Substitutional impurities replace the host atoms, e.g. B, As, P, are substitutional impurities in Si, and replace the Si atoms. Substitutional impurities act as dopants, i.e. donate electrons or holes to the conduction or valence bands. Not all impurities can act as dopants; the requirements are that the impurities have to be of nearly the same size as the host atoms, and should have one more or one less valence. Impurities much  smaller than the host atoms will occupy interstitial position in the host lattice; these may cause impurity levels in the band gap and act as recombination centres, but, normally cannot be effective and stable dopants. 

Doping by interstitial impurities have been tried in non-crystalline and 2D/1D semiconductors. An example is Na-doping of amorphous SiN. The efficacy of such doping in production is unknown. 

Substitutional impurities replace the host atoms, e.g. B, As, P, are substitutional impurities in Si, and replace the Si atoms. Substitutional impurities act as dopants, i.e. donate electrons or holes to the conduction or valence bands. Not all impurities can act as dopants; the requirements are that the impurities have to be of nearly the same size as the host atoms, and should have one more or one less valence. Impurities much  smaller than the host atoms will occupy interstitial position in the host lattice; these may cause impurity levels in the band gap and act as recombination centres, but, normally cannot be effective and stable dopants. 

Doping by interstitial impurities have been tried in non-crystalline and 2D/1D semiconductors. An example is Na-doping of amorphous SiN. The efficacy of such doping in production is unknown.