why cdo is white while cds is yellow?
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a painter’s palette is rich with colors, some of which arise from the band gaps in natural semiconductors.
The mechanism behind the color we perceive in semiconductors can be explained by the band theory that governs color in many metals. Like metals, semiconductors have a reflective surface when polished, but they do not conduct electricity as effectively. In fact, semiconductors frequently act as insulators, and require particular conditions in order to become conductors. While metals become more resistant to the flow of charge with increasing temperature, semiconductors become conductors only with sufficient thermal energy, performing better as temperature increases.
According to band theory, the electrons in some solids behave differently than those in individual atoms. In a single atom, an electron can occupy one of several distinct energy levels. When light strikes the surface, electrons in a lower energy level can be excited to a higher energy level. The distance between the levels represents the relative energy required to excite an electron. As atoms are brought closer together, these distinct energy levels interact. The low energy levels overlap and the high energy levels overlap. As the number of neighboring atoms increases, more overlap occurs and bands of low and high energy replace the distinct energy levels.
The portion of the band below the level of the gap, called the valence band, contains a full complement of electrons in the absence of thermal energy. The portion of the band above the gap is known as the conduction band, because electrons excited to this level are available to conduct electricity. This leaves a positively charged "hole" in the valence band into which an electron can move, so that the valence band can also contribute to the flow of charge.
The mechanism behind the color we perceive in semiconductors can be explained by the band theory that governs color in many metals. Like metals, semiconductors have a reflective surface when polished, but they do not conduct electricity as effectively. In fact, semiconductors frequently act as insulators, and require particular conditions in order to become conductors. While metals become more resistant to the flow of charge with increasing temperature, semiconductors become conductors only with sufficient thermal energy, performing better as temperature increases.
According to band theory, the electrons in some solids behave differently than those in individual atoms. In a single atom, an electron can occupy one of several distinct energy levels. When light strikes the surface, electrons in a lower energy level can be excited to a higher energy level. The distance between the levels represents the relative energy required to excite an electron. As atoms are brought closer together, these distinct energy levels interact. The low energy levels overlap and the high energy levels overlap. As the number of neighboring atoms increases, more overlap occurs and bands of low and high energy replace the distinct energy levels.
The portion of the band below the level of the gap, called the valence band, contains a full complement of electrons in the absence of thermal energy. The portion of the band above the gap is known as the conduction band, because electrons excited to this level are available to conduct electricity. This leaves a positively charged "hole" in the valence band into which an electron can move, so that the valence band can also contribute to the flow of charge.
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