What is the difference between id and ids of a MOS operating parameter

This will depend on which device model is being used, but if you look in the Spectre Circuit Simulator Components and Device Models Reference manual you should find some information. For bsim4 (if that's what you have here), then in the Component Parameters section at the end of the chapter, it has a description and links to relevant equations:

5 ids (A) Resistive drain-to-source current. Refer to equations 21-16 and 21-43.

49 id (A) Resistive drain current. The sum of the currents though D and DI node.

The second is a bit misleading, but if you look at the equivalent circuit early in the chapter, I'm sure id is the total resistive drain current (including that which flows to the bulk) and ids is just that between drain and source. That would seem logical given the names too.

Regards,

Andrew.

Dear RFStuff,

Andrew’s explanation is exactly correct. ID is the total drain current, including that to the bulk, and IDS is only the portion that flows from drain to source.

Shawn

Dear Andrew Beckett

I was plotting the currents through transistors during switching. I saw that when I plot the drain current by probing the drain terminal of the transistor, the current was much higher than id. Is it because the drain terminal current also include the c*d(v)/d(t) currents along with the current flowing through channel while id only includes the current flowing through channel?

This may depend on how the model has been implemented - sometimes there is an inline subckt model and what the id is reporting is the current through the drain of the internal inline device, whereas the terminal current might be including the path through other devices. I can't really comment on precisely why there might be a difference in general. That said, I would expect that the id curren (which is coming from the operating point) t is a "static" current (so not including any dynamic currents though the capacitances which are part of the device). The "id" is an operating point parameter which is what you would have get from a DC operating point with everything biased at the voltages at that instant of time, and the capacitors would have no effect in that case.

Andrew.