Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are common, three lead semiconductor devices. The two primary purposes of MOSFETs are to act as voltage controlled switches (pretty much all digital integrated circuits (ICs) are created from MOSFETs) or as amplifiers. The three leads of a MOSFET are called the gate, source, and drain. The basic physical operation involves applying a voltage between the gate and source to control the conductivity of a channel between the source and the drain. The videos below provide a little bit more detail for the two broad categories of MOSFETs which are called depletion MOSFETs and enhancement MOSFETs.
Depletion MOSFETs have a conductive channel between source and drain by default. In other words, they are “normally-ON” devices. The existing channel can be depleted by a voltage applied between gate and source to turn the channel off. Watch this video to find out more:
Enhancement MOSFETs must be “enhanced” to create a conductive channel between source and drain. In other words, E-MOSFETs are “normally-OFF”. To create this channel a voltage must be applied between source and drain.
E-MOSFETs are used primarily as switches, i.e., they are devices that are either on or off. In fact, pretty much all digital circuitry is created from complementary MOSFET (CMOS) circuits which are digital circuits created using P-channel and N-channel E-MOSFETs. This video focuses more on standalone switches that control current to a load (i.e., a voltage at the gate determines whether a load is turned on or off).