What is the difference between Vss and Vs in a transistor?
Going back a ways, Vs refers to any (V)oltage (s)ource. For early logic ICs, which could use odd supply voltages, Vss stood for "substrate voltage". Sometimes this meant ground, sometimes not. See, as an example, http://www.datasheetarchive.com/dl/Scans-022/ScansEX55919.pdf which is a the digital logic for a 4 1/2 digit A/D converter for making DPMs. It has two power pins, Vss and Vdd. Vss is set to +5, and Vdd to (typically) -15. Note that that there is no ground pin.