power switch cell

A power switch cell is the on-chip breaker that physically cuts a block off from its power supply — the transistor that actually performs power gating. It is a big, high-threshold MOSFET inserted between the always-on supply rail and the block's local rail (a 'header' switch on VDD) or between the block and ground (a 'footer' switch). When the block is needed, the switch is closed and current flows; when it sleeps, the switch opens and leakage collapses to almost nothing.

Designing these switches is a balancing act. The transistor must be wide enough that its on-resistance doesn't starve the block of current and cause IR-drop, yet small enough not to waste area and its own leakage. Hundreds of them are sprinkled across a domain and daisy-chained, and the enable is deliberately staggered (a slow turn-on sequence) so that thousands of gates don't all gulp current at once and crash the supply — a phenomenon called rush current or in-rush. The switch's own leakage when off is tiny because it is a high-Vt device, which is the whole point: a small always-leaking guard that saves a much larger always-leaking block.