MOSFET Calculator

The n-type MOSFET is a four terminal semiconductor device. It consists of a p-type substrate with two n+ doped wells. For a p-type MOSFET, the dopings are n-type substrate and p+ wells. This calculator will calculate the current through two terminals of an n-type MOSFET.

The source contact is generally used as the voltage reference for the MOSFET. The gate region is a metal contact deposited over a thermally grown field oxide that insulates the gate metal from the channel region. The drain contact of the n-type MOSFET is connected to a positive voltage point called the drain voltage.

The gate metal contact forms a Metal Oxide Semiconductor contact with the substrate below the oxide insulator. When the voltage applied to the gate rises above the threshold of the MOS contact an inversion layer is created in the substrate and the once p-type semiconductor now has n-type properties. This inversion layer is referred to as the "channel" of the MOSFET. The ideal threshold voltage is determined by the equation: where is the surface potential to cause an inversion layer, N_A is the semiconductor doping in the channel/substrate and C_o is the capacitance of the oxide layer. The surface potential to cause an inversion layer, , is given by the equation

Once an inversion layer has formed in the channel, a voltage can be applied to the drain contact of the MOSFET. As this voltage is small, the inversion layer has a constant resistance. This region of MOSFET operation is called the linear region because of the linear I-V characteristics. As the voltage difference from the drain to the gate increases to more than V_T a depletion region forms between the inversion layer in the channel of the MOSFET, and the drain well. This causes the current to reach a maximum current, called the saturation current (I_Dsat). In the ideal MOSFET model, the saturation current is constant for increasing levels of V_D.