Until Heaviside discovered that Maxwell's equations imply wave propagation when sufficient inductance is in the circuit, this square diffusion relationship was thought to provide a fundamental limit to the improvement of long-distance telegraph cables. Charge spreads by diffusion in such a wire, as explained by Lord Kelvin in the mid nineteenth century. The typical digital propagation delay of a resistive wire is about half of R times C since both R and C are proportional to wire length, the delay scales as the square of wire length.
This delay can be reduced by replacing the aluminum conducting wire by copper, thus reducing the resistance it can also be reduced by changing the interlayer dielectric (typically silicon dioxide) to low-dielectric-constant materials, thus reducing the capacitance. When the feature size becomes smaller and smaller to increase the clock speed, the RC delay plays an increasingly important role. Resistive-capacitive delay, or RC delay, hinders the further increasing of speed in microelectronic integrated circuits.
The signal delay of a wire or other circuit, measured as group delay or phase delay or the effective propagation delay of a digital transition, may be dominated by resistive-capacitive effects, depending on the distance and other parameters, or may alternatively be dominated by inductive, wave, and speed of light effects in other realms. In more complicated circuits consisting of more than one resistor and/or capacitor, the open-circuit time constant method provides a way of approximating the cutoff frequency by computing a sum of several RC time constants. The RC time constant, also called tau, the time constant (in seconds) of an RC circuit, is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads), i.e.
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