Semiconductor Laser Diodes

    A semiconductor laser diode is made up from p-type and n-type semiconductor layers that are separated by a thin active layer in which laser action takes place. The laser is pumped by passing an electric current across the active layer from the p-type layer to the n-type layer. The electric current density required to achieve laser operation (threshold current density) is typically in the region of several thousand amps per cm2 but, as these devices are very small, the actual current required is often below 100 mA. In order to achieve the required electrical and mechanical properties, the number of layers in practical devices is larger than shown in the diagram below.

Laser Diode

    Laser diodes can have efficiencies in the region of 30% and the optical gain within the active layer is high. Because of the high gain, the length of the active layer can be small and is typically below 1mm. In early laser diodes, the thickness of the active layer was between 100 and 200 nm but, in quantum well lasers, it is in the region of 10 nm. These small dimensions of the laser source give rise to appreciable diffraction effects with the result that the output beam of a laser diode is divergent and lens systems must be used to collimate it. A further consequence of the high gain is that cavity mirrors are not required for the laser to operate. In the case of gallium arsenide (GaAs), the large difference in refractive index between the semiconductor material and air at the lasing wavelength gives a reflectivty at the interface of about 30% and this is sufficient to allow laser action to occur. With this type of construction, laser radiation would be emitted from both ends of the active layer as indicated in the diagram, but only the radiation from one of the ends would be used as an output beam. The radiation emitted from the other end could be detected with a suitable photodetector and used to monitor the output of the laser or, by using an electrical feedback circuit, it could be used to control the current through the diode with the purpose of stabilising the laser output.

    The active layers in all laser diodes are made from compound semiconductor materials such as gallium arsenide rather than elemental semiconductors such as silicon or germanium. Many have been fabricated from compounds of elements in groups III and V of the periodic table. For example, gallium arsenide is a binary semiconductor compound of gallium (group III) and arsenic (group V). Some laser diodes that emit in the visible region of the spectrum have been made using compound semiconductors such as zinc selenide which is composed of elements from groups II and VI of the periodic table. Ternary semiconductors (composed of three elements) and quaternary compounds (composed of four elements) have been used to make diodes that emit at particular wavelengths.

    The type of laser illustrated above is often referred to as having a double heterojunction structure. In the case of a GaAs laser, the active layer of GaAs would be sandwiched between p and n-type layers of the ternary semiconductor gallium aluminium arsenide, GaAlAs.The term heterojunction refers to a junction between different semiconductor materials such as the GaAs in the active layer and the GaAlAs in one of the adjoining layers.

    In all laser diodes, the mechanism of light emission involves the recombination of electrons with holes in the region of the active layer.