The number of charge carriers, either an excess of or a deficiency of electrons (causing holes in a material), can be altered by doping, or adding very small quantities of impurities to a semiconductor material.

electrons              

Explanation:

             Semiconductor is a material whose electrical conductivity lies between the the values of a conductor and an insulator.

             Doping is the process in which impurities are added to the semi conductors to change the properties of the semi conductor. Excess of electrons in the semi conductor increases the electron carrier concentration making it N type.

            In the semi conductors, the deficiency of the electrons or the excess of the electrons can be changed by doping to alter the properties.

Hence the correct answer is -- electrons.