How Device Resistance change with respect to W and L in any technoogy

Hi, I know that Device resistance = (Resistor Length(L)/Resistor width(W))*Sheet resistance.



If a resistor from a particular technology has sheet resistance 1 Kohm and if I choose L/W as 1, it should give me 1 kOhm resistor. But that is not happening, when I change W, L is also changing accordingly and vice versa and not providing me with 1 Kohm resistor. In my case, as in the figure, If I choose W=1um and L=3um then it should give me the device resistance should be 990 ohm. Unfortunately that is not what I get because W and L are chanigng automatically.

Could anyone tell me how the parameters in the attached figure behave, their interrelation and limitations? Thanks

On you figure the sheet resistance is 330 Ohm, you set 2kOhm for the desired value with 2um desired width. Cadence will calculate the necessary length from these parameters by a function like this:

L = (R - 2*R_contact)*W/R_sheet = (2000 - 2*9.5)*2um/330 = 12um

The difference from your figure is like 0.9um, because Cadence and process engineers are smarter than me.

I think you have a switched off parameter in the properties window, where you can set how to change the resistor value. It should be a drop-down menu, and the possible choices as I remember are resistance vs. width, resistance vs. length, or a 3rd option where you can simply set arbitrarily the width & length, and that is what you want. Now you have set the resistance vs. width option.

The reason behind why W is changing can be that your technology has a minimum width for the device, and cadence automatically recognize if you set a smaller value.

Surely learn and understand "Entry Mode" of resistor.

circuitslave said:

In my case, as in the figure,
If I choose W=1um and L=3um

Click to expand...

It can never be.
How can you enter value of L, while entry area of L is gray out in your figure ?
You can not specify L, since your "Entry Mode" is "r & w".

Show us true situation.

circuitslave said:

Unfortunately that is not what I get because W and L are chanigng automatically.

Click to expand...

Minimum of W is 2um.
So W is changed to 2um, even if you enter 1um as W.

circuitslave said:

Could anyone tell me how the parameters in the attached figure behave,
their interrelation and limitations?

Click to expand...

Too easy.
Your "Entry Mode" is "r & w".
Minimum of W is 2um.
That's all.