2
\$\begingroup\$

I would like to ask if this sentence below is correct.I had find it, when I was reading a paper.

"PMOS transistors have lower flicker noise than similarly-sized nMOS transistors in most CMOS processes."

I am a little confused why PMOS transistors have lower flicker noise.I didn't find a good explanation for this.

\$\endgroup\$
6
  • \$\begingroup\$ As flicker noise is directly related to the underlying chemistry /topology of the device and different devices are different chemistries /topologies? Google "flicker noise" - there's a page... \$\endgroup\$
    – Paul Uszak
    Apr 30, 2017 at 12:40
  • \$\begingroup\$ I think PFET channels are deeper in the substrate than are NFETS. Thus surface traps, releasing and refilling the tiny charge buckets, affect NFETS more. \$\endgroup\$ Apr 30, 2017 at 13:01
  • \$\begingroup\$ thank you analogsystemsrf!!This sentence refer to MOSFET. I know that this question is little difficult for someone to answer, but I think is very important for someone to understand the reason. \$\endgroup\$
    – elecV1
    Apr 30, 2017 at 13:10
  • \$\begingroup\$ PFET and NFET in @analogsystemsrf's comment refer to p-channel and n-channel MOSFETs. \$\endgroup\$
    – Hearth
    Apr 30, 2017 at 13:28
  • 2
    \$\begingroup\$ @analogsystemsrf I think PFET channels are deeper in the substrate than are NFETS. I doubt that, both NMOS and PMOS are very much surface devices. A more likely cause for the difference in noise between N- and P-type MOS becomes clear when you look at the formulas describing 1/f noise: the K-parameter is in there. K is directly related to electron/hole mobility and higher for NMOS (electron mobility) than for PMOS (hole mobility) resulting in higher K for the NMOS resulting in higer noise for the NMOS. \$\endgroup\$ Apr 30, 2017 at 14:50

1 Answer 1

2
\$\begingroup\$

The answer lies in the mean time between collisions and this is based on mobility. Whichever device has a higher mobility, \$\mu\$, will have higher collisions because you have a greater probability of have a collision. This generally means that nFETs have higher collisions and therefore higher flicker noise, but on undoped channels, you will see similar mobilities. I have 10nm fins on my bench that show higher \$\mu_p\$.

Due to Brownian motion, you have movement whenever you have heat, and skipping a bunch of physics, you end up with the average net velocity for drift to be \$v_{dn}= -\mu_nE\$ and \$v_{dp}= \mu_pE\$ respectively. 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. To actually calculate \$\tau_c\$, you will need to pull out a device physics book and look at the density of states at a temperature under field conditions. This is one of those things that we just empirically measure. The math says it's proportional to \$T^{\frac{1}{2}}\$, but on the bench you see \$T^{\frac{3}{2}}\$ as your channel changes.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.