In a thin-film transistor (on insulating substrate) what is the difference between putting a load on the source or drain side of the transistor?
For a planar MOSFET like this the designation of the source and drain is not determined by the physical layout of the transistor...the layout is symmetric. If it is an NMOS then at any given instant the terminal with the lower voltage is the source and the terminal with the higher voltage is the drain. So, \$V_{GS}\$ has a consistent meaning regardless of which physical "end" of the transistor you connect to.