Such additions create Extrinsic Point Defects in the arrangement of atoms in silicon that impart special electrical properties to different parts of the silicon crystal. Point defects are intentionally added to semiconductors to control the type and concentration of charge carriers.
Given below is the illustration of such arrangement:-
As shown in the diagram above, the 5 valence electrons of P are sufficient to fill the local covalent bonds with one electron left over. This electron can only go into an excited state, and orbits about the P ion core somewhat as shown in the figure. It requires a relatively small energy increment to free this electron from the P core, in which case it can transport current by moving through the lattice.
The conductivity of Si rises dramatically if a small amount of P is added. Electron-excess solutes such as P in Si are called donors. Semiconductors whose electrical properties are controlled by electrically active solutes are called extrinsic semiconductors. Almost all of the semiconductors that are used in engineering devices are extrinsic.
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