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26th November 2010, 19:04 #1
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CMOS Transistor size ratio
Hi
I apologize if this is´t the right forum for this kind of question but i have been searching the forum and have´t been able to find a suitable answer to my problem .
The problem is that I have to size CMOS transistors accordingly to a specific ratio to (under my understanding) get equal rise and fall time.
The design is a four input pseudoNMOSgate with the ratio for the NMOS W/L = (1.8/1.2) and for the PMOS W/L = (3.6/1.2) with the foundery of 0.25um.
I just want to be clear that im not looking for the strait up answer to my problem but instead a push in the right direction..
Hope that you understand my description and that you have time to answer.
Best regards Erikwikt.

27th November 2010, 07:32 #2
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Re: CMOS Transistor size ratio
Hi,
Please go through this PDF for better understanding of sizing the transistor.
http://wwwi.elec.gla.ac.uk/teaching_...gn_4/unit3.pdf
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27th November 2010, 07:32

28th November 2010, 23:30 #3
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Re: CMOS Transistor size ratio
Hi vlsi123, thanks for the quick answer.
I have been reading the PDF that you linked to me and I think its great! But I have a couple of questions in my way of thinking.
In the problem that i descibled it was said that the ratio for the NMOS (as an example) transistor should be (1.8/1.2).
Because of the fact that it has decimal numbers in both over and under the division sign. Can I simplify it to 3/2? So that I get strait lambdas?
To my second question, I have read from other sources that a rule of thumb is that the with of the PMOS should be twice the with of the NMOS because of the mobility in the material.
Now I am using MicroWind as my design program and from the program I can extract the minimum length of the gate for both the NMOS and the PMOS, which is 2 lambda.
So if the with of the PMOS gate has to be 3 times larger then the length, will the total gate with be [(minimum length) * (the mobility rule) * (the with ratio)] = (2 * 2 * 3) = 12 lambda?
Yours ErikWikt

29th November 2010, 12:17 #4
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Re: CMOS Transistor size ratio
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29th November 2010, 17:19 #5
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Re: CMOS Transistor size ratio
Hi erikl
The size ratio for the NMOS transistor should be 3/2 and 3 (3/1) for the PMOS.
So wont the width for the PMOS be equal to 2 * 3 * 3 = 18 and 2 * (3/2) = 3 (or 6 if i chose the length for the NMOS gate to be 4)?Last edited by erikwikt; 29th November 2010 at 17:53.

29th November 2010, 18:29 #6
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Re: CMOS Transistor size ratio
Again: no, sorry! Why
(minimum length) * (the mobility rule) * (the width ratio) = 2 * 3 * 1.5 = 9
In your equation for the PMOS width, (the width ratio) = width ratio of NMOS. The mobility ratio of 3 already takes care
NMOS W/L = 1.5 . For similar electrical behavior of PMOS & NMOS use
W/L(PMOS) = (µn/µp) * W/L(NMOS) ≈ 3*W/L(NMOS)
You don't need to double the PMOS ratio (the doubling at your original source PMOS/NMOS W/L ratio is caused by a mobility ratio assumed to be =2); now we calculate with a mobility ratio=3.
So finally: W/L(NMOS) = 3/2 (or 6/4) and W/L(PMOS) = 9/2 (or 18/4)
Hope I could present this graspably!
BTW: If you design for a mobility ratio of 2 or of 3 depends on your personal feeling: quite a lot of books and papers use a ratio of 2 (hence the PMOS is a bit wispier than the NMOS), it is (somewhat) easier to calculate , and  last not least  it saves costly silicon area. A design ratio of 3, however, is closer to reality, better for NMOS/PMOS symmetry (concerning Ids, gm, fan out ...), achieves (somewhat) faster circuitry  just is (somewhat) more expensive.
So choose up to your requirements  or up to your taste!

