Re: what happens if we short drain-gate AND drain-sourse in
If you short the drain and the gate in a MOSFET, in the small signal analysis it becomes a diode-connected transistor i.e it is equivalent of a resistor of value 1/gm .Note that the diode connected MOSFET is always in saturation given that its Vgs is larger than Vth in other words it never operates in triode .If you short both the drain and source of a MOSFET and use them as one terminal along with the gate as another terminal you can use it as a MOS capacitor .One other application of the drain and source shorted together is at switches at capacitive switched ciruits to remove the charge injection .