Which is four terminals device? 

MOSFET

• MOSFET stands for metal oxide semiconductor field effect transistor.
• MOSFET is a four-terminal device.
• The four terminals are the Source, Gate, Drain, and Body.
• The gate is separated from the channel by a thin insulating layer of silicon dioxide.
• When a voltage is applied between the gate and body terminals, the electric field generated penetrates through the oxide and creates an inversion layer or channel at the semiconductor-insulator interface.
• The inversion layer provides a channel through which current can pass between the source and drain terminals.
• Varying the voltage between the gate and body modulates the conductivity of this layer and thereby controls the current flow between the drain and the source.