There have been other questions on the site about why PMOS devices are observed to have less flicker noise than NMOS devices, for example this one, but I'm not sure it's correct.
- On mobility - the accepted answer on the linked question states that the difference comes from the difference in mobility and mean free time (MFT) between PMOS and NMOS charge carriers, i.e. holes and electrons.
Whichever device has a higher mobility, \$\mu\$, will have higher collisions because you have a greater probability of have a collision. [...] The mobility \$\mu_x\$ has the "mean free time between collisions" term of \$\tau_c\$, as \$\mu_{n,p}=\frac{q\tau_c}{2m_{n,p}}\$. [...] Once you calculate \$\tau_c\$, what this tells you is that for a field, \$E\$, for similar devices you will have more collisions just due to higher mobility.
Although I agree with the given expression for mobility in terms of the MFT \$\tau_c\$ (can also see a derivation here, PDF page 8), I don't see why "you will have more collisions just die to higher mobility". On the contrary, since mobility is proportional to MFT, a higher mobility seems to correspond to fewer collisions. So I don't see how this explains PMOS devices with lower mobility having less flicker noise.
- In Razavi's Design of Analog CMOS Integrated Circuits 2nd edition, section 7.2.2, page 234, the following is stated:
Generally, PMOS devices exhibit less \$1/f\$ noise than NMOS transistors because the former carry the holes in a “buried channel,” i.e., at some distance from the oxide-silicon interface, and hence trap and release the carriers to a lesser extent.
This seems to make sense to me but I'm unsure about is, do only PMOS devices have buried channels and not NMOS devices? I can't find many sources talking about "buried channels" (maybe there's some other terminology). One source that I can find here discusses buried channels but not how they might differ between PMOS and NMOS devices, so the paper still doesn't really answer for me why PMOS flicker noise would be lower.
