Semiconductor is one of the most common—but least understood—terms in the tech world. Simply defined, semiconductors are generally certain elements (such as silicon) and chemical compounds (such as lead sulfide) that allow, but still resist the flow of electricity. Somewhere between good conductors, such as copper, and poor conductors, such as glass, lie semiconductors, which are just OK conductors. If the semiconductor is only a mediocre conductor, why is it so important? Because semiconductors have a unique atomic structure that allows their conductivity to be controlled by stimulation with electric currents, electromagnetic fields, or even light. This makes it possible to construct devices from semiconductors that can amplify, switch, convert sunlight to electricity, or produce light from electricity.
In electronics the usefulness of semiconductors stems from the structure of the atoms that make up semiconductor crystals. For example, a silicon atom has four electrons in its outer orbital (the top “shell” of orbiting electrons). When heated to the melting point and refrozen, silicon atoms tend to form organized crystal structures or lattices. In a process called doping, phosphorus or arsenic atoms are mixed into the silicon. This disturbs the silicon’s structure, giving the resulting crystal extra electrons. The crystal is changed from an OK conductor to a good conductor. Since electrons carry a negative charge, this type of crystal with extra electrons is known as an N-type or N-doped semiconductor.
Doping the crystal with boron or gallium also turns the crystal into a conductor, but it does so by leaving it with a shortage of electrons. Physicists say that the crystal has holes, which make the crystal positive or P-type. When N-type and P-type crystals come together, something surprising happens. The junction acts as a barrier to the flow of electricity in one direction but presents almost no resistance in the other direction. This one-way valve can be used in an electronic device called a diode. You can think of a diode as a door that only swings one way—you can go out, but you can’t go back in.
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