Most semiconductor circuits are built on a silicon substrate. When devices switch, especially those in the digital domain or involve high currents, transient noise can be injected into the substrate. Due to the nonzero conductivity of the substrate material, an intended coupling can be made devices. Analog devices, which are more susceptible to noise, can be affected by this noise in the substrate.
Designers can minimize this noise coupling using isolation techniques. Common techniques include deep trench, wells or guard rings.