Contact resistance
It occurs at electrical connections such as switches, connectors, breakers, contacts, and measurement probes.
Contact resistance values are typically small (in the microohm to milliohm range).
Contact resistance can cause significant voltage drops and heating in circuits with high current.
William Shockley[1] introduced the idea of a potential drop on an injection electrode to explain the difference between experimental results and the model of gradual channel approximation.
In two-electrode systems, specific contact resistivity is experimentally defined as the slope of the I–V curve at V = 0: where
The units of specific contact resistivity are typically therefore in ohm-square metre, or Ω⋅m2.
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.
The three electrode systems such as transistors require more complicated methods for the contact resistance approximation.
The slope of the linear function is related to the channel transconductance and can be used for estimation of the ”contact resistance-free” carrier mobility.
[2] Beside the TLM it was proposed the gated four-probe measurement[3] and the modified time-of-flight method (TOF).
[7] For given physical and mechanical material properties, parameters that govern the magnitude of electrical contact resistance (ECR) and its variation at an interface relate primarily to surface structure and applied load (Contact mechanics).
If a contact patch is sufficiently small, with dimensions comparable or smaller than the mean free path of electrons resistance at the patch can be described by the Sharvin mechanism, whereby electron transport can be described by ballistic conduction.
is Fermi wavevector of the conducting material, Ohm's law does not hold anymore.
Quantum point contacts behave more like waveguides than the classical wires of everyday life and may be described by the Landauer scattering formalism.
Measurements of thermal conductivity are also subject to contact resistance, with particular significance in heat transport through granular media.
Similarly, a drop in hydrostatic pressure (analogous to electrical voltage) occurs when fluid flow transitions from one channel to another.
Bad 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 low battery.
Unpredictable or noisy contacts are a major cause of the failure of electrical equipment.
