Excimer laser liftoff of AlGaN/GaN HEMTs on thick AlN heat spreaders

We report on 193 nm excimer laser-based liftoff (LLO) of Al0.26Ga0.74N/GaN high electron mobility transistors (HEMTs) with thick (t > 10 μm) 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 K mm/W for as-fabricated devices on sapphire, which is lower than the value of ∼25–50 K mm/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 K mm/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/V s). This is the highest drain current density and mobility reported to-date for LLO AlGaN/GaN HEMTs.