Laser diodes are electrically pumped semiconductor lasers in which the gain is generated by an electric current flowing through a p–n junction or (more frequently) a p–i–n structure. In such a heterostructure of a bipolar interband laser, electrons and holes can recombine, releasing the energy. Lasers are the stuff of science fiction: big, heavy boxes that make blazing blasts of light. These devices are currently used in the fields of telecommunications and medicine and in industrial cutting and welding applications. It operates similarly to a light-emitting diode (LED) but produces a focused, monochromatic, and coherent beam of light.