Analog Practical Design

Question

This is a very basic question. I hope it won't offend you.

One my prof in college, while giving us assignments, asked us to consider channel length of all the mosfets in the design to consider same. like L=200nm.

My question is it a good idea to set channel length (L) of all transistors same.

Also, I heard that if we vary channel length of transistors (like to get hight output resistance we may increase L) then there may be problem while making layout of the design. Can you please confirm this whether this is followed in practical analog design like in analog companies.

Answer 1

My question is it a good idea to set channel length (L) of all transistors same.

It depends on what you're designing and the CMOS technology you're using.

For logic circuits it is in general beneficial to use the same (minimum) channel length for all transistors. When you use the minimum channel length the transistors can be as small as possible saving you area and thus cost.

However, if you want logic circuits at a very low power and/or low leakage running at a low frequency then you could consider not using the minimum channel lenght. This might increase the size of your design but you should gain in power consumption (it will be lower).

For analog circuits you generally want and need to use transistors with a different length depending on how you're using a transistor. You already mentioned output impedance as a factor.

However some very small processes like 10 nm and below (with finFETs) might not even allow you to use a non-standard length. So in such a process you might have no choice.

then there may be problem while making layout of the design.

If the process allows for different lengths then this is not true, you can just make the transistor you want within practical limits of course. At least in my 20 years of CMOS IC design experience I have never seen a problem with this.

Answer 2

There is a tension when designing analogue ICs, and in fact circuits in general, between using the 'right' component in every place, and standardising on common components, which has cost and effort savings.

The compromise most of us arrive at is to have a range of standard components, usually in a log range like E6 1, 1.5, 2.2, 3.3, 4.7, 6.8, or binary scalings, or decade scalings.

The benefit of using a standard channel length is that the transistor layout can be reused in multiple places. There is a large 'cost' in each new transistor layout, mostly the testing, characterising and proving of the layout. Any new special length layout has the opportunity to introduce an error.

The benefit of using common component values is a reduction in holding different lines of stock, fewer stations on the board stuffing machine, higher volumes of specific components so better prices.

In a few critical places, you can't modify the design to accept your standard layout or component, and then you need to accept the extra cost of using exactly the right one. You decide these on a case by case basis.