Integration of diamond and AlGaN/GaN highelectron mobility transistors (HEMTs) terminated with an in situ grown SiN x interface layer via metal organic chemical vapor deposition is investigated. The effect of diamond growth on the structure and interface properties of the HEMT is studied using high-resolution X-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, and scanning transmission electron microscopy (STEM). No structural or physical damage is observed to the HEMT device layers as a result of the hot filament chemical vapor deposition diamond fabrication process. The TEM cross section confirms the smooth and abrupt interface of in situ SiN x /AlGaN/GaN before and after the diamond growth, with no detectable carbon diffusion into the GaN buffer layer. However, selective degradation of the in situ SiN x dielectric adhesion layer was observed at the SiN x /diamond interface. Using time domain thermoreflectance (TDTR), the effective isotropic thermal conductivity of the diamond was determined to be 176 −35 +40 W/m•K. The effective thermal boundary resistance of the diamond/ GaN interface (including the SiN x and additional layers) was 52.8 −3.2 +5.1 m 2 •K/GW.