Question about channel length in layout

[aabs]

salaam
i am working on Op-amp designing i want to know if we are designing in 0.18u can we have transistor with L=0.25u
i ask this question because i see some design that the channel length is not equal to technology L.
thanks.

 

[mberesin]

Hi,

Chanell length can be larger than technology dimension.
In fact in analouge designs most of transistors have chanell length larger than minimum value.
Go to:
**broken link removed**
for opamp design tutorial.

Marcin

 

[wizard of oz]

The techonlogy number gives you only the MINIMUM drawn size.

 

[prashantbabu]

The minimum channel length is defined by the foundry for a particular technology process.
More often than not, minimum channel lengths are used for digital blocks only, since they are the high speed switching blocks and also because they occupy, as a rule of thumb, around 60-70% of a die area.
In analog design, where high performance is a criteria, minimum channel lengths are seldom used. In fact higher lengths are required to increase the output impedance of a transistor which is required to achieve good gains, isolation from supply variation, etc.
Remember that Id is a function of channel length modulation factor "lambda" which in turn is inversely proportional to channel length 'L'. So, for a current source transistor, more the "L", lesser will be the variation in Id with supply.
Hope this helps. You can always refer to books like Razavi for more insight on analog design.

 

[gafsos]

prashantbabu;

what do you mean by "high performance"

Thanks

A+

 

[prashantbabu]

Hi gafsos,
By "high performance", I just meant high gain, supply noise isolation etc. which I mentioned in my post.
Didnt mean to offend digital design here

 

[k.gurunath]

High performance refers to stability,noise suppression,critical operations etc

 

[leo_o2]

Large length is usually adopted to suppress channel length modulation effect.

 

[drDOC]

i am working on Op-amp designing i want to know if we are designing in 0.18u can we have transistor with L=0.25u.

That really depends on your design software, company policy, layout policy... Ask someone.