The optical properties of GaN-based materials are essential to the optoelectronic devices such as light emitting diodes and laser diodes in visible and ultraviolet regions. In this work unintentionally doped GaNand Mg-doped p-GaN were grown on sapphire substrates by plasma-assisted molecular beam epitaxy (PA-MBE ). The photoluminescence (PL) characteristics were studied by means of RT and low-temperature PLspectra with λ

ex

=325 nm. The electrical properties of GaN:Mg were tested by Hall measurement. We especially studied the yellow luminescence (YL) observed in PLspectra in view of the difference between these two types of GaN. In as much as the exciton peaks of Ga-rich GaN exhibit a narrower FWHM than N-rich GaN

it is indicated that material quality of the former is better than the latter. The YLin Ga-rich GaNis related to point defects which is correlative with nitrogen vacancies. Mg doping concentration has a great effect on PLcharacteristics of p-GaN. The p-type samples show three shapes of PLlines with Mg-doping temperature varying. Hall measurements show that hole concentration of p-type GaNis increased up to a upper limit and then decreased along with the increasing of Mg doping temperature. In combination with the results of hall test

we proposed that the YLin highly Mg-doped GaNcould be attributed to self-compensation effect and the decrease of crystal quality caused by highly Mg doping. Through the study

we found that the YLoriginates from different sources by comparing the undoped GaN and Mg-doped GaN

and it also correlates with the material qualities commonly.