We report on the thermal study of AlGaN/GaN high electron mobility transistors (HEMTs) after substrate transfer from the original sapphire substrate onto Si using laser lift-off and metal bonding. Raman thermography determined a thermal resistance of about 8 degrees C/(W/mm) for these devices, which is similar to that for devices grown directly on Si substrates, and significantly improved over devices grown on sapphire substrates. The effective thermal boundary resistance between GaN and the new Si substrate was determined to be (1.0-2.5) x 10(-8) m(2) K/W at 145 degrees C, comparable to that in the GaN/Si system. These results illustrate that bonding layers between AlGaN/GaN HEMTs and the substrate can be as good as as-grown GaN/Si interfacial layers. This approach points to the potential benefit to devices transferred onto even higher thermal conductivity substrates such as diamond. (C) 2009 Elsevier Ltd. All rights reserved.