Symmetry of N-Channel MOSFET: VGD & VGS Thresholds

[jrive]

In an N-channel mosfet, for example, can the roles of the Drain and Source be reversed such that the FET turns on with positive VGD? Will this threshold be the same as the specified one for VGS?

 

[es1]

In the basic 2d model of a NFET usually given in most introductory texts, yes. And, if you're controlling the layout and force symmetry [1], then again, yes (obviously I guess due to the forced symmetry).

But there are many examples where this does not hold and the FET is not symmetrical to change parasitics or increase robustness.

An obvious example of a NFET which is not symmetrical is the standard trench layout used in power FETs. See the linked IR app note figure 5. For this layout it is obvious drain and source cannot be reversed. There are many other examples as well.

http://www.irf.com/technical-info/appnotes/mosfet.pdf

[1] The entire geometry, including the base tie, must be symmetrical. If only the base geometry is different then the source and drain can be reverse but the threshold will change.