what do the the finger and multiplicity do in ic5141?

[urian]

hi,there
i find that there are "Number of fingers" and "Multiplicity" options in "Edit Object properties" box of virtuoso.
what does this mean?

in my option,they are both timing the W in the W/L expression.but the simulation result shows that it seems wrong.
then what the exact meaning of them?
if i want to increase the W/L ratio, whether should i increase the finger number, or the multiplicity number?

Best regards
urian

 

[leozuo]

usually I use multiplier. But I do not know what do fingers do either..

 

[JoannesPaulus]

Let's say that you have a 100um wide transistor. If you set the number of fingers to 10 and the multiplier (m) to 1, your transistor will have one 100um wide device composed by ten 10um transistors abutted together (they share every other drain-source connection)
If you set n=10 and m=2, you will have two 50um wide devices composed by ten 5um wide transistors abutted together.
It does make a difference! Especially if you are using a sub-micron technology...

 

[oversea]

Let's say that you have a 100um wide transistor. If you set the number of fingers to 10 and the multiplier (m) to 1, your transistor will have one 100um wide device composed by ten 10um transistors abutted together (they share every other drain-source connection)
If you set n=10 and m=2, you will have two 50um wide devices composed by ten 5um wide transistors abutted together.
It does make a difference! Especially if you are using a sub-micron technology...

so which one is better?

 

[JoannesPaulus]

It depends on what you want to achieve.

 

[rfsystem]

The finger versus multiplicity have very clear context:

"Finger" mean a specific layout arrangement with n gate fingers where source and drain diffusion areas are shared. If you have n fingers you will have n+1 diffusions. So devil in the detail now is that you can assign either to the source or drain one more diffusion when the fingeris an even number.

The multiplicity is simple the parallel connection of multiple MOS devices.

More complex layout arrangements assume also 2D arrays of gate fingers. The more complex models take also specific parasitics into account.

For the design process that is a good way because you can have a very near physical model and the device layout process is automated.

 

[urian]

thanks,guys
my tutor told me that i should set multiplicity to 1 and only to change the finger to attain large W/L ratio,i doubt whether it is right.

 

[rfsystem]

That advice is right because it is near the physical values of the layout.

Sometimes the maximum finger lenght which is equal to the width of the MOS device is limited by design rules.

How that is handled is depend on the property editor and the underlying callback routines.