transition time.

Footer switch (NMOS) characteristics:

• Lower resistance due to higher mobility and more drive strength, so the slew is faster.
• Less short-circuit power because of the lower transition time.
• Higher leakage power because of the lower resistance.
• Footer gates are smaller for the same current, since NMOS has roughly twice the mobility of PMOS.
• Switch-on and switch-off take less time because of the lower transition time.

Why a PMOS header is generally preferred:

• It has lower leakage (higher resistance) and a slower switching rate; a fast switching rate would draw a huge rush current to turn on the block and cause power-integrity problems.
• Power-gating devices should use high-Vt cells for slower switching.
• NMOS is leakier than PMOS, and designs become more sensitive to ground noise on the virtual ground (VIRTUAL_VSS) coupled through the footer switch.
• The choice of footer versus header depends on three parameters - switching efficiency, area efficiency and body bias.
• Switching efficiency is the ratio of drain current in the ON and OFF states (Ion/Ioff); total leakage in the power switch is mainly set by switching efficiency.
• Area efficiency depends on the L x W product; switching efficiency falls as W increases in PMOS transistors, so a small W is preferred.
• Applying reverse body bias to the sleep transistor raises Vt, lowers Ioff and significantly reduces leakage; the cost of reverse body bias is much smaller for a header switch because the N-well for PMOS is readily available for bias tapping in a standard CMOS process, while NMOS has no well and needs higher fabrication cost and design complexity.
• Conclusion: a PMOS header is preferable, especially for reverse body bias applications.