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Method for monitoring process variation of PMOS and NMOS

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Advanced Member level 4
Oct 19, 2005
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I want to implement process monitor on chip. The easiest way is to count ring oscillator frequency to get process information. But in this case, I cannot tell it's PMOS or NMOS variation.

Is there any simple way to monitor process variation of PMOS and NMOS separately?


Re: CMOS process monitor

Hi bageduke,

You have the option to implement NMOS-only, and PMOS-only ring oscillators. In this case, you increase the power consumption because the active load always conducting.
In addition, you can use stacked-pair transistors. Two transistors connected in series, with the gate and bulk connected. Monitoring the output voltage (drain of M1 and source of M2) you have information about variability. Take a look in the following references:

[1] Rahul Rao; Jenkins, K.A.; Jae-Joon Kim, "A Completely Digital On-Chip Circuit for Local-Random-Variability Measurement," Solid-State Circuits
Conference, 2008. ISSCC 2008. Digest of TechnicalPapers. IEEE International , vol., no., pp.412-623, 3-7. 2008.

[2] Wils, N.; Tuinhout, H.P.; Meijer, M., "Characterization of STI Edge Effects on CMOS Variability," Semiconductor Manufacturing, IEEE
Transactions on , vol.22, no.1, pp.59-65, Feb. 2009.

[3] Test Circuit for Evaluating Characteristics Mismatch in Metal–Oxide–Semiconductor Field-Effect Transistor Pairs by Estimating Conductance Variation through Voltage Measurement Mamoru Terauchi and Kazuo Terada Jpn. J. Appl. Phys. 47 (2008).

Finally, you can have a matrix of transistors.... and measure each of them in order to have the statistic distribution.


Re: CMOS process monitor

In fact, both pmos and nmos include many aspects for characteristic, such as Vt, mobility, capacitances, on-resistance and so on. For different aspects, different methods are needed for monitoring. In most cases, foundries will have test keys in scribe line for process monitoring.

Can anyone explain me how the RO modules on a chip monitors a chip's performance? Especially, predict the speed of operation of the chip.


Basically RO is composed of inverters. It reflects PMOS and NMOS switching characteristics. So it mimics the process and temperature variation for the other logic circuits.
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