A MOSFET is a transistor that uses the effects of an electric field to control the flow of current; it acts as a switch and a signal amplifier. Unlike a junction transistor, which controls a large current with a smaller one, a MOSFET controls current with a voltage. MOSFETs come in two polarities, P channel and N channel, where “P” stands for positive and “N” stands for negative.
A MOSFET is a three-terminal device; the terminals are called the gate, drain and source. A voltage applied at the gate controls the flow of electrons from the source to the drain. As the gate voltage passes a threshold value, the transistor goes from non-conducting to conducting. The gate resistance is extremely high, on the order of millions of megohms. Because of this high resistance, the MOSFET’s current consumption at the gate is very low. The resistance between the source and drain becomes low when the device conducts; a MOSFET can handle tens of amps of current with very little loss. In addition to P and N channel types, MOSFETs are made as enhancement-mode or depletion-mode devices. An enhancement-mode transistor is normally off and turns on with a voltage; a depletion-mode device is normally on and turns off with a voltage. The descriptions that follow apply to enhancement-mode MOSFETs.
To turn a P channel MOSFET on, you apply a negative voltage to the gate. This voltage is negative relative to ground. In a circuit, you connect the P channel MOSFET’s source terminal to a positive voltage supply and the drain to a resistor connected to ground; the resistor limits the current flowing through the transistor. The circuit diagram symbol for a P channel MOSFET has an arrow pointing away from the gate.
An N channel MOSFET turns on when you apply a positive voltage at its gate terminal. The voltage is greater than the positive voltage supply at the drain terminal. A resistor between the positive supply and the drain limits current; for an N channel MOSFET, the source terminal connects to ground. The circuit symbol for an N channel MOSFET has an arrow pointing toward the device’s gate.
Low Side and High Side
A circuit called a low side driver uses an N channel MOSFET; it is called “low side” because the transistor connects to the circuit ground connection. A positive supply voltage drives a device when the MOSFET turns on. A high side driver, on the other hand, has a P channel MOSFET connected to the positive supply, with the switched device connected to the transistor’s drain terminal and ground. The low side driver is a simpler circuit; the high side driver, however, lets you switch the direction of current through the device.