A diode is semiconductor device consisting of a PN-junction. The Khan Academy India provides an excellent and very clear introduction on highschool level.

Have a look at the YT playlist on Intro to semiconductors by Khan Academy India

A few example videos from the tutorial series:

To fully understand PN-junction one has to study the Fermi charge distribution function which is a result of the statistical theory of quantum mechanics. This is beyond the scope of the course Phy1 / EEng.

Anyway, if you want to dive deeper have a look at the full YT playlist of Jordan Edmunds on Semiconductor Physics (challenging!).