The values of RD and RS in small-signal MOSFET model

Dear Hensin,

Hensin said:

Rd and Rs represent the ohmic resistance of the drain and source, respectively. It seems that they are only dependent on technology and the transistor's length and width. I

It is quite difficult to include a "general" estimate of a MOS device's drain and source resistance in a schematic based model since, a you noted, their values are highly dependent on the physical dimensions of the drain and soure. Further, they also may include contact resistances. The resistance of a "contact" will depend on the number of contacts, the metal layers of the contacts, and their physical shape - all of which are all highly variable. Often a set of MOS  "macro" models is offered that consists of the basic BSIM model in addition to a wrapper containing estimates for the added source and drain resistances, gate capacitances, etc... to better model a post-layout environment. As such, the model parameters displayed in your post do not contain the source and drain resistances. You might check with your modeling team to see if a device macro model is available for your technology. Alternately, include discrete resistors in your schematic in series with the device source and drain terminals to study the impact of their values on your circuit's performance.

I hope this helps Hensin,

Shawn

Dear Shawn,

Thank you so much for your detailed explanation. It is really helpful.

Zhenxin

Dear Shawn,

Thank you so much for your detailed explanation. It is really helpful.

Zhenxin