Regarding fingers in MOS transistors

Hi ,

In the design , it was given as 5(10/25) , which means 10 =width and 25 = length,

I am confused whether 5 represents the no of fingers or multiplier , in schematic i assumed it as the multiplier and designed , and i tried with fingers also , but the output didnt vary.

Now in designing the layout , im confused how it need to be considered , whether 5 fingers which together of drain and source , or 5 individual mos transistors with separate drains and sources?

And also if i need to divide the length can i divide it with out dividing the width ?? or need to divide the width also of the same factor ?? after dividing the length need to connect the split transistors in parallel or series ?

Is there any rules which states if width divided connect in parallel , if length divided connect them in series ??

Please help me in these doubts , so that i can proceed ...

Thanks in advance to all

Regards

You need to be clear what 5(10/25) means. I don't know what software you are using, but 5 may be the multiplier, it may be the number of fingers. 10/25 may be the final w/l ratio, it may be the w/l of a single finger. Depending on your software you should be able to click on the device and see or edit a number of parameters which will allow you to determine what each of the number means.

Keith.

Hi keith,

thanks for reply , I am using cadence , W/L is the width/length of the single finger but i need to know more about exact difference between fingers and multiplier...

5(10/25), in this case whether 5 is multiplier or fingers....

Thanks
Swetha..

snowgal said:

Hi keith,


5(10/25), in this case whether 5 is multiplier or fingers....

Click to expand...

Swetha,

I cannot tell you that, unfortunately. I can tell you the difference between fingers and multiplier, but not what the number 5 means in your example.

A multiplier makes the transistor effectively wider. A transistor which is w/l=10um/1um with M=5 is similar to saying w/l=50um/1um. It is not quite the same due to fact that a transistor with a width of 10um drawn may only have a real width of say 9.9um. So 9.9um*5 isn't the same as 49.9um - it all depends on how it is laid out.

Fingers are really like saying the same thing, but the other way round. In other words, a 50um/1um transistor with 5 fingers would be made as five 10um/1um transistors in parallel.

Somewhere I would expect there to be an "effective width" and length shown. However, with some many possible parameters to display, they may be hidden.

I am not sure how rigorous Cadence is in implementing this in the simulation and the LVS. Hopefully a Cadence person can help you and tell you how to determine what your figure 5 is.

Keith.

Thanks a lot for helping me.. please let me know where can i post to know more about cadence...

thanks in advance

Regards
swetha

I am pretty sure there is a support forum on Cadence's web site.

cadence.com/community/forums I think.

Keith