Explanation: Semiconductor diodes are fundamentally categorized by how their rectifying junction is formed. **Junction diodes** are the most common type, created by joining a P-type semiconductor material with an N-type semiconductor material to form a PN junction. This junction is where the diode's rectifying action occurs, allowing current to flow predominantly in one direction. Examples include rectifiers, Zener diodes, and LEDs. **Point contact diodes** utilize a sharpened metal wire (the "cat's whisker") making direct, physical contact with a small semiconductor crystal (often germanium). This creates a very small, localized junction at the point of contact. They are known for low capacitance and were historically used in crystal radios and for microwave detection. Other options are incorrect: * **Electrolytic** refers to a type of capacitor, not a diode. * **Vacuum** describes vacuum tubes, which are an older, non-semiconductor technology distinct from semiconductor diodes.