Impure polycrystalline material has the appearance of pale orange or grayish pieces.

Undoped single crystals are orange, but strongly doped wafers appear darker due to free-carrier absorption.

Gallium phosphide has been used in the manufacture of low-cost red, orange, and green light-emitting diodes (LEDs) with low to medium brightness since the 1960s.

Gallium phosphide is transparent for yellow and red light, therefore GaAsP-on-GaP LEDs are more efficient than GaAsP-on-GaAs.

In crystal growth from a 1500 °C melt (for LED wafers), this must be prevented by holding the phosphorus in with a blanket of molten boric oxide in inert gas pressure of 10–100 atmospheres.