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Multiplier parameter of a MOSFET

Pyroblast
Hi there community.

I have a doubt regarding the multiplier parameter on the MOSFET properties.

In a book that I have they say that if we have a big transistor we can split him several gates (fingers) and then place them together. Thinking with m=1 (multiplier), I would say that I would instantiate a mosT with W=1000um with number of finger equal to 10. That would translate into 100um per finger times 10 equals 1000um.

Now, if I want a device with a width of 2000um, I should use a W=2000um and 20 fingers? With m=1 or I should use a W=1000um with 10 fingers and m=2?

I have done a small test using the above mentioned parameters. When I use a multiplier of 2 the Layout XL generates two transistor with the same W, L and number of fingers.

I read in here: citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf (page 34) that the width of the transistor is: "Describes the width of each MOSFET finger. Total Device Width = Width * Number of finger * Multiplier", however it says at the same time that "“Number of Fingers” and “Multiplier” differ in simulation but are interpreted the same by this program. “Number of Fingers” implies merged sources/drains while “Multiplier” does not imply sharing. It is recommended that the user only change the “Number of Fingers” field so that layout and simulation will more closely match."

I don't know if this is a reliable source, but I couldn't find any useful information on the web, even in here, the Cadence forum community website.

Just for the record: here: www.designers-guide.org/Forum...num=1351774957 the guy says that when he uses other multiplier value (16) when he does the LVS it gives an error. I didn't understood if it might have something to do with the multiplier or he must be doing aome kind of mistake.

What I intend is to know which one of these parameters is better to use, multiplier or fingers for the better sizing of MOS?

I am confused!

Kind regards and  thanks in advance.

  • Hi Pyroblast

    Setting of m or nf affects the calculation of bsim stress parameters sa, sb, etc. The general rule I follow is as follows:

    a. If the devices are meant to be abutted together in layout, use nf in schematic
    b. If the devices are meant to be separated, use m.

    It is hard to comment on which is a "better" parameter because layout editors also allow the splitting of nf devices to make them look like m devices. : )  This certainly adds to the confusion.

    Best regards
    Quek

  • Hi Quek and thank you very much for your reply. The goal is to design a power switch best optmizide as possible. In this case I shouldn't care if I use the nf or the m?

  • Hi Pyroblast

    Yes, that's what I think. It should not really matter as long as the layout and schematic are consistent as explained in my previous post.

    Best regards
    Quek

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