Why PMOS aspect ratio is greater than NMOS in CMOS?

Reasons:

1. We know, Electrons has mobility 2.7 times higher the holes.

2. Since hole mobility is lesser than electron mobility, PMOS width must be greater to compensate

    and make the pull-up network stronger.

3.If W to L of PMOS is same as the corresponding NMOS, the charging time of the output node

   would be higher than that of the discharging time (related to NMOS)

     

       W/L to PMOS is 2 or 3 times the W to L of NMOS.