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A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator. Semiconductors differ from metals in their characteristic property of decreasing electrical resistivity with increasing temperature.[1] Semiconductor materials are useful because their behavior can be manipulated by the addition of impurities, known as doping. Current conduction in a semiconductor occurs via mobile or "free" electrons and holes, collectively known as charge carriers. Doping a semiconductor with a small amount of impurity atoms, greatly increases the number of charge carriers within it. When a doped semiconductor contains excess holes it is called "p-type", and when it contains excess free electrons it is known as "n-type". Semiconductors are the foundation of modern electronics, including radio, computers, and telephones. Semiconductor-based electronic components include transistors, solar cells, many kinds of diodes including the light-emitting diode (LED), the silicon controlled rectifier, photo-diodes, and digital and analog integrated circuits.

The electronic properties and the conductivity of a semiconductor can be changed in a controlled manner by adding very small quantities of other elements, called “dopants”, to the intrinsic material. This is typically achieved in crystalline silicon by adding impurities of boron or phosphorus to the melt and then allowing it to solidify into the crystal. This process is called "doping" and the semiconductor is termed "extrinsic".[4]

A simplified diagram illustrating the energy band levels of an insulator, a semiconductor, and a conductor. Electrons can only exist in certain energy levels.

The property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping.

Mass action law Under thermal equilibrium the product of the free electron concentration and the free hole concentration  is equal to a constant equal to the square of intrinsic carrier concentration. The intrinsic carrier concentration is a function of temperature. The equation for the mass action law for semiconductors is: