Doping (semiconductors)

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nl:Doteren In semiconductor production, doping refers to the process of intentionally introducing impurities into a pure semiconductor substrate, in order to change its electrical properties. The desired impurities are dependent upon the basic type of semiconductor used.

Some dopants are generally added as a boule is grown, giving each wafer an almost uniform initial doping. To define circuit elements, selected areas (controlled by i.e. photolithography) are further doped by such processes as diffusion and ion implantation, the latter method being more popular in large production runs due to higher speed and greater overall control.

Contents

Doping elements

Group IV semiconductors

For such semiconductors such as silicon, germanium, and silicon carbide, the most common dopants are group III or group V elements, referring by Roman numerals to the columns in the Periodic Table of the Elements. Arsenic, boron, gallium, phosphorus are all commonly used to dope silicon.

By doping pure silicon with group V elements such as Phosphorus, extra valence electrons are added which become unbonded from individual atoms and allow the compound to be electrically conductive, n-type material. Doping with group III elements, such as Boron, which are missing the forth valence electron creates "broken bonds", or holes, in the silicon lattice that are free to move. This is electrically conductive, p-type material. In this context then, a group V element is said to behave as an electron donor, and a group III element as an acceptor.

III-V and II-VI semiconductors

Other compound semiconductors such as gallium arsenide are quite often doped with group IV elements such as Si. In most cases, a low concentration of Si will preferrentially substitute for the group III element, and function as a donor.

Compensation

In most cases, many types of impurity will be present. If an equal number of donors and acceptors are present in the semiconductor, the extra electrons provided by the former will be used to satisfy the broken bonds due to the other, so that doping produces no free carriers of either type. This phenomenon is known as compensation, and occurs at the p-n junction in the vast majority of semiconductor devices. Partial compensation, where donors outnumber acceptors or vice-versa, allows device makers to repeatedly reverse the type of a given portion of the material by applying successively higher doses of dopants.

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