Real output resistance of MOSFET
@Keno Let's put it this way. When you operate your MOSFET in the saturation regime, as an amplifier, you use r0 in the small-signal analysis of the circuit. If you operate MOSFET as a switch (as in digital circuitry), and the switch is turned ON, you can use Ron, as long as the transistor is in the triode region. For detailes on MOS regimes of operation, i suggest looking up one of great books from Baker, Sedra &amp; Smith, Gray &amp; Meyer etc., because you can't really cut corners with these stuff if you want to build proper circuits.