Contact Resistance Evaluation and Mechanisms
– Contact resistance can be evaluated in two-electrode systems (diodes) and three-electrode systems (transistors).
– Specific contact resistivity is defined as the slope of the I-V curve at V = 0 in two-electrode systems.
– Specific contact resistivity is typically measured in ohms-square meter.
– Ohmic contacts are characterized by a linear relationship between current and voltage.
– Four-terminal and two-lead measurements can be used to estimate contact resistance.
– Electrical contact resistance is influenced by surface structure and applied load.
– Metallic contacts have oxide layers and adsorbed water molecules, leading to different types of contact resistance.
– Contact patches at weakly contacting surfaces can relax over time through current-induced welding and dielectric breakdown.
– The mechanistic evaluation of electrical contact resistance considers surface chemistry, contact mechanics, and charge transport mechanisms.
– Resistance creep refers to the relaxation of contact resistance over time.
Quantum Limit and Other Forms of Contact Resistance
– When a conductor’s spatial dimensions are close to 2π/kF, where kF is the Fermi wavevector, Ohm’s law does not hold.
– Quantum point contacts are small devices where conductance must be an integer multiple of 2e^2/h.
– Quantum point contacts behave more like waveguides and can be described by the Landauer scattering formalism.
– Point-contact tunneling is a technique used to characterize superconductors.
– Contact resistance can also affect measurements of thermal conductivity and hydrostatic pressure in fluid flow.
– Thermal conductivity measurements through granular media are particularly affected by contact resistance.
– Transitioning from one channel to another can cause a drop in hydrostatic pressure due to contact resistance.
Significance of Contact Resistance
– Bad contacts can lead to failure or poor performance in electrical devices.
– Corroded contacts in jumper cables can prevent starting a vehicle with a low battery.
– Dirty or corroded contacts on fuses can give the false impression of a blown fuse.
– High contact resistance can cause substantial heating in high current devices.
– Unpredictable or noisy contacts are a common cause of electrical equipment failure.
Contact Force
– Contact force refers to the pressure or mechanical load applied to ensure proper contact between electrical contacts.
– It is crucial for establishing a low-resistance electrical connection.
– Insufficient contact force can result in increased contact resistance and poor electrical performance.
– Excessive contact force can lead to wear, deformation, and damage to the contacts.
– The optimal contact force depends on the specific application and contact material.
Contact Materials and Reliability
– Contact materials play a significant role in determining the electrical and mechanical properties of electrical contacts.
– Common contact materials include copper, silver, gold, and alloys such as silver-cadmium oxide (AgCdO) and silver-nickel (AgNi).
– The choice of contact material depends on factors such as electrical conductivity, thermal conductivity, corrosion resistance, and cost.
– Some materials, like silver, offer excellent electrical conductivity but may be prone to oxidation.
– Contact materials can also be coated or plated to enhance their performance and durability.
– Contact reliability refers to the ability of electrical contacts to maintain consistent and stable electrical connections over time. Source: https://en.wikipedia.org/wiki/Contact_resistance
Electrical contact resistance (ECR, or simply contact resistance) is resistance to the flow of electric current caused by incomplete contact of the surfaces through which the current is flowing, and by films or oxide layers on the contacting surfaces. 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. Because contact resistance adds to the intrinsic resistance of the conductors, it can cause significant measurement errors when exact resistance values are needed.
Contact resistance may vary with temperature. It may also vary with time (most often decreasing) in a process known as resistance creep.
Electrical contact resistance is also called interface resistance, transitional resistance, or the correction term. Parasitic resistance is a more general term, of which it is usually assumed that contact resistance is a major component.
William Shockley 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.
