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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.
 

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.

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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