MOSFET sizing (channel length) in cadence

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ansonyeap

Joined: 29 Oct 2009
Posts: 7

29 Oct 2009 2:56 MOSFET sizing

Hello. I doing some analog IC design simulation using cadence. but the transistor size that the library have is too small(in term of channel length) for my design. i need a larger channel length.

My question is how do I connected the smaller transistors in series so that i get can get the channel length i want? i know that connecting smaller transistors in parallel will increase the width. thank you.

dick_freebird

Joined: 04 Mar 2008
Posts: 313
Helped: 46
Location: USA

29 Oct 2009 5:28 MOSFET sizing

Are you saying that you can't edit the properties to change L?

I have never seen such a limitation before.

ansonyeap

Joined: 29 Oct 2009
Posts: 7

29 Oct 2009 6:39 Re: MOSFET sizing

ya. I can not edit both the width and length. The software didn't allow to do so. so any suggestion on how to obtain the length that required? thank..

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snafflekid

Joined: 09 May 2007
Posts: 120
Helped: 16
Location: USA

29 Oct 2009 7:09 Re: MOSFET sizing

you can put the mosfets in series to get the L you want, but it is better to adjust L in your model

Last edited by snafflekid on 29 Oct 2009 21:24; edited 1 time in total

ansonyeap

Joined: 29 Oct 2009
Posts: 7

29 Oct 2009 9:56 Re: MOSFET sizing

as I know paralleling the transistor can only make the width wider right? but the length is still fixed right?

I not really get what you mean... can you explain more on how to parallel the transistor can vary the length? thank you...

deepak242003

Joined: 24 Dec 2008
Posts: 206
Helped: 10

29 Oct 2009 12:35 MOSFET sizing

I blieve taht transisitor lenght cant be changes however we connect it .. parallel or series.. only effective will be altered......

Please correct me if I am wrong....

oermens

Joined: 19 Nov 2005
Posts: 332
Helped: 37
Location: canada

29 Oct 2009 12:56 MOSFET sizing

connecting identical transistors in series will increase L
connecting identical transistors in parallel will increase W

ramya19

Joined: 29 Oct 2009
Posts: 3

29 Oct 2009 13:14 MOSFET sizing

Placing transistors parallelly will only increase the width of the transistor but not the length. To increase the length the transistors (fingers) have to be connected in series in concept . To check the max length you can get for a transistor you can check in the cdf callback or the model file. If you are using a particular technology for your design you cannot cross the limits of length or width pertaining to that technology.