What is VDD of a MOSFET Transistor?

VDD of a MOSFET transistor is the DC voltage that is supplied to the drain of the transistor.

It is a very important voltage when biasing the transistor because it determines how much the AC signal can be amplified to in the transistor. If VDD is too low, the transistor will not have enough power and the amplified AC signal will come out clipped.

VDD normally does not have to be calculated in a transistor circuit, as it's normally just given. However, it is very important when calculating other values in a transistor such as ID, the current through the drain and IS, the current through the source of the MOSFET. VDD is also important for calculating the load line of the transistor and thus its Q-point.