||Fluorine plasma (F-plasma) treatment technique can effectively incorporate F-atoms into the AlGaN barrier, depleting the two-dimensional electron gases (2DEG) in AlGaN/GaN high electron mobility transistors (HEMT) and converting the device from depletion-mode to enhancement-mode. In this thesis, the effects of F-plasma treatment on the transport and optoelectronic properties of AlGaN/GaN heterostructures were studied. It was found that the F-related centers were deep electron traps, which dominated the 2DEG scattering mechanism in F-plasma treated AlGaN/GaN heterostructures. Gated Hall measurements revealed that both 2DEG density and mobility increased when positive gate voltage was applied. It was obtained that the photoionization energy of the F-related centers was about 1.85 eV, while the energy barrier for electron recapture by F-related centers was determined to be 0.624 eV. The configuration-coordinate diagram of F-related centers in AlGaN/GaN heterostructure was obtained. The effects of F-plasma treatment on the performance of AlGaN/GaN heterostructure based UV-photodetectors were also investigated. Transport behaviors of the Ni/Au-AlGaN/GaN Schottky diodes in bias range far beyond the turn-on voltage were studied for the first time. A second turn-on was observed in the current-voltage (I-V) characteristics. Electroluminescence (EL) corresponding to the bandedge emission from GaN layer was detected when applied bias was larger than the second turn-on voltage, indicating that the second turn-on in the I-V characteristics was due to the hole injection into GaN layer. A model based on the Fermi level depinning due to the interface states ionization by hot electrons was proposed to explain the observed second turn-on in transport behaviors and the corresponding electroluminescence characteristics. The effects of F-plasma treatment on the transport as well as the EL behaviors of the Ni/Au-AlGaN/GaN Schottky diode were also investigated. It was found that the F-related deep centers in AlGaN barrier could assist the hole injection processes through hopping conduction.