Semiconductor technology.

Most semiconductor devices are made by introducing controlled numbers of impurity atoms into a crystal. This process is called doping. To improve the semiconductor crystal the impurities known as dopant a are added to the silicon. They produce a special type of conductivity, characterized by either positive(p-type) charge carriers or negative/ (n-type) ones. The dopants are diffused into semiconductor crystals at high temperature.

In the furnace the crystals are surrounded by vapour containing atoms of the desired dopant. These atoms enter the crystal by substi­tuting for the semiconductor atoms at regular sites in the crystal lattice and moving into the interior of the crystal by jumping from one site to an adjacent vacancy.

Silicon crystals may be doped with different elements. Suppose silicon is doped with boron. Each atom inserted in the silicon lattice creates a deficiency of one electron. This state is called a hole. A ho­le can become mobile in response to an applied voltage. It is not a real particle but merely the absence of an electron in a pure lattice of silicon atoms. Nevertheless the hole has a positive electric charge and can carry electric current. The hole moves through the lattice in the same way that the bubble moves through a liquid medium. An adjacent atom transfers an electron to the impurity atom "filling" the hole there but creating a new one in its own cloud of electrons. This process is then repeated, so that the hole is passed along from atom to atom.

Silicon doped with phosphorus or another pentavalent element is called an n-type semiconductor. Doping with boron or another trivalent element gives rise to p-type semiconductor.

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