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An ohmic contact is a non-rectifying junction: an electrical 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 (p–n junction, Schottky barrier, rectifying heterojunction, breakdown junction, etc.). Generally the term "ohmic contact" implicitly refers to an ohmic contact of a metal to a semiconductor, where achieving ohmic behaviour 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 or oxidation; various techniques are used to create ohmic metal-metal junctions (soldering, welding, crimping, deposition, electroplating, etc.). This article focuses on metal–semiconductor ohmic contacts. Low-resistance, stable ohmic contacts to semiconductors 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 of the semiconductor 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. == Physics of formation of metal–semiconductor ohmic contacts == (詳細はSchottky barriers are dependent on the Schottky barrier height, which sets the threshold for the excess energy an electron requires to pass from the semiconductor to the metal. For the junction to admit electrons easily in both directions (ohmic contact), the barrier height must be small in at least some parts of the junction surface. To form an excellent ohmic contact (low resistance), the barrier height should be small everywhere and furthermore the interface should not reflect electrons. 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.〔http://academic.brooklyn.cuny.edu/physics/tung/Schottky/systematics.htm〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ohmic contact」の詳細全文を読む スポンサード リンク
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