P-Channel MOSFET: Implications of Bulk Connected to +Vdd

[seang]

What are the implications of connecting the bulk of a p-channel mosfet to a +Vdd. Up until now, we've only had examples where the bulk has been shorted to the source of the mosfet, or shorted to ground. I don't know how a bulk voltage changes the source voltage or the drain voltage.

Thanks for your help

 

[es1]

Long story short, changing the builk voltage changes the threshold value of the transistor.

The threshold value is in both the linear and quadratic models of the FET so this has implications to Ids as well but they are usually minor unless you are dealing with low power electronics or operating in weak inversion.

The value that really matters is the difference between the source and the bulk. Tying bulk to the source shorts this so the threshold never changes.

This link has more details and the math:
http://ece-www.colorado.edu/~bart/book/book/chapter7/ch7_4.htm#7_4_2

However, in practice people usually tie the bulk to ground (or to vdd for pfets) to avoid biasing the body diodes. The drain and the source form pn junctions (aka a diode) with the substraight (aka bulk). Connecting the bulk to the lowest voltage in your system for nfets (or highest for pfets) insures these diodes never get biased and thus you get to ignore them which is nice when one is starting out with electronics.