier concentration and can be used as pure semiconductors.

Small amounts of dopants can change the properties of a semiconductor and allow electron flow with less resistance. The measurement of electron capacity for doped semiconductors is known as the extrinsic carrier concentration. This value is used to calculate the electrical properties of the semiconductor in an electronic circuit. Changes in carrier concentration from controlling the doping will affect the electrical properties of the semiconductor.

A semiconductor contains three sections. The conduction band is material doped with trace molecules that have excess electrons. Gap material, normally a pure material without doping, is placed in the middle. The last layer is the valence layer, where material is doped with trace molecules lacking electrons.

There are many common uses for semiconductors other than in electronic devices. Solar panels are comprised of amorphous silicon cells connected to electrical circuits. The energy of sunlight releases electrons in the conduction band that passes through the silicon semiconductors and creates an electrical current. Electricity created from solar panels is typically used to charge battery banks for later use.

Light-emitting diodes, or LEDs, are common devices used for lighting homes, businesses and vehicles. An electrical current activates a semiconductor containing dopants that give visible light when electrons pass through them. LEDs create very little excess heat, are energy efficient, and have long useful lifetimes.

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