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.
Around the middle 1950s, engineers discovered that junction diodes made from a material called gallium arsenide emitted light (although it was only much later that usable lasers and LEDs were made this way). Alas, explaining this phenomenon introduces more vocabulary terms. Free electrons traveling through a semiconductor crystal have a fairly high level of energy, so they are said to be in the conduction band. When an electron meets a hole in the crystal, it tends to stay there. Holes are where an electron would normally be, and when a free electron “falls in,” it releases energy in the form of a photon of light. When the energy difference or band gap between the high conduction band state and the lower state is small, as it is in silicon, the light is released at the invisible infrared frequencies. When the band gap is large, the emission is visible light. This happens in all types of diodes, but in an ordinary silicon diode the silicon itself absorbs most of the light. Light emitting diodes are constructed so that most of the light radiates outward. The device is usually mounted in a small reflector cup to help direct the light, and the whole assembly is packaged in translucent plastic.
A semiconductor laser diode, like the kind in a DVD player and other common systems, uses much the same principle, but uses special materials to create a larger band gap. A laser diode uses heterostructures, which are junctions of two different types of semiconductor materials, chosen so that the band gap is very large. The device also uses mirrors and other means to reflect light emitted from the junctions in order to stimulate the laser effect.
While a semiconductor diode is the simplest type of electronic device, semiconductors are also used to make transistors, integrated circuits, and many other types of electronic devices.