Abstract: The present disclosure relates to use of 193-nm excimer laser-based lift-off (LLO) of Al0.26Ga0.74N/GaN High-electron mobility transistors (HEMTs) with thick (t>10 ?m) AlN heat spreading buffer layers grown over sapphire substrates. The use of the thick AlN heat spreading layer resulted in thermal resistance (Rth) of 16 Kmm/W for as-fabricated devices on sapphire, which is lower than the value of ?25-50 Kmm/W for standard HEMT structures on sapphire without the heat-spreaders. Soldering the LLO devices onto a copper heat sink led to a further reduction of Rth to 8 Kmm/W, a value comparable to published measurements on bulk SiC substrates. The reduction in Rth by LLO and bonding to copper led to significantly reduced self-heating and drain current droop. A drain current density as high as 0.9 A/mm was observed despite a marginal reduction of the carrier mobility (?1800 to ?1500 cm2/Vs). This is the highest drain current density and mobility reported to-date for LLO AlGaN/GaN HEMTs.