gate or transistor sizing in detail

Yes sir ur right that it depends upon w/L but we cannot change the feature size that is a technology which is given by the foundry. that is why we are changing the width thats it ?? if i am wrong correct me....


I need complete concept of gate or transistor sizing can anybody else explain this in detail ?

you gotta understand one thing... The W and L of all the transistors in a IC are not the same and this variation is done through transistor sizing and also once the IC is out of the foundry you cannot change anything in the IC i.e. the width of the channel can get modulated that is all... it cannot be changed....

You can change and W and L of transistor... But often we just change W because it is easyer to make such ICs, cause of production proces of that IC... But when designing circuit you can change and W and L. There are λ rules that must be respected... Diferent tehnology gives you difrent λ... Minimum size of L is 2λ, and minimum size of W is 3λ...

For most digital circuit, what you say is true. Since you want the circuit to work as fast as possible, you use the mininum size L a technology will support and size W to get the drive you need.

In certain applications, you do want to change the L as well. Examples:
1. Delay cells - you want your gates to work slow.
2. I/O driver - for ESD reasons you might not want minimum L.
3. Any analog circuits - lots of reasons, but mainly to get higher output impedance.

The minimum length is determined by the process. For digital circuit, you can use the minimu length to save die size.

Well, many very common circuits utilize the difference in W to achieve different objectives.

For e.g. In a current mirror, the output current is equal to input current times the ratio of W's of output and input transistor.

Similarly, many a times we want to have different W's to have different gains (transconductance).......also when you want to use a small capacitor in your design, you simply short-ckt. the drain and source, thus you have a two-terminal device that can be used as a capacitor whose value to be adjusted using W.



In most circuits, we want to keep L the same (minimum value), because if L is larger the path of the charges in going from drain to source increases, slowing down the circuit. However, still we do have some special-purpose circuits, that utilize higher values of L; for e.g. to increase the delay (when we intentionally want a delay)....You must keep in mind that going from one technology node (one particular Lmin) to another technology node (one with a small Lmin), is a very costly business. The reason we keep trying to decrease L, is to achieve higher speeds and less power consumption. Thus, when working with a particular Lmin based technology, you want to use that Lmin for most of the transistors in your design.

HOPE THIS HELPS.