Doping (semiconductor)
In semiconductor production, doping is the intentional introduction of impurities into an intrinsic (undoped) semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic semiconductor.
Small numbers of dopant
In the context of phosphors and scintillators, doping is better known as activation; this is not to be confused with dopant activation in semiconductors. Doping is also used to control the color in some pigments.
History
The effects of impurities in semiconductors (doping) were long known empirically in such devices as
Similar work was performed at
Woodyard's prior
Carrier concentration
The concentration of the dopant used affects many electrical properties. Most important is the material's charge carrier concentration. In an intrinsic semiconductor under thermal equilibrium, the concentrations of electrons and holes are equivalent. That is,
In a non-intrinsic semiconductor under thermal equilibrium, the relation becomes (for low doping):
where n0 is the concentration of conducting electrons, p0 is the conducting hole concentration, and ni is the material's intrinsic carrier concentration. The intrinsic carrier concentration varies between materials and is dependent on temperature. Silicon's ni, for example, is roughly 1.08×1010 cm−3 at 300 kelvins, about room temperature.[6]
In general, increased doping leads to increased conductivity due to the higher concentration of carriers. Degenerate (very highly doped) semiconductors have conductivity levels comparable to metals and are often used in integrated circuits as a replacement for metal. Often superscript plus and minus symbols are used to denote relative doping concentration in semiconductors. For example, n+ denotes an n-type semiconductor with a high, often degenerate, doping concentration. Similarly, p− would indicate a very lightly doped p-type material. Even degenerate levels of doping imply low concentrations of impurities with respect to the base semiconductor. In intrinsic crystalline silicon, there are approximately 5×1022 atoms/cm3. Doping concentration for silicon semiconductors may range anywhere from 1013 cm−3 to 1018 cm−3. Doping concentration above about 1018 cm−3 is considered degenerate at room temperature. Degenerately doped silicon contains a proportion of impurity to silicon on the order of parts per thousand. This proportion may be reduced to parts per billion in very lightly doped silicon. Typical concentration values fall somewhere in this range and are tailored to produce the desired properties in the device that the semiconductor is intended for.
Effect on band structure
Doping a semiconductor in a good crystal introduces allowed energy states within the
Dopants also have the important effect of shifting the energy bands relative to the
Relationship to carrier concentration (low doping)
For low levels of doping, the relevant energy states are populated sparsely by electrons (conduction band) or holes (valence band). It is possible to write simple expressions for the electron and hole carrier concentrations, by ignoring Pauli exclusion (via Maxwell–Boltzmann statistics):
where EF is the Fermi level, EC is the minimum energy of the conduction band, and EV is the maximum energy of the valence band. These are related to the value of the intrinsic concentration via[7]
an expression which is independent of the doping level, since EC – EV (the band gap) does not change with doping.
The concentration factors NC(T) and NV(T) are given by
where me* and mh* are the density of states effective masses of electrons and holes, respectively, quantities that are roughly constant over temperature.[7]
Techniques of doping and synthesis
Doping during crystal growth
Some dopants are added as the (usually silicon) boule is grown by Czochralski method, giving each wafer an almost uniform initial doping.[8]
Alternately, synthesis of semiconductor devices may involve the use of
Post-growth doping
To define circuit elements, selected areas — typically controlled by photolithography[12] — are further doped by such processes as diffusion[13] and ion implantation, the latter method being more popular in large production runs because of increased controllability.
Spin-on glass
Spin-on glass or spin-on dopant doping is a two-step process of applying a mixture of SiO2 and dopants (in a solvent) onto a wafer surface by spin-coating and then stripping it and baking it at a certain temperature in the furnace at constant nitrogen+oxygen flow.[14]
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 and semiconductor detectors. It is based on the conversion of the Si-30 isotope into phosphorus atom by neutron absorption as follows:
Dopant elements
Group IV semiconductors
(Note: When discussing
For the
By doping pure silicon with
A 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.[17] Lower dosage of doping is used in other types (NTC or PTC) thermistors.
Silicon dopants
- Acceptors, p-type
- Boron is a p-type dopant. Its diffusion rate allows easy control of junction depths. Common in CMOS technology. Can be added by diffusion of diborane gas. The only acceptor with sufficient solubility for efficient emitters in transistors and other applications requiring extremely high dopant concentrations. Boron diffuses about as fast as phosphorus.
