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Contact resistance
Additional recommended knowledge
Origin of contact resistanceThere are two major determinants of contact resistance: geometry and insulating layers between the contacting surfaces (better known as "dirt.") The resistance of a contact is inversely proportional to its area, which is in turn dependent on the force holding the two surfaces together and their stiffness. Joining methods like soldering or wire bonding keep the force between two halves of a contact constant. Softer metals like gold tend to form larger area contacts and are therefore preferred for bonding pads. Gold also has the advantage of being nonreactive, which prevents the formation of oxides or other poorly conducting reaction products. Experimental characterizationThe specific contact resistance is experimentally defined as the slope of the IV curve at V=0:
where J is the current density = current/area. The units of contact resistance are typically therefore in Ω.cm^{2} where Ω stands for ohms. When the current is a linear function of the voltage, the device is said to have ohmic contacts. The resistance of contacts can be crudely estimated by comparing the results of a four terminal measurement to a simple twolead measurement made with an ohmmeter. In a twolead experiment, the measurement current causes a potential drop across both the test leads and the contacts so that the resistance of these elements is inseparable from the resistance of the actual device, with which they are in series. In a fourpoint probe measurement, one pair of leads is used to inject the measurement current while a second pair of leads, in parallel with the first, is used to measure the potential drop across the device. In the fourprobe case, there is no potential drop across the voltage measurement leads so the contact resistance drop is not included. The difference between resistance derived from twolead and fourlead methods is a reasonably accurate measurement of contact resistance assuming that the leads resistance is much smaller. Specific contact resistance can be obtained by multiplying by contact area. For development of integrated circuit fabrication processes, far more sophisticated measurements of contact resistance are used, the most popular being the transmission line measurement. The basic idea of the transmission line measurement is to plot the resistance of strips of constant width and varying length that are terminated by similar contacts. The slope of the resulting line is a function of the bulk film resistivity while the intercept is the contact resistance. Inductive and capacitive methods could be used in principle to measure an intrinsic impedance without the complication of contact resistance. In practice, direct current methods are more typically used to determine resistance. Quantum limitWhen a conductor has spatial dimensions close to (2 * π) / k_{F}, where k_{F} is Fermi wavevector of the conducting material, Ohm's law does not hold any more. These small devices are called quantum point contacts. Their conductance must be an integer multiple of the value 2e^{2} / h, where e is the electronic charge and h is Planck's constant. Quantum point contacts behave more like waveguides than the classical wires of everyday life and may be described by the Landauer scattering formalism.^{[1]} Pointcontact tunneling is an important technique for characterizing superconductors. Other forms of contact resistanceMeasurements of thermal conductivity are also subject to contact resistance. Similarly, a drop in hydrostatic pressure (analogous to electrical voltage) occurs when fluid flow transitions from one channel to another. SignificanceBad contacts are the cause of failure or poor performance in a wide variety of electrical devices. For example, corroded jumper cable clamps can frustrate attempts to start a vehicle that has a dead battery. Dirty or corroded contacts on a fuse or its holder can give the false impression that the fuse is blown. A sufficiently high contact resistance can cause substantial heating in a high current device. Unpredictable or noisy contacts are a major cause of the failure of electrical equipment. An intermittent contact which alternates rapidly between a high and low resistance is the worst nightmare of anyone who has to troubleshoot equipment. Notes


This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Contact_resistance". A list of authors is available in Wikipedia. 