Hello all,

Before asking my question, could someone give me a reference or expression about Id for transistor operating in subthreshold. In the books I have it is assumed that Vbs = 0. Unfortunately it is not so in my case, so I am looking for more complete equation to better understand the influence from the bulk.

Now to the question: it is about the subthreshold slope parameter ‘n’ which appears in the equation for Id. I have a circuit, which shows big device to device spread on a silicon especially at low temperatures. I tried to reproduce this behavior on a simulation with no success until now - MC simulation shows much less spread of the output. The point is that the output is directly proportional to ‘n’ and I suspect that namely ‘n’ is the main source of spread. From a simulation I extract n=1.27 and run Spectre Process&Mismatch MC analysis on it. The result tells me that the parameter ‘n’ has almost no process, temperature and device to device variations, which is quite doubtful in my eyes. My question is: Could it be that this factor ‘n’ is really so stable over different devices (and I have to look for the problem on another place) or it is simply not properly modeled in Monte Carlo?

Thanks in advance!

"Tradeoffs and Optimization in Analog CMOS Design" by David M . Binkley is an other good book on device modeling using the EKV MOS model.