Minimum Channel Length for Analog IC Design

[isaacnewton]

In CMOS digital design, we need to use the minimum length L to save die area, right? If it is 0.25um technology, we should let L=0.25 um. How about the CMOS analog design? Should we use the minimun channel length in analog IC design? If so, the channel length modulation could be large. Can some experienced analog designers explain it to me. Thank you very much.

 

[sunking]

Re: Minimum Channel Lengthfor Analog IC Design

As a normal, the digital use the minimum channel length.
But in analog IC design, most of device is not the minimum channel length, due to noise, match and so on. In fact, it depond you spec. But the output stage you can use minimum channel length

 

[Japp]

Re: Minimum Channel Lengthfor Analog IC Design

One thing we have gained in the analog area as the dimension has scaled down is speed!. Not many years ago you could not have made all these >GHz design because the Ft and the associated parasitics were to large. Now you can design above 10´s of GHz So that is the good part. But as stated before the matching of the devices is very poor for small dimensions so it is actually limited how often it is used. Another main concern is the flicker noise that is larger for the small dimensions.

 

[isaacnewton]

Re: Minimum Channel Lengthfor Analog IC Design

As a normal, the digital use the minimum channel length.
But in analog IC design, most of device is not the minimum channel length, due to noise, match and so on. In fact, it depond you spec. But the output stage you can use minimum channel length

Hi sunking. Could you give a little explaination why the output stage can have the minimum channel length? Thank you.

 

[penghan]

Re: Minimum Channel Lengthfor Analog IC Design

With output stage of minimum channel length,we have high output impedance,then high output.

And,noise are mainly due to first stage,output stage with small dimension is not dominant.

 

[mady79]

Re: Minimum Channel Lengthfor Analog IC Design

Clarify how high output impedance is achieved with minimal length .Higher L would give higher output impedance .

Coming back to the query ,People use channel lengths typically 4 times the minimal channel length ,there is not rule of thumb but one could measure the slope of Id vs Vds curve in saturation region ,look at the slope and decide on the length .

Again Area (W*L) is more important than the Length alone while considering the short channel effects .

 

[stefano2m]

Re: Minimum Channel Lengthfor Analog IC Design

In Analog design you always have to make compromises. This is the case of chosing L.
For example, in differential inputs you would like high gain - Gm which is proportional with square of W/L. So high gain means small L. This is the same for bandwidth. Parasitic capacitance at the input depend on the area W*L. So again small L.
On the other hand, the technology has tolerances which are in absolute value, lets say dL. In % dL is more for minimum length than for a larger L.
So for matching and offset reasons, you chose a larger L.
This is the basic, there are many thing to discuss on chosing the L in an Analog design

 

[penghan]

Re: Minimum Channel Lengthfor Analog IC Design

Sorry,i am wrong .I take it as channel width unconsciously.

 

[eda4you]

Re: Minimum Channel Lengthfor Analog IC Design

Topic has been treated also in



:lol:

 

[sunking]

Re: Minimum Channel Lengthfor Analog IC Design

The reason that ouput stage of L can be minimum is reduce the output resistor, then improve the ablity of drive. At the same time, it has no more effect on other char.