- Aluminum, used for deep p-diffusions. Not popular in VLSI and ULSI. Also a common unintentional impurity.[18]
- Gallium is a dopant used for long-wavelength infrared photoconduction silicon detectors in the 8–14 μm atmospheric window.[19] Gallium-doped silicon is also promising for solar cells, due to its long minority carrier lifetime with no lifetime degradation; as such it is gaining importance as a replacement of boron doped substrates for solar cell applications.[18]
- Indium is a dopant used for long-wavelength infrared photoconduction silicon detectors in the 3–5 μm atmospheric window.[19]
- Donors, n-type
- Phosphorus is a n-type dopant. It diffuses fast, so is usually used for bulk doping, or for well formation. Used in solar cells. Can be added by diffusion of phosphine gas. Bulk doping can be achieved by nuclear transmutation, by irradiation of pure silicon with neutrons in a nuclear reactor. Phosphorus also traps gold atoms, which otherwise quickly diffuse through silicon and act as recombination centers.
- Arsenic is a n-type dopant. Its slower diffusion allows using it for diffused junctions. Used for buried layers. Has similar atomic radius to silicon, high concentrations can be achieved. Its diffusivity is about a tenth of phosphorus or boron, so it is used where the dopant should stay in place during subsequent thermal processing. Useful for shallow diffusions where well-controlled abrupt boundary is desired. Preferred dopant in VLSI circuits. Preferred dopant in low resistivity ranges.[18]
- Antimony is a n-type dopant. It has a small diffusion coefficient. Used for buried layers. Has diffusivity similar to arsenic, is used as its alternative. Its diffusion is virtually purely substitutional, with no interstitials, so it is free of anomalous effects. For this superior property, it is sometimes used in VLSI instead of arsenic. Heavy doping with antimony is important for power devices. Heavily antimony-doped silicon has lower concentration of oxygen impurities; minimal autodoping effects make it suitable for epitaxial substrates.[18]
- Bismuth is a promising dopant for long-wavelength infrared photoconduction silicon detectors, a viable n-type alternative to the p-type gallium-doped material.[20]
- Lithium is used for doping silicon for radiation hardened solar cells. The lithium presence anneals defects in the lattice produced by protons and neutrons.[21] Lithium can be introduced to boron-doped p+ silicon, in amounts low enough to maintain the p character of the material, or in large enough amount to counterdope it to low-resistivity n type.[22]
- Other
- Germanium can be used for gettering, and improves wafer mechanical strength.[18]
- Silicon, germanium and xenon can be used as ion beams for pre-amorphization of silicon wafer surfaces. Formation of an amorphous layer beneath the surface allows forming ultrashallow junctions for p-MOSFETs.
- Nitrogen is important for growing defect-free silicon crystal. Improves mechanical strength of the lattice, increases bulk microdefect generation, suppresses vacancy agglomeration.[18]
- Gold and platinum are used for minority carrier lifetime control. They are used in some infrared detection applications. Gold introduces a donor level 0.35 eV above the valence band and an acceptor level 0.54 eV below the conduction band. Platinum introduces a donor level also at 0.35 eV above the valence band, but its acceptor level is only 0.26 eV below conduction band; as the acceptor level in n-type silicon is shallower, the space charge generation rate is lower and therefore the leakage current is also lower than for gold doping. At high injection levels platinum performs better for lifetime reduction. Reverse recovery of bipolar devices is more dependent on the low-level lifetime, and its reduction is better performed by gold. Gold provides a good tradeoff between forward voltage drop and reverse recovery time for fast switching bipolar devices, where charge stored in base and collector regions must be minimized. Conversely, in many power transistors a long minority carrier lifetime is required to achieve good gain, and the gold/platinum impurities must be kept low.[24]
- Germanium can be used for
Other semiconductors
[25] In the following list the "(substituting X)" refers to all of the materials preceding said parenthesis.
