Why are FETs similar to diodes?

Answer 1

Junction Field Effect Transistors (JFET) have a single junction between the gate and channel, like a diode but they do not operate like diodes! The junction in a JFET must always be kept reverse biased for it to operate, the junction in a diode is usually expected to be forward biased sometimes and reverse biased at other times while operating.

Metal Oxide Semiconductor Field Effect Transistors (MOSFET) have no junction between the gate and channel! Instead they have a silicon dioxide insulator layer. Diodes do not have an insulator layer.

The feature that makes FETs operate entirely different from diodes is the channel between the source and drain contactson the semiconductor. By varying the potential on the gate of a FET relative to the potential on the channel the channel can be made to "pinch off" or "open up" thus varying its resistance. This can be used to create amplification in the circuit, which cannot be done using a diode.

MOSFETs usually are built on a substrate and use a junction that is always reverse biased to prevent the substrate from short circuiting the channel.

Note: most integrated circuits use this same reverse biased substrate junction structure to insulate the components of the circuit on the chip from the substrate semiconductor material to prevent it from short circuiting those components. These junctions are parasitic diodes not shown on the schematic of the integrated circuit, as they have no function except as insulation between components.