Which type of diode is capable of both amplification and oscillation?

The tunnel diode is unique among the various types of diodes due to its ability to exhibit negative resistance, which allows it to both amplify signals and produce oscillations. This capability stems from the quantum-mechanical tunneling effect that occurs at very high carrier concentrations. Because of this property, tunnel diodes are often used in high-frequency applications and in circuits that require oscillation.

Other diodes, such as Zener diodes, Schottky diodes, and photodiodes, have specific applications but do not possess the same negative resistance behavior that facilitates amplification and oscillation. Zener diodes are primarily used for voltage regulation, Schottky diodes are known for their rapid switching speed and low forward voltage drop, and photodiodes convert light into electrical current, but none exhibit the dual functionality of amplification and oscillation like tunnel diodes do.