Sheet resistance is a popular and convenient integrated circuits concept. The resistance of a strip of material is given by
R = ρL/A,
where ρ is the material's resistivity, L is the strip's length, and A is the strip's cross-sectional area. Ideally, each layer in an integrated circuit has no thickness variation across the wafer. The resistance can then be rewritten as
R = (ρ/t)(L/W) = Rs(L/W),
where Rs is the sheet resistance of the film. Thus the resistance of a strip in an integrated circuit is fully characterized by its sheet resistance and its aspect ratio, L/W.
The concept of sheet resistance can also be applied to the ion implanted regions of a crystalline wafer, even though the resistivity of these regions varies with thickness. In this case, the sheet resistance is very well approximated by
Rs = 1 / ∫Tdt/ρ(t),
where the lower limit is the wafer surface and T (this integration limit is rather arbitrary) is the point at which the implantation doping concentration has fallen to the wafer's background doping concentration.