Irrespective of the application, a MOSFET has basically three terminals namely Gate, Drain and Source. The structure of a MOSFET varies based on the application i.e., MOSFETs in IC technology are fairly lateral, while the structure of Power MOSFETs is more of a vertical channel. The main difference between Enhancement Mode MOSFET and Depletion Mode MOSFET is that in depletion mode, the channel is already formed i.e., it acts as a Normally Closed (NC) switch and in case of enhancement mode, the channel is not formed initially i.e., a Normally Open (NO) switch. The symbols for each of these types of MOSFETs are shown in the image below. Each of these types are further divided into N-channel MOSFET and P-channel MOSFET. MOSFET can be classified into Enhancement type MOSFET and Depletion type MOSFET. Speaking of terminals, a MOSFET is typically a 3-terminal device, which are Gate (G), Source (S) and Drain (D) (even though there is a 4th terminal called Substrate or Body, it is usually not used in either input or output connection). It is a type of field effect transistor with an insulated gate from the channel (hence, sometimes called as Insulated Gate FET or IGFET) and the voltage at the gate terminal determines the conductivity. A MOSFET or Metal Oxide Semiconductor Field Effect Transistor, unlike a Bipolar Junction Transistor (BJT) is a Unipolar Device in the sense that it uses only the majority carriers in the conduction.
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