What is a Power MOSFET?

What is a Power MOSFET?

We all know how to use diodes to make switches, but we can only turn them on and off, not control the flow of a signal over time. Furthermore, the diode's ability to act as a switch depends on the direction of the signal flow; we can't program it to pass or block a signal. For applications such as "flow control" or programmable switching, we need a three-terminal device and the bipolar transistor. We've all heard of Bardeen &
Brattain, who invented the transistor by accident, like so many other great discoveries.
Structurally, it's made of two back-to-back junctions (not a big deal, we probably had a common cathode with the same structure long before Bardeen), but functionally it's a completely different device, like a "faucet" that controls the flow of emitter current - the "hand" that operates the faucet is the base current. The bipolar transistor is therefore a current-controlled device.

Field effect transistors (FETs) provide the same "faucet" function, albeit with different constructions. The difference is that FETs are voltage controlled devices; you don't need base current, but rather voltage to implement current control. The bipolar transistor was invented in 1947, and soon after, a brilliant father and son team, Shockley and Pearson, invented (at least the concept) the FET. To distinguish it from its earlier bipolar "twin", the three electrodes of the FET are called drain, gate, and source, while the three electrodes of the corresponding transistor are collector, base, and emitter. There are two main variants of FETs, which are optimized for different types of applications. JFETs (junction FETs) are used for small signal processing, while MOSFETs (metal oxide semiconductor FETs) are mainly used in linear or switching power supply applications.