You have probably never seen gallium arsenide (GaAs) and may not have even heard of it but every day you likely encounter devices that use this metallic compound and its related compounds aluminum gallium arsenide and indium gallium arsenide. Gallium arsenide has a similar crystal structure to silicon, but each atom of gallium has an arsenic atom nearest neighbor and vice versa. These materials are the core, along with the compound indium phosphide and its derived compounds (which are mostly used in telecommunications), of semiconductor lasers, which are also sometimes called semiconductor diodes or injection lasers. The function of a semiconductor laser is to turn electrical energy into light energy in a directed narrow beam. The best semiconductor lasers can turn around 70% of the input electrical power into light.