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[SOLVED] CMOS device characterization of Vth [Ref: CMOS Analog Circuit Design - Holberg]

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rmanalo

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Hello everyone,

I'd like your opinion on the parameter extraction of the threshold voltage used for hand calculation, please see the snipped image from the book and simulated graph (L=5um for a 0.18um technology) below.

k-parameter.JPGplot.JPG

The threshold voltage is extrapolated following the procedure in appendix C in the book. From the data points in the simulated graph, I obtained a Vth of 0.33V. However, when I performed operating point analysis the Vth is showed to be 0.45V. I'd like your opinion on this and it's implication for hand calculation for first-cut design.
 

I have Vth = ~0.40V using your simulation. You should draw the interpolation line on your simulation graph. We would be able to see how you are getting 0.33V.
 

hello @ t4_v, in the appendix the procedure involved a "linear regression" using data points, i.e. the use of numerical methods as shown in the image below,

equation.JPG

for the extrapolation of Vth, a similar method is used say, y= a + bx where b=(ks*Weff/2*Leff)^1/2 * Vth

aa.JPG

The extracted value of Vth does match well with the square law equation, but I'm curious if this is a normal occurrence in transistor behavior modeling. If this is true then won't it affect the boundary for saturation, i.e. vds>vgs-vth?
 

So you calculated Vth using math? I used an interpolation line, hence I got 0.40V.

I do not model transistors. I am analog IC designer. What I do, I take the foundry documentation where Vth is written down. There is a different Vth for different dimensions, but does not vary a lot. Additionally, EDA tool like Cadence may plot transistor .op on a schematic.
 
Most of the time I use the vth listed in operating point analysis in HSPICE. I was interested in how different textbooks have different parameter extraction procedures. For example in baker's book, he used an interpolation line such as how you did it, while in allen&holberg's book they used numerical methods (e.g. extrapolating vth through data points). I'm curious on which gives more accurate results.
 

rmanalo said:
Hello everyone,

I'd like your opinion on the parameter extraction of the threshold voltage used for hand calculation, please see the snipped image from the book and simulated graph (L=5um for a 0.18um technology) below.

View attachment 152162View attachment 152161

The threshold voltage is extrapolated following the procedure in appendix C in the book. From the data points in the simulated graph, I obtained a Vth of 0.33V. However, when I performed operating point analysis the Vth is showed to be 0.45V. I'd like your opinion on this and it's implication for hand calculation for first-cut design.
Click to expand...

Hi rmanalo,
I suppose that your predicted VT0 is handled well with linear regression model and your calculating step is correct. However, take notice that there is always a tolerance between prediction value and real stuff when you do linear regression, this error depends on how you choose W and L to generate your data above ( you could find something related to "linear regression" for more detail, i took this stuff in machine learning course). Then, your hand calculation of VT0 relied on square model that makes difference vs your predictive VT0 is a completely proper result.
Let try to modify W and L to generate more accurate data points close to linear line n do linear regression again.
Hope it helps,
TM
 
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