What is the substrate tap or substrate contact? Do I have to use it?
The substrate tap is a metal conductor that grounds the surface of the substrate. Most integrated circuits use substrate taps to collect the substrate current injected by passive and active devices. You are not required to use a substrate tap but due to substrate conductivity, lateral currents flow into the tap and the substrate loss will depend on the location of the substrate tap. You can model this effect by creating a substrate contact metal layer in the technology file and using the pix or 2portx commands to correctly extract the substrate effects.
