Disclosed is a light emitting device having a plurality of light emitting cells. The light emitting device comprises a thermally conductive substrate, such as a SiC substrate, having a thermal conductivity higher than that of a sapphire substrate. The plurality of light emitting cells are connected in series on the thermally conductive substrate. Meanwhile, a semi-insulating buffer layer is interposed between the thermally conductive substrate and the light emitting cells. For example, the semi-insulating buffer layer may be formed of AlN or semi-insulating GaN. Since the thermally conductive substrate having a thermal conductivity higher than that of a sapphire substrate is employed, heat-dissipating performance can be enhanced as compared with a conventional sapphire substrate, thereby increasing the maximum light output of a light emitting device that is driven under a high voltage AC power source. In addition, since the semi-insulating buffer layer is employed, it is possible to prevent an increase in a leakage current through the thermally conductive substrate and between the light emitting cells.