MOSFET TRANSISTORS

Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET) are the most important electronic device in modern world. A MOSFET is a voltage-controlled switch in which the flow of current between its drain and source terminals is turned on or off depending on the voltage applied to the gate terminal. After learning how an MOS capacitor works, we are ready to see how a MOSFET operates.

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Apply drain voltage

Let’s assume no voltage is applied to the gate. Apply a voltage between the source and drain and see what happens. You can hover over the transistor to see the vertical and horizontal components of the electric field. Can you explain what you see by looking at the band diagram or the electric field at the two PN junctions?





Graph electric field direction:

Dynamic Scale for Electric Field

Apply gate voltage

Now, let’s assume no voltage is applied to the drain and instead apply a voltage to gate and see what happens. Can you explain what you see based on what you learned about MOS capacitors? Can you guess what is the threshold voltage of this device?





Graph electric field direction:

Dynamic Scale for Electric Field

Apply gate and drain voltages

Apply a voltage to the gate and then increase the drain voltage set by step and see what happens. Do you see the saturation of the drain current (pinch off)? Can you explain what happens at the pinched off region?





Graph electric field direction:

Dynamic Scale for Electric Field