Incoming links: Direct and indirect band gaps
Direct and indirect band gaps is an important part of why diodes don't emit light apparently.
Bibliography:
- www.quora.com/What-is-the-difference-between-an-LED-and-a-diode
- youtu.be/9BDTtcRMxpA?t=388 from Video "How LEDs work by VirtualBrain" explains the geometry aspect well