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Design >> Analog Design >> Layout of a very long transistor
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Message started by mixed_signal on Jul 19^th, 2012, 2:13pm

Title: Layout of a very long transistor
Post by mixed_signal on Jul 19^th, 2012, 2:13pm

Hi,
I have a transistor of very long length! w/l=1u/40u. How do I layout? As a single block or as a series of small transistors.

Some say series is good as it will match the models well
Some say at low currents there can be noise injection in the middle transistors in series connection.

My current level is in nanoamps. So i m confused. How do low power circuit designers do such layout?

Title: Re: Layout of a very long transistor
Post by Dan Clement on Jul 19^th, 2012, 8:42pm

Be careful with a transistor like this... The model guys may not have characterized a structure like this and the models may not be accurate no matter how you lay it out.

I normally don't rely on anything but matching when I use devices like this. I have only really used very long transistors in startup circuits, diode stacks, and hysteresis.

I've seen all kinds of layout for long transistors but I don't recall ever hearing of someone having an issue between silicon and simulation with the exception of the power maybe.

Title: Re: Layout of a very long transistor
Post by AnalogDE on Jul 20^th, 2012, 9:23am

Ask for guidance from your foundry on what device sizes fit the model well. Limit your device aspect ratio to devices that fit the model.

For example, they might come back and say 1u/10u fits well but any larger L they won't guarantee. If that's the case, use 4 1u/10u in series.

Title: Re: Layout of a very long transistor
Post by mixed_signal on Jul 20^th, 2012, 11:05am

Hi,
Thank you AnalogDE and Dan Clement! That was helpful!