If a conducting electron reduces to the valence band in an LED, where does it get the energy to go back to the conductance band upon leaving the diode so current can flow? I'm confused as to how current can flow completely through an LED if all electrons are reduced to the valence electron state, meaning they are not conducting electricity. Do the electrons return back to the conductance band energy state?
Dirac’s generalised ‘theory of holes’ in a stack of electron energy levels as corresponding to positive quasi- particles now forms the basis of our understanding of the quantum theory of solid state semiconductors and layered semiconductors known as ‘transistors’ . The purity of the crystalline lattice structure of silicon gives rise to the valence and conduction energy levels in a semiconductor. An ‘n-p’ junction between a silicon wafer doped with excess electrons (n-type) and a wafer with few electrons (p-type) – equivalently with an excess of positively charged ‘holes’ - forms a diode since electrons can only flow one way; from the excess n-side to the deficit p-side; under an applied voltage. Creating light as it flows turns our diode into a Light Emitting Diode or LED. Adding another n-layer turns this diode into an n-p-n two state transistor ‘sandwich’.
 W Shockley (1951) ‘Electrons and Holes in Semiconductors with Applications to Transistor Electronics’, Bell Labs/van Nostrand.