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Substitutional doping is a process used to alter the electrical properties of semiconductor materials. It involves intentionally introducing impurity atoms, known as dopants, into the crystal lattice of a semiconductor. These dopant atoms replace some of the original semiconductor atoms in their lattice positions. The key aspect is that the dopant atom occupies a substitutional site, meaning it takes the place of a host atom.

The type of dopant chosen determines whether the semiconductor becomes n-type or p-type. In n-type doping, dopants with more valence electrons than the semiconductor atoms are used. For example, in silicon (which has four valence electrons), Group V elements like phosphorus or arsenic, with five valence electrons, are used. When a phosphorus atom substitutes a silicon atom, it brings an extra electron that is loosely bound and can move freely, increasing the conductivity by providing extra charge carriers (electrons).

In p-type doping, dopants with fewer valence electrons than the semiconductor atoms are introduced. Group III elements like boron or aluminum, with three valence electrons, are commonly used with silicon. When a boron atom substitutes a silicon atom, it creates a "hole," which is essentially a missing electron in the covalent bond. These holes can act as positive charge carriers, as an electron from a neighboring atom can jump into the hole, effectively moving the hole. This process also increases the conductivity of the semiconductor. The precise control over the concentration and type of dopants is crucial for manufacturing various semiconductor devices like transistors and diodes.