Diodes allow electricity to flow in one direction and block it in the other. They are used in circuits for protection and applications like converting AC to DC power. There are different types of diodes but their basic function is the same. Bipolar junction transistors and MOSFETs are semiconductor devices with three terminals - gate, drain, and source - that allow current through the drain and source to be controlled by the voltage at the gate. They have different operating regions including cutoff, active, and saturation. JFETs are also field-effect transistors that operate in a similar manner.