12th December 2010, 19:31 #7
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Re: CMOS Transistor size ratio
Hi again guys. Sorry for the terrible late answer but my studies has been eating me alive....
I just want to thank you guys for the help with my question! =D
It really was a homework if that wasn't clear and I got total points on it as well!
Thanks again for the help!

13th December 2010, 14:05 #8
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Re: CMOS Transistor size ratio
if you just want to get equal rise and fall time,
you can use a capacitor load and tansient analysis,
find the rise and fall time of Vout,
then adjust the W/L of NMOS and PMOS.
usualy,the mobility ratio µn/µp is not equal for different fab process.
but the design theory is the same as what erikl's said.

5th October 2012, 09:46 #9
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Re: CMOS Transistor size ratio
Hello,
In a circuit I have changed PMOS of W from 240nm to W/L=540nm/180nm. I want to know for NMOS how much the W from 240nm has to be changed if the L remains the same i.e.180nm? The circuit is being implemented in Cadence.

5th October 2012, 09:46

5th October 2012, 13:25 #10
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Re: CMOS Transistor size ratio
Just keep the old PMOS/NMOS ratio. If you want to consider the µn/µp ratio (which is about 4.7 for 180nm CMOS), you could adjust the PMOS(W/L) / NMOS(W/L) ratio for this factor.

6th October 2012, 03:42 #11
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6th October 2012, 12:37 #12
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Re: CMOS Transistor size ratio
As the mobility ratio for 180nm CMOS bulk processes is about 4.7 (s. the image below), the PMOS(W/L) ratio should be this factor greater than the NMOS(W/L) ratio, if you want the same drive strength from both transistors. So if you selected W/L=540nm/180nm for your PMOS, your NMOS needs only a W/L=115nm/180nm. For more info see e.g. the book named at the image below.
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6th October 2012, 19:08 #13
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26th November 2012, 14:08 #14
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Re: CMOS Transistor size ratio
Please consider that all these ratio considerations are just good approximations but not accurate and don't consider effects of layout, temperature and process variation (ss/ff/sf/fs). If you really want a minimal difference between rise/fall times at all corners (for example when creating clock buffers), then you have to numerically optimize the netlist, preferrably an extracted one postlayout.

16th March 2013, 06:03 #15
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Re: CMOS Transistor size ratio
I am designing a 3 input nand gate on microwind( layout level) and on Dsch27 (transistor level).
i am given the Nmos sizes: W=4.5 pi L=2.5 pi
with the foundary = CMOS018
I am told to use the same length of PMOS as i used for NMOS. however, i have to find the width of PMOS so that i have equal rise and fall time.
is there a special relationship between width of NMOS and PMOS?? please help out. or direct me to a link.

16th March 2013, 10:57 #16
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Re: CMOS Transistor size ratio
Everything you need is described above in this thread: read it attentively :)
I have a comment, though, on the page you show as an image from David Binkely's book. The values for carrier mobility are strange. Both UMC 0.18u and TSMC 0.18u have a carrier mobilty ratio of approx 2.6, not 4.7 as in Binkley's table.
Slainte!
H

16th March 2013, 19:30 #17
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Re: CMOS Transistor size ratio
Last edited by erikl; 16th March 2013 at 19:35.

16th March 2013, 19:30

16th March 2013, 22:21 #18
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Re: CMOS Transistor size ratio
Since the µ depend on doping, and customers of foundries normally want to use similar designs when going to a smaller technology node, I can scarcely imagine what could bring a foundry to doing this 4.7 ratio ... they'd have to do it on purpose, but for what purpose?
Scottish ... I studied English in Edinburgh, a long time ago :)

13th April 2013, 17:47 #19
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Re: CMOS Transistor size ratio
Sir have you parameters value related to 22nm technology??
and value for voltage suply for this 22nm technology??
thanking you...
   Updated   
Sir I am not able to download this pdf is there any other pdf like this available??
   Updated   
value of lambda would be???
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