Doping (semiconductor)

In semiconductor production, doping is the intentional introduction of impurities into an intrinsic 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 atoms can change the ability of a semiconductor to conduct electricity. When on the order of one dopant atom is added per 100 million intrinsic 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 high or heavy. This is often shown as n+ for n-type doping or p+ for p-type doping. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as a degenerate semiconductor. A semiconductor can be considered i-type semiconductor if it has been doped in equal quantities of p and n.
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 were long known empirically in such devices as crystal radio detectors and selenium rectifiers. For instance, in 1885 Shelford Bidwell, and in 1930 the German scientist Bernhard Gudden, each independently reported that the properties of semiconductors were due to the impurities they contained.
A doping process was formally developed by John Robert Woodyard working at Sperry Gyroscope Company during World War II. Though the word doping is not used in it, his US Patent issued in 1950 describes methods for adding tiny amounts of solid elements from the nitrogen column of the periodic table to germanium to produce rectifying devices. The demands of his work on radar prevented Woodyard from pursuing further research on semiconductor doping.
Similar work was performed at Bell Labs by Gordon K. Teal and Morgan Sparks, with a US Patent issued in 1953.
Woodyard's prior patent proved to be the grounds of extensive litigation by Sperry Rand.

Carrier concentration

The concentration of the dopant used affects many electrical properties of the semi-conductor. 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 :
where n₀ is the concentration of conducting electrons, p₀ is the conducting hole concentration, and n_i is the material's intrinsic carrier concentration. The intrinsic carrier concentration varies between materials and is dependent on temperature. Silicon's n_i, for example, is roughly 1.08×10¹⁰ cm⁻³ at 300 kelvins, about room temperature.
In general, increased doping leads to increased conductivity due to the higher concentration of carriers. Degenerate 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 band gap, but very close to the energy band that corresponds to the dopant type. In other words, electron donor impurities create states near the conduction band while electron acceptor impurities create states near the valence band. The gap between these energy states and the nearest energy band is usually referred to as dopant-site bonding energy or E_B and is relatively small. For example, the E_B for boron in silicon bulk is 0.045 eV, compared with silicon's band gap of about 1.12 eV. Because E_B is so small, room temperature is hot enough to thermally ionize practically all of the dopant atoms and create free charge carriers in the conduction or valence bands.
Dopants also have the important effect of shifting the energy bands relative to the Fermi level. The energy band that corresponds with the dopant with the greatest concentration ends up closer to the Fermi level. Since the Fermi level must remain constant in a system in thermodynamic equilibrium, stacking layers of materials with different properties leads to many useful electrical properties induced by band bending, if the interfaces can be made cleanly enough. For example, the p-n junction's properties are due to the band bending that happens as a result of the necessity to line up the bands in contacting regions of p-type and n-type material.
This effect is shown in a band diagram. The band diagram typically indicates the variation in the valence band and conduction band edges versus some spatial dimension, often denoted x. The Fermi level is also usually indicated in the diagram. Sometimes the intrinsic Fermi level, E_i, which is the Fermi level in the absence of doping, is shown. These diagrams are useful in explaining the operation of many kinds of semiconductor devices.

Relationship to carrier concentration (low doping)

For low levels of doping, the relevant energy states are populated sparsely by electrons or holes. It is possible to write simple expressions for the electron and hole carrier concentrations, by ignoring Pauli exclusion :
where is the Fermi level, is the minimum energy of the conduction band, and is the maximum energy of the valence band. These are related to the value of the intrinsic concentration via
an expression which is independent of the doping level, since does not change with doping.
The concentration factors and are given by
where and are the density of states effective masses of electrons and holes, respectively, quantities that are roughly constant over temperature.

Techniques of doping and synthesis

Doping during crystal growth

Some dopants are added as the boule is grown by Czochralski method, giving each wafer an almost uniform initial doping.
Alternately, synthesis of semiconductor devices may involve the use of vapor-phase epitaxy. In vapor-phase epitaxy, a gas containing the dopant precursor can be introduced into the reactor. For example, in the case of n-type gas doping of gallium arsenide, hydrogen sulfide is added, and sulfur is incorporated into the structure. This process is characterized by a constant concentration of sulfur on the surface. In the case of semiconductors in general, only a very thin layer of the wafer needs to be doped in order to obtain the desired electronic properties.

Post-growth doping

To define circuit elements, selected areas — typically controlled by photolithography — are further doped by such processes as thermal diffusion doping and ion implantation, the latter method being more popular in large production runs for integrated circuits because of increased controllability.
Thermal diffusion doping, simply known as diffusion, is widely used in silicon photovoltaics and uses chemicals such as Boron tribromide or diborane as a source for doping with boron. With the diffusion process, the wafer is placed in a quartz tube furnace, using a quartz holder called a boat at a temperature of 1200 °C in which a chemical compound containing the dopant, such as Boron tribromide for doping with boron to create p-type semiconductor regions, or Phosphoryl chloride to create n-type regions, is introduced into the furnace. This creates a layer of the dopant on the surface of the wafer and this step is called pre-deposition. Then a second step, called drive-in, is performed in which the wafer is heated at a higher temperature of 1300 °C to introduce the dopant into the structure of the wafer. Diffusion can use solid, liquid or gaseous sources with dopant atoms, such as solid boron nitride for boron, arsenic trioxide for arsenic, liquid arsenic trichloride, gaaseous arsine or phosphine. If using a gaseous source, it is carried to the furnace using a carrier gas such as nitrogen, and then allowed to decompose on the hot surface of the wafer, depositing the desired dopant, such as arsenic for example. If a liquid source is used, its vapors are carried to the furnace using nitrogen. The furnace can be either horizontal or vertical.

Spin-on glass

Spin-on glass or spin-on dopant doping is a two-step process. First, a mixture of SiO₂ and dopants is applied to a wafer surface by spin-coating. Then it is stripping and baked at a certain temperature in a furnace with constant nitrogen+oxygen flow.

Neutron transmutation doping

doping 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:
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.