Definition
The negative terminal of a semiconductor diode. It is the end of the diode that connects to the N-type material, where conventional current flows out when the diode is forward-biased. On most diodes, the cathode is marked with a painted band or stripe near that end of the case.
Plain English
The cathode is the negative end of a diode. A diode lets electricity pass through it in only one direction, and the cathode is the side current flows out of. It is usually marked with a stripe so you can tell which end is which.
Context Anchor
Seen in aircraft electrical system diagrams, alternator rectifier discussions, and maintenance descriptions of diode checks.
Derivation
From the Greek kathodos, meaning 'a way down.' The term was coined in the 1830s to describe the terminal where current was thought to flow 'down' into a device. The same word is used today for the negative terminal of diodes, batteries, and vacuum tubes, even though the underlying physics is better understood now.
Why Pilots Care
Diodes only work in one direction. If a diode is installed backwards because the cathode end was misidentified, the circuit will not function and may damage other components. Knowing which end is the cathode (the banded end) is essential when replacing diodes during maintenance.
Analogy
Think of a diode as a one-way door for electricity. The cathode is the side the current walks out of. Try to push current in the wrong direction and the door stays shut.
Intuition Check
Do not assume “cathode” always means “the negative terminal.” In a normal forward-biased diode it is usually on the more negative side, but the word itself means a specific diode end: the bar or stripe end.
Example Sentence 1
The technician identified the cathode by the silver band on the diode's case before soldering it into the circuit board.
Example Sentence 2
Current leaves the semiconductor diode through the cathode when the circuit is complete.