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PMOS has low flicker noise why?

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Hello

Can some one tell me why PMOS has low flicker noise compare to MOS. I have read in Behzad razavi book that the flikcer noise of PMOS is low because PMOS has a buried layer which makes the flikcer noise low. What is this buried layer and why is it not in NMOS? I want to know the reason of low flicker noise in PMOS due to buried layer?
 

As a first step, read the book exactly.

The respective sentence starts
It is also believed that PMOS devices exhibit less 1/f noise than NMOS
Click to expand...
and ends
Nonetheless, this difference between PMOS and NMOS transistors is not consistently observed.
Click to expand...

Razavi is also quoting literature, if you are seriously interested, read it.

The reasoning, why transport of the holes in a "buried channel" below the Oxyde-silicon interface could cause less 1/f noise is obvious if you read the flicker noise chapter from the start.
 

As a supplement to FvM’s hint I found a well-fitting explanation in R.J. Baker’s book ”CMOS Circuit Design, Layout, and Simulation”, 3rd Edition, p.304, regarding the build of a buried channel in old processes’ PMOS devices, and the context of PMOS/NMOS flicker noise contribution in newer submicron processes in general:

R.J.Baker_CMOS_Circuit_Design,_Layout_and_Simulation_flicker_noise__3rd_Edition_p.304.png
 
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