What is a Mosfet Used for?

MOSFET stands for metal-oxide semiconductor field effect transistor. What sets a MOSFET apart from its transistor counterpart, BJT (bipolar junction transistor), is that it is voltage controlled as opposed to current controlled. This makes a MOSFET more capable to handle high levels of voltage as opposed to a BJT.

There are three pins on a MOSFET, a “gate”, a “drain”, and a “source” terminal. When voltage is applied to the gate, an electrical field is created to control a current flow through the drain and the source. A few connections are made when voltage is added, Gate-Source and Drain-Source. When the Gate-Source connection does not have applied voltage between them the Drain-Source resistance is high, but when the Gate-Source potential difference is applied, the Drain-Source resistance is reduced. This causes these connections to transition from an open circuit to a closed circuit. Think of this device as a variable resistor where the electric resistance value can be adjusted, specifically controlled by voltage. This is important to note because FETs, or field effect transistors, are controlled by Gate-Source voltage, making for larger currents and turning the MOSFET into a switch.

To utilize the MOSFET as a switch, the gate voltage (Vgs) has to be higher than the source of the voltage. For example, a “standard” MOSFET will only turn on if the Vgs is between 10 and 20 volts. A “logic-level” MOSFET, however, only has a Vgs of 5 volts. This level is not the same for all MOSFET types but can be used as a guide.

There are 2 modes that the MOSFET can enter into, enhancement or depletion. Depletion mode occurs when there is a closed current meaning the electrical current can flow when there is no voltage applied. Enhancement mode can have two channels, N-Channel (NMOS) or P-Channel (PMOS). When voltage is applied between the gate and the source, current is able to flow between the drain and the source, creating that connection mentioned above (Drain-Source). This is for N-Channel MOSFET and the most common type of MOSFET, but for P-Channel MOSFET, current will only flow if the gate has minimal voltage applied to it.

The basis for this device is to increase conductivity of the circuit. The steps to utilize a MOSFET are as follows. The first step is to indicate which pin is the Gate, Drain, and Source. The second step is to find the gate threshold indicated on the datasheet as Vgs to allow the device to be used as a switch. Step three is to figure out what the Drain-Source resistance is to be able to maintain an open or closed current. Once all of this is found, you can begin to add voltage. The secret behind its conductivity is truly the voltage. When voltage is applied to the gate, the width of the Drain-Source increases making for better conductivity.

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