Insulated Gate Field-Effect Transistors

The “substrate” connection in a MOSFET is often internally connected to the source, like this:

This turns the MOSFET from a four-terminal device into a three-terminal device, making it easier to use. One consequence of this internal connection, though, is the creation of a parasitic diode between the source and drain terminals: a PN junction that exists whether we want it to or not.

Add this parasitic diode to the MOSFET symbol shown here (representing the MOSFET cross-section shown above), and explain how its presence affects the transistor’s use in a real circuit:

A technician is using a digital multimeter (with a “diode check” feature) to identify the terminals of a power MOSFET:

The technician obtains the following “diode check” voltage measurements, in this order:

1. Black lead on middle terminal, Red lead on right terminal = 0.583 volts (shown in illustration)
2. Red lead on middle terminal, Black lead on right terminal = O.L. (open)
3. Black lead on middle terminal, Red lead on left terminal = O.L. (open)
4. Black lead on middle terminal, Red lead on right terminal = 0.001 volts
5. Red lead on middle terminal, Black lead on right terminal = 0.001 volts

Explain why the fourth and fifth measurements are so different from the first and second, respectively, when they were taken between the same terminals on the MOSFET. Hint: this particular MOSFET is an N-channel, enhancement-type.

An important consideration when working around circuits containing MOSFETs is electrostatic discharge, or ESD. Describe what this phenomenon is, and why it is an important consideration for MOSFET circuits.