Structurally, what are the two main categories of semiconductor diodes?

Semiconductor diodes are primarily categorized by their internal physical structure into two types: junction diodes and point contact diodes. A junction diode features a flat interface (the PN junction) where P-type and N-type semiconductor materials meet, commonly used for modern rectification and switching. A point contact diode, on the other hand, consists of a sharp metallic wire (the "cat's whisker") pressing against a semiconductor crystal. This older, specialized structure has very low junction capacitance, making it physically and functionally distinct from the planar PN junction. Distractors like "vacuum" and "electrolytic" refer to entirely different types of components (vacuum tubes and electrolytic capacitors, respectively) and are not structural categories of solid-state semiconductor diodes.