When VGS > Vth and VDS < (VGS - Vth)
The transistor is turned on, and a channel has been created that allows current to flow in between the drain and the source. The MOSFET operates such as a resistor, controlled by the gate voltage that relative to both the source and drain voltages. The current from drain to source is modeled like:
ID = μn Cox W/L ((VGS - Vth) VDS - V2DS /2)
In which μn is the charge-carrier effective mobility, W is the gate width, L is the gate length and Cox is the gate oxide capacitance per unit area. The transition from the exponential sub threshold region to the triode region is not as very sharp as the equations suggest.