Doping (semiconductor)

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

In the context of phosphors and scintillators, doping is better known as activation.

History

Its effects were long known empirically in such devices as crystal radio detectors and selenium rectifiers. However, semiconductor doping was formally first developed by John Robert Woodyard working at Sperry Gyroscope Company during World War II.^[1] The demands of his work on radar denied Woodyard the opportunity to pursue research on semiconductor doping. However, after the war ended, his patent proved the grounds of extensive litigation by Sperry Rand.^[2] Related work was performed at Bell Labs by Gordon K. Teal and Morgan Sparks.^[3]

Process

Some dopants are added as the (usually silicon) boule is grown, giving each wafer an almost uniform initial doping.^[4] To define circuit elements, selected areas — typically controlled by photolithography^[5] — are further doped by such processes as diffusion^[6] and ion implantation, the latter method being more popular in large production runs because of increased controllability.

Small numbers of dopant atoms can change the ability of a semiconductor to conduct electricity. When on the order of one dopant atom is added per 100 million atoms, the doping is said to be low or light. When many more dopant atoms are added, on the order of one per ten thousand atoms, the doping is referred to as heavy or high. This is often shown as n+ for n-type doping or p+ for p-type doping. (See the article on semiconductors for a more detailed description of the doping mechanism.)

Dopant elements

Group IV semiconductors

(Note: When discussing periodic table groups, semiconductor physicists always use an older notation, not the current IUPAC group notation. For example, the carbon group is called "Group IV", not "Group 14".)

For the Group IV semiconductors such as silicon, germanium, and silicon carbide, the most common dopants are acceptors from Group III or donors from Group V 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 because it diffuses at a rate that makes junction depths easily controllable. Phosphorus is typically used for bulk-doping of silicon wafers, while arsenic is used to diffuse junctions, because 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 that become unbonded from individual atoms and allow the compound to be an electrically conductive n-type semiconductor. Doping with Group III elements, which are missing the fourth valence electron, creates "broken bonds" (holes) in the silicon lattice that are free to move. The result is an electrically conductive p-type semiconductor. In this context, a Group V element is said to behave as an electron donor, and a group III element as an acceptor. This is a key concept in the physics of a diode.

Very heavily doped semiconductor behaves more like a good conductor (metal) and thus exhibits more linear positive thermal coefficient. Such effect is used for instance in sensistors.^[7] Lower dosage of doping is used in other types (NTC or PTC) thermistors.

Compensation

In most cases many types of impurities will be present in the resultant doped semiconductor. 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 oxidize, or sometimes reduce, the system so that electrons are pushed into the conducting orbitals within the already potentially conducting system. There are two primary methods of doping a conductive polymer, both of which use an oxidation-reduction (i.e., redox) process.

1. Chemical doping involves exposing a polymer such as melanin, typically a thin film, to an oxidant such as iodine or bromine. Alternatively, the polymer can be exposed to a reductant; this method is far less common, and typically involves alkali metals.
2. Electrochemical doping involves suspending a polymer-coated, working electrode 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 that causes a charge and the appropriate counter ion from the electrolyte to enter the polymer in the form of electron addition (i.e., n-doping) or removal (i.e., p-doping).

N-doping is much less common because the Earth's atmosphere is oxygen-rich, thus creating an oxidizing environment. An electron-rich, n-doped polymer will react immediately with elemental oxygen to de-dope (i.e., reoxidize to the neutral state) the polymer. Thus, chemical n-doping must be performed 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, it is unlikely that n-doped conductive polymers are available commercially.

