Why using transistor sizes larger than the minimum length?

[ezt]

Re: transistor size

Hi.
I think using bigger L in analog design is not always necessary. But sometimes we don't have any other way. Some of problems are about fab but don't forget in each Process, we have a max limit over L. I mean L can't be larger than a special value in each Process. On the other hand, enlarging the size of devices would make the Frequency Response worse. and as a result that would reduce the speed. there's a good expression for fT in Razavi book about Analog IC Design.
fT = (µn/2Π) * (Vgs - Vth) / L^2
Afterall we should remember, by increasing the L we can have less short channel effects, and the world is more beautiful. (don't you agree with me!)

Regards,
EZT

 

[skal81]

Re: transistor size

To try to answer to konqueror, in RF design one important factor is the parasitic capacitances (Gate, Drain....). Indeed with a few pF cap, it's become hard to have circuit working in the GHz (for eg. WLAN). In many case it seems that for the input transistor L is chosen minimum, and the W is size with consideration for speed,Noise Factor, linearity. For example you can find a tutorial here: **broken link removed**.
And I'd like to add that in recent articles even for non RF circuit minimum size is used (for eg. **broken link removed**). So it is not always true that analog = 2~5 times the minimum size.

 

[xz781122]

Re: transistor size

in analog design L is ofter 2 times larger than minimus value