How to determine the channel length?

[pugongying]

Say,if I want to design an op amp using tsmc0.18um process,how can I determine the channel length for each MOSFET,are the lengths in discrete values?
What is the general principle in determining the channel length?

Thanks in advance

 

[elbadry]

I think the length step is determined by the process grid, 0.005 um for 0.18 um technology.

That is to say, channel lengths are: 0.18 um, 0.18um + 0.005um, 0.18um + 2*0.005 um..etc

 

[pd]

In a stable process like .18u process u can have ideally any channel length as now a days λ based rules are not followed in analog design.But when u layout u have to check out what is the minimum resolution of the lenses of the foundry can have for design implementation of the layout into silicon.And generally it is in the range of .005u ie 50nm.

 

[Mistico_Pereyra]

Your foundry tells you the minimun chanel length for your transistors, 50nm in your process!! Usually you use the minimun length in order to achieve the highest frecuency if you are designing gates!! But if you need more current driving in your circuit, you need increase the length and the width of the transistor channel!!

Regards!!

 

[walker5678]

"as now a days λ based rules are not followed in analog design"

Is this due to the short channel length effect?

As i know , the gain of amplifier is proportionally related to the channel length, so if you want to get higher gain, then as a choice, you can choose larger channel length. [/list]

 

[engrvip]

basically in analog design channel lengths are choosen to be atelast 2 to 3 times of minimum channel lengths provided by ur process. this is used as a thumb rule and done to keep in check channel length modulation.

 

[ambreesh]

Your OPAMp frequencu would be critical in choosing length for input pair.
Assue you wnat an opamp to have a UGB of 800MHZ you would go for minimum L
If high frequency is not what you want then ,best would be to have transfer characteristic for varing L', find from what L onwards the % increase in rds is small
use that L for design.

 

[Material]

It depends on your spec
The selection of L should be a trade off in gain, bandwidth, and noise.

 

[davidwong]

It depends on your spec
The selection of L should be a trade off in gain, bandwidth, and noise.

Hi Material,

how do the L affect noise?
do you mean larger W/L cause large noise.
thus try to use min. L to attend min. noise
am I right

 

[boooser]

It depends on your spec
The selection of L should be a trade off in gain, bandwidth, and noise.

Hi Material,

how do the L affect noise?
do you mean larger W/L cause large noise.
thus try to use min. L to attend min. noise
am I right

I guess for 1/f noise of the transistor, the L value will affect the noise.

 

[Hooda]

hi
1/f flicker noise affected by both W L
flicker corner frequency is inversly proportional to W*L
so increasing both will reduce flicker corner in MOS device
that ' s what i think ."may this help"

 

[Areky_qin]

when 0.18 process is used fab analog circuit,
the channel length is not commendatory to use the minim channel length, the minim is used for digital.
to get high performance circuit, the large channel length can be used, otherwise there is a balance between area with performance.