gallium arsenide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A dark-gray crystalline compound, GaAs, used in transistors, solar cells, semiconductor lasers, and other semiconductor applications.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun inorganic chemistry A binary compound of gallium and arsenic, GaAs, used as a semiconductor, especially in light-emitting diodes.