We have investigated the effect of the growth temperature and magnesium flux on Mg incorporation in ammonia-MBE grown GaN epilayers. Secondary ion mass spectroscopy revealed that the incorporation of Mg is more sensitive to the growth temperature than to Mg flux. Simultaneously, the available amount of Mg at the substrate surface has to be accurately balanced in order to achieve the optimum electrical (p similar to 3 X 10(17) cm(-3), mu similar to 12cm(2)/V s, rho similar to 2 ohm cm) and structural properties [omega-scan FWHM((0002)) similar to 550 arcsec]. The surface morphology of the Mg-doped GaN epilayers, using various growth temperatures and Mg fluxes, has been studied by atomic force microscopy showing a considerable change in the GaN average grain size. The optimum growth window for achieving high quality, p-type conductivity in GaN using ammonia-MBE will be discussed. As an application, n-p-n bipolar junction transistors were grown and fabricated. A current gain of 10 with V-BC =0 V was achieved using these devices. (c) 2005 American Vacuum Society.