What is the crystal diode of a capacitor?

A crystal diode of a capacitor, a semiconductor device at both ends of a solid state electronic device. The main feature of these devices is their non-linear current-voltage characteristics. Since then, with the development of semiconductor materials and process technology, a variety of crystal diodes with various structures and different functional uses have been developed using different semiconductor materials, doping profiles, and geometric structures. The materials used are germanium, silicon and compound semiconductors. Crystal diodes can be used to generate, control, receive, transform, amplify signals, and perform energy conversion.