Silicon doped with electron-rich impurity is an n-type semiconductor, where n stands for negative because of the excess of delocalized electrons. The increase in conductivity is because of negatively charged electrons. Hence, (i) is incorrect while (ii) and (iii) are correct. Electron vacancies are formed in p-type semiconductors where the vacancies are formed due to addition of electron-deficient impurities. Option (iv) is incorrect.