Why gm is not zero even when iDS is a constant DC current?

This is because gm is a parameter for the small signal model of the transistor - so effectively it's what the small signal dId/dVgs would be at that bias point. It's computed in a DC analysis (in your case you've swept the voltage, but it could just be a DC operating point). It would be completely useless if it was based on the actual change of current at a fixed bias point (since that would always be zero).

This is pretty clear in numerous text books - I pulled one off the shelf at home (most of my electronics texts are in my work office) - Analog Integrated Circuit Design by David A. Johns, Ken Martin (Wiley, 1997, ISBN 0-471-14448-7) and in the first chapter on Integrated Circuit Devices and Modelling, section 1.2, MOS Transistors in the subsection headed Small-Signal Modelling in the Active Region it shows various alternative formulations based on the operating point, including a form that is dependent upon Id. So I suggest you read the literature on this. The point of knowing the gm is to know what the transconductance at a particular bias point would be if you were to apply a changing (small signal) current through the drain, or to apply a varying Vgs and then the current through the drain varies too. It's a number to feed into the small signal model of the transistor, not something that is measured from a large signal sweep that you were doing (essentially you're plotting the small-signal gm found at each discrete value of the DC sweep).

There is some coverage of this in the Virtuoso® Simulator Components and Device Models Reference manual in the documentation, but I think a basic book talking about how transistors work covers this more simply (bear in mind that most texts use a much simplified model compared with real life modern transistor models which are highly complex).

Regards,

Andrew.

This is because gm is a parameter for the small signal model of the transistor - so effectively it's what the small signal dId/dVgs would be at that bias point. It's computed in a DC analysis (in your case you've swept the voltage, but it could just be a DC operating point). It would be completely useless if it was based on the actual change of current at a fixed bias point (since that would always be zero).

This is pretty clear in numerous text books - I pulled one off the shelf at home (most of my electronics texts are in my work office) - Analog Integrated Circuit Design by David A. Johns, Ken Martin (Wiley, 1997, ISBN 0-471-14448-7) and in the first chapter on Integrated Circuit Devices and Modelling, section 1.2, MOS Transistors in the subsection headed Small-Signal Modelling in the Active Region it shows various alternative formulations based on the operating point, including a form that is dependent upon Id. So I suggest you read the literature on this. The point of knowing the gm is to know what the transconductance at a particular bias point would be if you were to apply a changing (small signal) current through the drain, or to apply a varying Vgs and then the current through the drain varies too. It's a number to feed into the small signal model of the transistor, not something that is measured from a large signal sweep that you were doing (essentially you're plotting the small-signal gm found at each discrete value of the DC sweep).

There is some coverage of this in the Virtuoso® Simulator Components and Device Models Reference manual in the documentation, but I think a basic book talking about how transistors work covers this more simply (bear in mind that most texts use a much simplified model compared with real life modern transistor models which are highly complex).

Regards,

Andrew.