Semiconductors are unique materials, solids whose electrical conductivity can be changed deliberately, usually in a dynamic (reversible) fashion. Semiconductors are used to make semiconductor devices, which led to the Information Age of the late 20th century. Today, semiconductors are ubiquitous, and continue to penetrate further into our daily lives. These devices include actuators and control systems in cars, MP3 players, cell phones, and computers of all kinds. Semiconductors are arguably one of the most important technologies of the 20th century, and continues to be a central aspect of developed economies. The most common semiconductors are made of silicon, as it is relatively cheap to extract from sand, and gets the job done. 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 twentieth century. They were called "cat's whiskers" and the semiconducting element was lead sulfide, but no one 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.

8 years later, in 1947, other scientists at Bell Labs used semiconducting materials to create a point-contact device which they called a transistor. The semiconductor material used was germanium. The whole device was about half a foot tall, and required extremely purified germanium.

The structure that underlies any transistor, or semiconductor device, is the p-n junction. A p-n junction 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" - 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 semiconducting 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 transistor. 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. And the computer you are using right now to access this web page probably has billions of transistors, of several different types. Although silicon is the most common transistor today, the transistor of the future may be diamond, which can be configured in a 3D matrix more easily than silicon transistors.