Fingerwidth of the Transistor

[bsrin]

Hi Everybody,
I heard that finger width of the transistor in layout is limited (around 10um or 5um ) for nm technologies.My doubt is , whether I heard is real ? If it so, may I know the reason for limiting the finger width of transistors. I hope the detail answer.

Thanks in Advance.
.

 

[JoannesPaulus]

if the finger is too long, it will get skinny in the middle, like the waist of a wasp.

 

[dick_freebird]

Because you are looking at a very short gate and the gate
length is the "W" of the gate resistor, a wide FET is a long "L"
and a high Rg which is going to degrade transient / high
frequency response. This is not so easily / accurately
modeled because Rg is really distributed, the simulator
can't know whether it's fed from one end or both and so
on. So it's probably best to constrain users from doing
dumb things, rather than trying to report their dumbness
accurately over all possible absurdities.

In RF you see large numbers of relatively narrow W FETs
because Rg and Cdg are your device's corner frequency.
Rg responds nicely to paralleling and Cdg doesn't care (much).