In this study, Al0.3Ga0.7N/GaN high electron mobility transistor (HEMT) structure is investigated grown over c- oriented sapphire substrate by using Metal Organic Chemical Vapor Deposition (MOCVD) method. Structural, optical, morphological and electrical characteristics of this structure are determined by X-ray diffraction (XRD), Photoluminescence (PL), Ultraviolet (UV-Vis.), Atomic Force Microscopy (AFM) and Hall-Resistivity measurements. By using XRD method, 20, Full Width at Half Maximun (FWHM), lattice parameters, crystallite size, strain, stress and dislocation values are calculated on symmetric and asymmetric planes. Direct band gap of GaN is determined by PL measurements as 3.24 eV. It is seen that conduction of AlGaN layer starts at 360 nm in UV-Vis. In Hall-Resistivity measurements, it is noticed that carrier density of HEMT structure is not effected by temperature and mobility value is high. Carrier density and mobility values are determined as 5.82x10(15) 1/cm(3) and 1198 cm(2) N.s at room temperature, respectively. At the lowest temperature point (25 K) they are calculated as 5.19x10(15) 1/cm(3) and 6579 cm(2)/Vs, respectively.