Nature Reviews Physics, Published online: 17 June 2026; doi:10.1038/s42254-026-00949-2 Because gallium nitride light-emitting diodes (LEDs) have made most of the visible spectrum available to LEDs, they have become the backbone of modern lighting applications. This Review examines three effects governing the physics of GaN-based LEDs: polarization fields, carrier localization and non-radiative defects.