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An ohmic contact is a non-rectifier electrical junction: a junction between two conductors that has a linear current–voltage (I–V) curve as with Ohm's law. Low-resistance ohmic contacts are used to allow charge to flow easily in both directions between the two conductors, without blocking due to rectification or excess power dissipation due to voltage thresholds.
By contrast, a junction or contact that does not demonstrate a linear I–V curve is called non-ohmic. Non-ohmic contacts come in a number of forms, such as p–n junction, Schottky barrier, rectifying heterojunction, or breakdown junction.
Generally the term "ohmic contact" implicitly refers to an ohmic contact of a metal to a semiconductor, where achieving ohmic contact resistance is possible but requires careful technique. Metal–metal ohmic contacts are relatively simpler to make, by ensuring direct contact between the metals without intervening layers of insulating contamination, excessive roughness or oxidation; various techniques are used to create ohmic metal–metal junctions (soldering, welding, Crimp connection, deposition, electroplating, etc.). This article focuses on metal–semiconductor ohmic contacts.
Stable contacts at semiconductor interfaces, with low contact resistance and linear I–V behavior, are critical for the performance and reliability of semiconductor devices, and their preparation and characterization are major efforts in circuit fabrication. Poorly prepared junctions to semiconductors can easily show rectifying behaviour by causing Depletion region near the junction, rendering the device useless by blocking the flow of charge between those devices and the external circuitry. Ohmic contacts to semiconductors are typically constructed by depositing thin metal films of a carefully chosen composition, possibly followed by annealing to alter the semiconductor–metal bond.
The Schottky barrier height between a metal and semiconductor is naively predicted by the Schottky–Mott rule to be proportional to the difference of the metal-vacuum work function and the semiconductor-vacuum electron affinity. In practice, most metal–semiconductor interfaces do not follow this rule to the predicted degree. Instead, the chemical termination of the semiconductor crystal against a metal creates electron states within its band gap. The nature of these metal-induced gap states and their occupation by electrons tends to pin the center of the band gap to the Fermi level, an effect known as Fermi level pinning. Thus, the heights of the Schottky barriers in metal–semiconductor contacts often show little dependence on the value of the semiconductor or metal work functions, in stark contrast to the Schottky–Mott rule. Different semiconductors exhibit this Fermi level pinning to different degrees, but a technological consequence is that high quality (low resistance) ohmic contacts are usually difficult to form in important semiconductors such as silicon and gallium arsenide.
The Schottky–Mott rule is not entirely incorrect since, in practice, metals with high work functions form the best contacts to p-type semiconductors, while those with low work functions form the best contacts to n-type semiconductors. Unfortunately experiments have shown that the predictive power of the model doesn't extend much beyond this statement. Under realistic conditions, contact metals may react with semiconductor surfaces to form a compound with new electronic properties. A contamination layer at the interface may effectively widen the barrier. The surface of the semiconductor may reconstruct leading to a new electronic state. The dependence of contact resistance on the details of the interfacial chemistry is what makes the reproducible fabrication of ohmic contacts such a manufacturing challenge.
The fundamental steps in contact fabrication are semiconductor surface cleaning, contact metal deposition, patterning and annealing. Surface cleaning may be performed by sputter-etching, chemical etching, reactive gas etching or ion milling. For example, the native oxide of silicon may be removed with a hydrofluoric acid dip, while GaAs is more typically cleaned by a bromine-methanol dip. After cleaning, metals are deposited via sputter deposition, evaporation or chemical vapor deposition (CVD). Sputtering is a faster and more convenient method of metal deposition than evaporation but the ion bombardment from the plasma may induce surface states or even invert the charge carrier type at the surface. For this reason the gentler but still rapid CVD may be used. Post-deposition annealing of contacts is useful for relieving stress as well as for inducing any desirable reactions between the metal and the semiconductor.
Because deposited metals can themselves react in ambient conditions, to the detriment of the contacts' electrical properties, it is common to form ohmic contacts with layered structures, with the bottom layer, in contact with the semiconductor, chosen for its ability to induce ohmic behaviour. A diffusion barrier-layer may be used to prevent the layers from mixing during any annealing process.
The measurement of contact resistance is most simply performed using a four-point probe although for more accurate determination, use of the transmission line method is typical.
Modern ohmic contacts to silicon such as titanium-tungsten disilicide are usually made by CVD. Contacts are often made by depositing the transition metal and forming the silicide by annealing with the result that the silicide may be non-stoichiometric. Silicide contacts can also be deposited by direct sputtering of the compound or by ion implantation of the transition metal followed by annealing.
Formation of contacts to compound semiconductors is considerably more difficult than with silicon. For example, GaAs surfaces tend to lose arsenic and the trend towards As loss can be considerably exacerbated by the deposition of metal. In addition, the volatility of As limits the amount of post-deposition annealing that GaAs devices will tolerate. One solution for GaAs and other compound semiconductors is to deposit a low-bandgap alloy contact layer as opposed to a heavily doped layer. For example, GaAs itself has a smaller bandgap than AlGaAs and so a layer of GaAs near its surface can promote ohmic behavior. In general the technology of ohmic contacts for III-V and II-VI semiconductors is much less developed than for Si.
| Silicon | aluminum, Al-Si, TiSi2, titanium nitride, tungsten, MoSi2, PtSi, CoSi2, WSi2 |
| Germanium | indium, AuGa, AuSb |
| Gallium arsenide | AuGe, PdGe, PdSi, Ti/Pt/Au |
| Gallium nitride | Ti/Al/Ni/Au, Pd/Au |
| indium | |
| ZnO | InSnO2, aluminum |
| CuIn1−xGaxSe2 | molybdenum, InSnO2 |
| indium | |
| titanium/gold,molybdenum/gold |
Transparent or semi-transparent contacts are necessary for active matrix LCD displays, optoelectronic devices such as and photovoltaics. The most popular choice is indium tin oxide, a metal that is formed by reactive sputtering of an In-Sn target in an oxide atmosphere.
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