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A quantum-well laser is a laser diode in which the active region of the device is so narrow that quantum confinement occurs. Laser diodes are formed in compound semiconductor materials that (quite unlike silicon) are able to emit light efficiently. The wavelength of the light emitted by a quantum-well laser is determined by the width of the active region rather than just the bandgap of the materials from which it is constructed.[1] This means that much shorter wavelengths can be obtained from quantum-well lasers than from conventional laser diodes using a particular semiconductor material. The efficiency of a quantum-well laser is also greater than a conventional laser diode due to the stepwise form of its density of states function.