Croaker wrote on Nov 23rd, 2006, 9:06am:That intuitively makes sense to me. I mean, physically, it seems right, since two in series have almost the same structure as a double-length one (except there is an implant in the middle of the series chain).
Well, I just worked out the equivalence and if you include body effect you'll get the result that two MOS in series kind of looks like a single 2*L MOSFET, but with an error term of:
k*W/2*(2*L)*(Vg - Vth)^2 + k*W/2*(2*L)*( -2*dV*(Vg - Vth) + Vx*dV + dV^2 )
^ looks like 2*L device ^ body effect error term
Vth2 = Vth1 + dV, and Vx is the drain voltage of M1. M2 is the top device, M1 is the bottom device.
Hi Croaker,
Quite many replies on this topic I see. I just have a quick question. How did you do the above derivation? I think you would need to make certain assumptions about the region of operation of each transistor which may/may not hold. Anyway, regarding the BSIM3 models being source-based, your understanding of this is correct (model source-oriented instead of bulk-oriented in BSIM3). Just to point out, the calculations that you do using a long-channel MOS model are implicitly source-oriented as well and should suffer from some errors that the simulator will also make.
I could try to explain a bit about why the body effect does not come into play with series transistors when looking at MOS operation in a bulk-oriented viewpoint, but I think there are others who can do a much better job.
Tsividis and the EKV model people both provide very good literature on how the MOS transistor should be modelled. Perhaps a look at their books/papers would be useful for you, but as Rob points out, don't hope to be able to simulate series transistors accurately anytime soon. Acually, simulation models always will have errors, and although BSIM3 has quite many, it is unlikely to be replaced soon as all companies and foundries have invested too heavily into these.
For now, the best answer to your original question may be found by delving into:
Yannis Tsividis - Operation & Modelling of the MOS Transistor
EKV model literature can be found online by searching their homepage.
Regards
Vivek