Magnetic doping

Research on magnetic doping has shown that considerable alteration of certain properties such as specific heat may be affected by small concentrations of an impurity; for example, dopant impurities in semiconducting ferromagnetic alloys can generate different properties as first predicted by White, Hogan, Suhl and Nakamura.^[8][9]

Single dopants in semiconductors

The sensitive dependence of a semiconductor’s electronic, optical, and magnetic properties on dopants has provided an extensive range of tunable phenomena to explore and apply to devices. Recently it has become possible to move past the tunable properties of an ensemble of dopants and to identify the effects of a solitary dopant on commercial device performance as well as locally on the fundamental properties of a semiconductor. New applications have become available that require the discrete character of a single dopant, such as single-spin devices in the area of quantum information or single-dopant transistors. Dramatic advances in the past decade towards observing, controllably creating and manipulating single dopants, as well as their application in novel devices have allowed opening the new field of solotronics (solitary dopant optoelectronics).^[10]

Neutron transmutation doping

Neutron transmutation doping (NTD) is an unusual doping method for special applications. Most commonly, it is used to dope silicon n-type in high-power electronics. It is based on the conversion of the Si-30 isotope into phosphorus atom by neutron absorption as follows:

$^{30}\mathrm{Si} \, (n,\gamma) \, ^{31}\mathrm{Si} \rightarrow \, ^{31}\mathrm{P} + \beta^- \; (\mathrm{T}_{1/2} = 2.62 h).$

In practice, the silicon is typically placed near a nuclear reactor to receive the neutrons. As neutrons continue to pass through the silicon, more and more phosphorus atoms are produced by transmutation, and therefore the doping becomes more and more strongly n-type. NTD is a far less common doping method than diffusion or ion implantation, but it has the advantage of creating an extremely uniform dopant distribution.^{[11] [12]}

See also

• Extrinsic semiconductor
• Intrinsic semiconductor
• p-n junction
• List of semiconductor materials

References

1. ^ U.S. Patent 2,530,110 filed, 1944, granted 1950
2. ^ "John Robert Woodyard, Electrical Engineering: Berkeley". University of California: In Memoriam. 1985. http://content.cdlib.org/view?docId=hb4d5nb20m&doc.view=frames&chunk.id=div00182&toc.depth=1&toc.id=&brand=calisphere. Retrieved 2007-08-12. 
3. ^ Sparks, Morgan and Teal, Gordon K. “Method of Making P-N Junctions in Semiconductor Materials,” U.S. Patent 2,631,356 (Filed June 15, 1950. Issued March 17, 1953)
4. ^ Levy, Roland Albert (1989). Microelectronic Materials and Processes. Dordrecht: Kluwer Academic. pp. 6–7. ISBN 0792301544. http://books.google.com/?id=wZPRPU6ne7UC&pg=PA248#PPA1,M1. Retrieved 2008-02-23. 
5. ^ Computer History Museum - The Silicon Engine|1955 - Photolithography Techniques Are Used to Make Silicon Devices
6. ^ Computer History Museum - The Silicon Engine 1954 - Diffusion Process Developed for Transistors
7. ^ Dharma Raj Cheruku, Battula Tirumala Krishna, Electronic Devices and Circuits, 2nd edition, 2008, Delhi, India, ISBN 978-81-317-0098-3
8. ^ Hogan, C. Michael (1969). "Density of States of an Insulating Ferromagnetic Alloy". Physical Review 188 (2): 870. Bibcode 1969PhRv..188..870H. doi:10.1103/PhysRev.188.870. 
9. ^ Zhang, X. Y.; Suhl, H. (1985). "Spin-wave-related period doublings and chaos under transverse pumping". Physical Review A 32 (4): 2530–2533. Bibcode 1985PhRvA..32.2530Z. doi:10.1103/PhysRevA.32.2530. PMID 9896377. 
10. ^ Koenraad, Paul M.; Flatté, Michael E. (2011). "Single dopants in semiconductors". Nature Materials 10 (2): 91–100. Bibcode 2011NatMa..10...91K. doi:10.1038/nmat2940. PMID 21258352. 
11. ^ B. J. Baliga, Modern Power Devices, p.32, John Wiley & Sons, New York (1987).
12. ^ P.E. Schmidt and J. Vedde, “High Resistivity NTD Production and Applications,” Electrochemical Society Proceedings, Vol.98-13, 3 (1998).