Tap cell (well tap)
Beginner
A tiny helper part placed at regular spacing that connects the chip’s silicon base to power, so its switches behave reliably.
Novice
A small cell with no logic that connects the silicon under the transistors (the wells and the substrate) to the power and ground wires. Taps are laid down in a regular pattern before any logic, so no transistor is too far from one; this prevents latch-up, a fault in which a stray path through the silicon shorts power to ground.
Expert
LEF CLASS CORE WELLTAP. The foundry sets a design rule on the distance from any active area to the nearest tap, so taps go in a checkerboard at a fixed pitch (OpenROAD tapcell -distance) and are FIXED before placement. They cut rows into segments that legalization must treat separately.