15 The concentration of hole-electron pairs in pure silicon at T-300 K is 7x 10 per cubic metre. Antimony is doped into silicon in a proportion of 1 atom in 10 atoms. Assuming that half of the impurity atoms contribute electrons in the conduction band, calculate the factor by which the number of charge carriers increases due to doping. The number of silicon atoms per cubic metre is 5 x 1028.