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"Length of MOSFET" is a design variable or not......

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Ramakrishna_444

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I have seen somewhere that the length of all the MOSFET's in the design are kept at 2.5*L_min (roughly 2-3 times of L_min)...
and
Some other design have follow some other strategy like for "Digital blocks length is fix at L_min and for analog blocks it is 2.5*L_min.

For example:
In 180nm Technology
for Digital blocks length is at 180nm fixed...
for Analog blocks Length is fix at 500nm (which is 2-3 times of Lmin)....

SO
Is Length is a design variable or not if so what might be the strategy for playing with it...?
please explain it ....
Thnax in advance..............
 

In analog design I would be choosing the length based on matching requirements. Minimum length would have poor matching so larger lengths would be used. For some parts of the circuit where matching is not critical and large gm or high current handling are required I might be using minimum length.

Keith
 
Hello

the minimum length cannot be used in the analog design circuits, usually you need minimum of 2*min L and that because of :
1. minimum length has worst case matching
2. has higher channel length modulation that we never like since it will degrade the mos transistor output impedance and hence degrade the design of the current mirror/reference circuits and op-amp

at the beginning of the design and before even starting it you have to select your minimum length and this will be the same for all other transistors in the circuit, so what left for you to play is the Width (W), it mean you are playing with ratio of W to a certain constant L
 

For high speed, as AD/DA, we need to use the minimum length for some key transistors.
 

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