p-type semiconductors
If an impurity element with three outer shell electrons, such as Indium, is added to silicon in similar small quantities, the impurity atoms will fit into the crystal structure but there will be one electron missing.
This doping allows more spaces for electrons above the valence band. This increases the conductivity of the material.
This impurity causes the material to conduct and is called p-type silicon. p-type refers to the slight positive charge of the gap caused by the missing electron.