- Gallium arsenide
- n-type: tellurium, sulfur (substituting As); tin, silicon, germanium (substituting Ga)
- p-type: beryllium, zinc, chromium (substituting Ga); silicon, germanium, carbon (substituting As)
- Gallium phosphide
- n-type: tellurium, selenium, sulfur (substituting phosphorus)
- p-type: zinc, magnesium (substituting Ga); tin (substituting P)
- isoelectric: nitrogen (substituting P) is added to enable luminescence in older green indirect band gap)
- Gallium nitride, Indium gallium nitride, Aluminium gallium nitride
- n-type: silicon (substituting Ga), germanium (substituting Ga, better lattice match), carbon (substituting Ga, naturally embedding into MOVPE-grown layers in low concentration)
- p-type: magnesium (substituting Ga) - challenging due to relatively high interstitialMg, hydrogen complexes passivating of Mg acceptors and by Mg self-compensation at higher concentrations)
- n-type: silicon (substituting Ga), germanium (substituting Ga, better lattice match), carbon (substituting Ga, naturally embedding into
- Cadmium telluride
- n-type: indium, aluminium (substituting Cd); chlorine (substituting Te)
- p-type: phosphorus (substituting Te); lithium, sodium (substituting Cd)
- Cadmium sulfide
- n-type: gallium (substituting Cd); iodine, fluorine (substituting S)
- p-type: lithium, sodium (substituting Cd)
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
Partial compensation, where donors outnumber acceptors or vice versa, allows device makers to repeatedly reverse (invert) the type of a certain layer under the surface of a bulk semiconductor by diffusing or implanting successively higher doses of dopants, so-called counterdoping. Most modern semiconductor devices are made by successive selective counterdoping steps to create the necessary P and N type areas under the surface of bulk silicon.[26] This is an alternative to successively growing such layers by epitaxy.
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 conductive polymers
- Chemical doping involves exposing a polymer such as alkali metals.
- Electrochemical doping involves suspending a polymer-coated, working potential difference is created between the electrodes that causes a charge and the appropriate counter ion from the electrolyteto 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
Doping in organic molecular semiconductors
Molecular dopants are preferred in doping molecular semiconductors due to their compatibilities of processing with the host, that is, similar evaporation temperatures or controllable solubility.[27] Additionally, the relatively large sizes of molecular dopants compared with those of metal ion dopants (such as Li+ and Mo6+) are generally beneficial, yielding excellent spatial confinement for use in multilayer structures, such as OLEDs and Organic solar cells. Typical p-type dopants include F4-TCNQ[28] and Mo(tfd)3.[29] However, similar to the problem encountered in doping conductive polymers, air-stable n-dopants suitable for materials with low electron affinity (EA) are still elusive. Recently, photoactivation with a combination of cleavable dimeric dopants, such as [RuCp∗Mes]2, suggests a new path to realize effective n-doping in low-EA materials.[27]
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
Single dopants in semiconductors
The sensitive dependence of a semiconductor's properties on dopants has provided an extensive range of tunable phenomena to explore and apply to devices. It is possible to identify the effects of a solitary dopant on commercial device performance as well as on the fundamental properties of a semiconductor material. 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).[33]
Modulation doping
Electrons or holes introduced by doping are mobile, and can be spatially separated from dopant atoms they have dissociated from. Ionized donors and acceptors however attract electrons and holes, respectively, so this spatial separation requires abrupt changes of dopant levels, of band gap (e.g. a
See also
- Extrinsic semiconductor
- Intrinsic semiconductor
- List of semiconductor materials
- Monolayer doping
- p-n junction
References
- ^ "Faraday to Shockley – Transistor History". Retrieved 2016-02-02.
- ^ Wilson, A. H. (1965). The Theory of Metals (2md ed.). Cambridge University Press.
- ^ US patent 2530110, Woodyard, John R., "Nonlinear circuit device utilizing germanium", issued 1950
- ^ US patent 2631356, Sparks, Morgan & Teal, Gordon K., "Method of Making P-N Junctions in Semiconductor Materials", issued March 17, 1953
- ^ "John Robert Woodyard, Electrical Engineering: Berkeley". University of California: In Memoriam. 1985. Retrieved 2007-08-12.
- doi:10.1063/1.349645.
- ^ doi:10.1063/1.345414.
- ISBN 978-0-7923-0154-7. Retrieved 2008-02-23.
- ISBN 978-0-521-01784-8.
- ISBN 978-0-07-041853-0.
- ISBN 978-0-19-508494-8.
- ^ "Computer History Museum – The Silicon Engine|1955 – Photolithography Techniques Are Used to Make Silicon Devices". Computerhistory.org. Retrieved 2014-06-12.
- ^ "1954: Diffusion Process Developed for Transistors". Computer History Museum.
- ^ "Spin-on Glass". inside.mines.edu. Retrieved 2022-12-22.
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- ISBN 9781566772075.
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- ISBN 978-94-011-7915-7.
- ^ US patent 4608452, Weinberg, Irving & Brandhorst, Henry W. Jr., "Lithium counterdoped silicon solar cell"
- ^ "2. Semiconductor Doping Technology". Iue.tuwien.ac.at. 2002-02-01. Retrieved 2016-02-02.
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External links
- Media related to Doping (semiconductor) at Wikimedia Commons