The procedure of doping tiny amounts of the metals chromium (Cr), neodymium (Nd), erbium (Er), thulium (Tm), ytterbium (Yb), and a few others, into transparent crystals, ceramics, or glasses is used to produce the active medium for solid-state lasers.
In addition, the rare-earth element erbium can readily be used as the dopant rather than neodymium, giving a different wavelength of its output.
In many optically-transparent hosts, such active centers may keep their excitation for a time on the order of milliseconds, and relax with stimulated emission, providing the laser action.
Usually the relative atomic percent is assumed in the calculations, taking into account that the dopant ion can substitute in only part of a site in a crystalline lattice.
[citation needed] This results in a material with predominantly negative (n-type) or positive (p-type) charge carriers depending on the dopant variety.
Dopants are introduced into semiconductors in a variety of techniques: solid sources, gases, spin on liquid, and ion implanting.