Doping (semiconductor)

In semiconductor production, doping refers to the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor in order to change its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic. A semiconductor which is doped to such high levels that it acts more like a conductor than a semiconductor is called degenerate.

Some dopants are generally added as the (usually silicon) boule is grown, giving each wafer an almost uniform initial doping. To define circuit elements, selected areas (typically controlled by photolithography)^[1] are further doped by such processes as diffusion^[2] and ion implantation, the latter method being more popular in large production runs due to its better controllability.

The number of dopant atoms needed to create a difference in the ability of a semiconductor to conduct is very small. Where a comparatively small number of dopant atoms are added (of the order of 1 every 100,000,000 atoms) then the doping is said to be low, or light. Where many more are added (of the order of 1 in 10,000) then the doping is referred to as heavy, or high. This is often shown as n+ for n-type dopant or p+ for p-type doping. A more detailed description of the mechanism of doping can be found in the article on semiconductors.

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Dopant elements

Group IV semiconductors

For the group IV semiconductors such as silicon, germanium, and silicon carbide, the most common dopants are group III or group V elements. (Group number refers to the Roman numerals of the columns in the periodic table of the elements.) Boron, arsenic, phosphorus and occasionally gallium are used to dope silicon. Boron is the p-type dopant of choice for silicon integrated circuit production, since it diffuses at a rate which makes junction depths easily controllable. Phosphorus is typically used for bulk doping of silicon wafers, while arsenic is used to diffuse junctions, since it diffuses more slowly than phosphorus and is thus more controllable.

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

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 core electrons provided by the former will be used to satisfy the broken bonds due to the latter, 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.

Although compensation can be used to increase or decrease the number of donors or acceptors, the electron and hole mobility is always decreased by compensation because mobility is affected by the sum of the donor and acceptor ions.

Doping in organic conductors

Main article: Conductive polymer

Conductive polymers can be 'doped' by adding chemical reactants to oxidise (or sometimes reduce) the system to push electrons into the conducting orbitals within the already (potentially) conducting system. (In a silicon lattice, the system is far from conducting to begin with!) There are two primary methods of doping a conductive polymer, both through an oxidation-reduction (redox) process. The first method, chemical doping, involves exposing a polymer, such as melanin (typically a thin film), to an oxidant (typically iodine or bromine) or reductant (far less common, but typically involves alkali metals). The second is electrochemical doping in which a polymer-coated, working electrode is suspended in an electrolyte solution in which the polymer is insoluble along with separate counter and reference electrodes. An electric potential difference is created between the electrodes which causes a charge (and the appropriate counter ion from the electrolyte) to enter the polymer in the form of electron addition (n doping) or removal (p doping).

The reason n doping is so much less common is that Earth's atmosphere is oxygen-rich, which creates an oxidizing environment. An electron-rich n doped polymer will react immediately with elemental oxygen to de-dope (re-oxidize to the neutral state) the polymer. Thus, chemical n doping has to be done in an environment of inert gas (e.g., argon). Electrochemical n doping is far more common in research, because it is easier to exclude oxygen from a solvent in a sealed flask; however, there are likely no commercialized n doped conductive polymers.

History

Doping was originally developed by John Robert Woodyard working at Sperry Gyroscope Company during World War II.^[3] The demands of his war work on radar denied Woodyard the opportunity to pursue this line of research but, post-war, his patent proved the grounds of extensive litigation by Sperry Rand.^[4]. Related work was done at Bell Labs by Teal and Sparks.^[5]

See also

• Intrinsic semiconductor
• Extrinsic semiconductor

References