The main difference between a JFET (Junction Field-Effect Transistor) and a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) lies in their construction and operation principles.
JFETs are typically constructed from a single piece of semiconductor material, forming a channel between two terminals (source and drain) with a third terminal (gate) controlling the width of this channel through an applied voltage. They operate in depletion mode, meaning the channel is normally conducting when no voltage is applied to the gate.
On the other hand, MOSFETs have a gate insulated from the channel by a thin layer of oxide (hence the name Metal-Oxide-Semiconductor). They can operate in enhancement mode (normally off without a gate voltage) or depletion mode (normally on without a gate voltage), depending on their design. The gate voltage controls the conductivity of the channel between the source and drain.
In terms of speed, MOSFETs generally offer faster switching speeds compared to JFETs due to their insulated gate structure, which allows for faster charge/discharge of the gate capacitance.
Regarding the term “FET,” it is a broad term encompassing both JFETs and MOSFETs. Therefore, while all MOSFETs are FETs, not all FETs are MOSFETs. JFETs and MOSFETs differ significantly in their construction, characteristics, and operational modes, catering to different circuit requirements and applications despite their shared classification as field-effect transistors.