The semiconductor laser is another technology that uses pn junctions. Creating a pn junction within a semiconductor brings about "population inversion" by means of the electrons that flow into n-type silicon and the electron holes in p-type silicon.

By skillfully placing two perpendicular mirrors with cleavage planes of semiconductor crystal on either end of the pn junction, we can intensify light by making it bounce back and forth between the planes, thus producing a laser beam comprising light with uniform phase and direction.

Such semiconductor lasers are also called laser diodes. These devices are only about 300 micrometers square and 80 micrometers thick. Laser diodes using gallium arsenide phosphide, which emit a laser beam with a wavelength of 700 nanometers, are being mass produced for use in compact disc (CD) players and laser beam printers.