We have grown undoped, Si- and Mg-doped GaN epilayers using metalorganic chemical vapor deposition. The grown samples have electron Hall mobilities (carrier concentrations)of 798 cm(2)/V s (7 x 10(16) cm(-3)) for undoped GaN and 287 cm(2)/V s (2.2 x 10(18) cm(-3)) for Si-doped GaN. Mg-doped GaN shows a high hole concentration of 8 x 10(17) cm(-3) and a low resistivity of 0.8 Ohm cm. When compared with undoped GaN, Si and Mg dopings increase the threading dislocation density in GaN films by one order and two orders, respectively. Besides, it was observed that the Mg doping causes an additional biaxial compressive stress of 0.095 GPa compared with both undoped and Si-doped GaN layers, which is due to the incorporation of large amount of Mg atoms (4-5 x 10(19) cm-3).