The structure and working principle of semiconductor lasers are now taking gallium arsenide (GaAs) lasers as an example to introduce the working principle of injection homojunction lasers.
1. The principle of oscillation of an injection homojunction laser. Since the semiconductor material itself has a special crystal structure and an electronic structure, the mechanism of forming a laser has its particularity.
(1) The energy band structure of a semiconductor. Semiconductor materials are mostly crystal structures. When a large number of atoms are regularly and tightly bound into a crystal, those valence electrons in the crystal are on the crystal energy band. The energy band in which the valence electron is located (corresponding to lower energy). The high-energy band closest to the valence band is called the conduction band, and the airspace between the energy bands is called the forbidden band. When an external electric field is applied, the electrons in the valence band transition into the conduction band, and can move freely in the conduction band to conduct electricity. At the same time, the loss of an electron in the valence band is equivalent to the appearance of a positively charged cavity, which can also conduct electricity under the action of an external electric field. Therefore, the holes in the valence band and the electrons in the conduction band have a conducting effect, collectively referred to as carriers.
(2) Doped semiconductor and pn junction. A pure semiconductor without impurities is called an intrinsic semiconductor. If impurity atoms are doped in the intrinsic semiconductor, impurity levels are formed below the conduction band and above the valence band, referred to as the donor level and the acceptor level, respectively.
A semiconductor having a donor level is referred to as an n-type semiconductor; a semiconductor having an acceptor level is referred to as a p-type semiconductor. At normal temperature, heat can cause most of the donor atoms of the n-type semiconductor to be ionized, where the electrons are excited onto the conduction band and become free electrons. Most of the acceptor atoms of the p-type semiconductor capture electrons in the valence band and form holes in the valence band. Therefore, the n-type semiconductor is mainly electrically conducted by electrons in the conduction band; the p-type semiconductor is mainly conducted by holes in the valence band.
The semiconductor material used in the semiconductor laser has a large doping concentration, and the number of n-type impurity atoms is generally (2-5) × 1018 cm-1; and the p-type is (1-3) × 1019 cm-1.
In a piece of semiconductor material, the region that suddenly changes from the p-type region to the n-type region is called a pn junction. A space charge region will be formed at the interface. In the n-type semiconductor strip, electrons are diffused toward the p region, and holes in the p-type semiconductor valence band are diffused toward the n region. In this way, the n-type region near the structure is positively charged due to the donor, and the p-type region near the junction region is negatively charged due to being the acceptor. An electric field from the n region to the p region is formed at the interface, which is called a self-built electric field. This electric field prevents the continued diffusion of electrons and holes.
(3) pn junction injection injection excitation mechanism. If a forward bias is applied to the semiconductor material on which the pn junction is formed, the p region is connected to the positive electrode and the n region is connected to the negative electrode. Obviously, the electric field of the forward voltage is opposite to the self-built electric field of the pn junction, which weakens the hindrance of the self-built electric field to the electron diffusion motion in the crystal, so that the free electrons in the n region are under the action of the forward voltage. Constantly diffusing through the pn junction to the p region. When there are a large number of electrons in the conduction band and holes in the valence band, they will recombine in the implantation region, and the electrons in the conduction band transition to the valence band. When the excess energy is emitted in the form of light. This is the mechanism of semiconductor electroluminescence, which is called spontaneous emission.
In order for the pn junction to generate laser light, a particle inversion distribution state must be formed in the structure, and a heavily doped semiconductor material is required, and the current required to inject the pn junction is required to be sufficiently large (for example, 30000 A/cm 2 ). Thus, in a partial region of the pn junction, it is possible to form an inverted distribution state of electrons in the conduction band more than the number of holes in the valence band, thereby generating stimulated composite radiation to emit laser light.
2. Semiconductor laser structure. Its shape and size are similar to those of a small power semiconductor transistor, with only one laser output window on the housing. The p region and the n region sandwiching the junction region are layered, the junction region is tens of micrometers thick, and the area is less than about 1 mm 2 .
The optical cavity of a semiconductor laser is constructed using a natural cleavage plane (110 faces) perpendicular to the plane of the pn junction, which has a reflectivity of 35, which is sufficient to cause laser oscillation. If you need to increase the reflectivity, you can plate a layer of silicon dioxide on the crystal surface, and then plate a layer of metal silver film to obtain a reflectivity of more than 95%.
Once the forward bias is applied to the semiconductor laser, the population inversion occurs in the junction region and is compounded.