MOSFET as a Switch

We saw previously, that the N-channel, Enhancement-mode MOSFET (e-MOSFET) operates using a positive input voltage and has an extremely high input resistance (almost infinite) making it possible to interface with nearly any logic gate or driver capable of producing a positive output.

We also saw that due to this very high input (Gate) resistance we can safely parallel together many different MOSFET until we achieve the current handling capacity that we required.

While connecting together various MOSFETS in parallel may enable us to switch high currents or high voltage loads, doing so becomes expensive and impractical in both components and circuit board space. To overcome this problem Power Field-Effect Transistors or Power FET’s were developed.

We now know that there are two main differences between field-effect transistors, depletion-mode only for JFET and both enhancement-mode and depletion-mode for MOSFETs. In this tutorial, we will look at using the Enhancement-mode MOSFET as a Switch as these transistors require a positive gate voltage to turn “ON” and a zero voltage to turn “OFF” making them easily understood as switches and also easy to interface with logic gates.

The operation of the enhancement-mode MOSFET, or e-MOSFET, can best be described using its I-V characteristics curves shown below. When the input voltage, ( VIN ) to the gate of the transistor is zero, the MOSFET conducts virtually no current and the output voltage ( VOUT ) is equal to the supply voltage VDD. So the MOSFET is “OFF” operating within its “cut-off” region.