hmsheng said:
Why it is believed that PMOS devices exhibit less 1/f noise than NMOS transistors?
There are two theories behind the physical origins of 1/f (flicker) noise: Hooge's mobility fluctuation theory, and McWhorter's carrier density (or number fluctuation) model. The number fluctuation model is based on the random trapping and release of mobile carriers in traps located at the Si-SiO2 interface of a MOSFET.
In a PMOS device, the mobile carriers (holes) move in a "buried channel" rather than right along the oxide-silicon interface. Since the carriers are distanced from the traps in the gate oxide, the 1/f noise is reduced.
However, reduced 1/f noise is not consistently observed in PMOS vs. NMOS devices on the same process. I have seen plenty of process data sheets which show 1/f noise of the PMOS and NMOS devices to be the same. On one process I recently used, the PMOS devices had worse 1/f noise than the NMOS devices.