What property of an LED's semiconductor material determines its forward voltage drop?
Why In an LED, electrons crossing the junction recombine with holes and release their energy as photons. The energy released, and thus the forward voltage needed to push carriers across, is set by the band gap of the semiconductor, the energy separation between the valence and conduction bands. Wider band gap materials emit shorter wavelengths (blue, near 3 V) while narrower ones emit red or infrared (around 1.6 to 1.8 V).
Watch out Junction capacitance is a reverse-bias property that matters in varactors and high-speed switching, not something that sets the forward turn-on voltage; intrinsic resistance only adds a small extra drop at high current.
Band gap sets both the color and the forward voltage: bigger gap means bluer light and higher volts.