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Semiconductors are unique materials: solids whose electrical conductivity can be changed deliberately, usually in a dynamic (reversible) fashion. They are used to make semiconductor devices, which led to the Information Age of the late 20th century. Today, these materials are everywhere and continue to penetrate further into most people's daily lives.
Devices made with semiconductors include actuators and control systems in cars, MP3 players, cell phones, and computers of all kinds. These materials are arguably one of the most important technologies of the 20th century, and they continue to be a central aspect of developed economies. The most common are made of silicon, as it is relatively cheap to extract from sand. The semiconductor industry sells several hundred billion US Dollars of product per year.
The first semiconductors were little detectors on radios popular around the beginning of the 20th century. They were called "cat's whiskers" and the semiconducting element was lead sulfide. No one at the time really understood how they worked; they just did. It was not until 1939 that Richard Ohl, an inventor at Bell Labs who was also the first to patent solar cells, discovered that certain crystals with small impurities have conductivity that varies based on exposure to light. His work grew out of an effort to find practical high-frequency amplifiers for applications in radio.
Eight years later, in 1947, other scientists at Bell Labs used semiconducting materials to create a point-contact device which they called a transistor. The material used was germanium. The whole device was about half a foot tall, and required that the element be extremely purified.
The structure that underlies any transistor is the p-n junction. It has two regions: a p region and an n region. The p region is "doped" with small amounts of boron, causing the material to become filled with numerous electron "holes," which are an absence of electrons where electrons should be. This happens because boron has a valence of three, which causes it to absorb weakly-bound outer electrons from the valence-four semiconductor atoms, leaving voids in its place. The n region is doped with a material that has a valence of five, causing the reverse effect, where the impurities donate their extra electron to the material, causing an abundance of electrons.
This relative abundance and absence of electrons is exploited in the transistor. A series of two p-n junctions makes up the heart of the device. By manipulating the junctions, charge flow can be regulated precisely, allowing for complex electronics. Variations on the transistor can be used to make LEDs and very delicate sensors, while most computers have billions of transistors of several different types. Although silicon is the most common transistor today, diamond, which can be configured in a 3D matrix more easily than silicon transistors, may be used in the future.