We have investigated the growth of Ga-polarity (0001) and N-polarity (000 (1) over bar) Si-doped GaN on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. For a given Si effusion cell temperature and under similar growth conditions, the Hall carrier concentration was found to be consistently higher for samples with N polarity. Secondary ion mass spectrometry measurements show no significant difference for the incorporation of Si into GaN of either polarity. However, the incorporation of background impurities, C and O, was found to be higher for the (000 (1) over bar) GaN samples.