![]() Even though photons have no mass, they have an observable momentum which follows the de Broglie equation. The speed of a photon through space can be directly derived from the speed of an electric field through free space. Photons travel at the speed of light, 2.997x10 8 m/s in empty space. These packets of energy can be transmitted over vast distances with no decay in energy or speed. Although they have never been observed, the longest theoretical wavelength of light is the size of the universe, and some theories predict the shortest possible at the Planck length. These fields may oscillate at almost any frequency. This energy is stored as an oscillating electric field. This is a very fitting analogy, as a photon contains energy that cannot be divided. Photons are often described as energy packets. In the different case of semiconductors, higher energy photons typically are absorbed more strongly, while low energy photons pass right through the semiconductor.\) As you can see, light in the higher and lower energies penetrate the least. The absorption coefficient is an important quantity that will show up in the following sections in the various models we have for semiconductor charge carrier generation, so it is good to keep in mind that it depends on both the incoming light and the intrinsic qualities of the material.Ībove is an image of the ocean and the depth of absorbance by various wavelengths (energies) of light. Where f is the frequency of the monochromatic light (related to the wavelength by λ= v/ƒ, where v is the velocity of the light wave), c is the speed of light, and π is a constant (≈ 3.14). The absorption and extinction coefficients are related by the following equation 1: ![]() κ > 0 means absorption, while κ = 0 means the light travels straight through the material. This coefficient κ is an optical property of the semiconductor material and is related to the index of refraction n, which merely determines how much light is absorbed by the material. The absorption coefficient is related to the wavelength of light and another quantity called the extinction coefficient, which is also related to the wavelength of light (the electromagnetic waves propagated from the sun). The wavelengths most important for solar application are in the infrared and visible parts of the electromagnetic spectrum. Where F(x) is the intensity at a point x below the surface of a semiconductor, F(x 0) is the intensity at a surface point x 0, and α is the absorption coefficient, which determines the depth at which light of a certain wavelength penetrates the semiconductor. The following equation models the exponential decay of monochromatic (one-color or approximately single-wavelength) light as it travels through a semiconductor 1: Therefore, the amount of photons that reach a certain point in the semiconductor depends on the wavelength of the photon and the distance from the surface. The rate of absorption of light is proportional to the intensity (the flux of photons) for a given wavelength in other words, as light passes through the material the flux of photons is diminished by the fact that some are absorbed on the way through. ![]() Light that is transmitted through the semiconductor material is attenuated by a significant amount as it passes through.
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