Excimer laser liftoff of AlGaN/GaN HEMTs on thick AlN heat spreaders
Md. Didarul Alam, Mikhail Gaevski, Mohi Uddin Jewel, Shahab Mollah, Abdullah Mamun, Kamal Hussain, Richard Floyd, G. Simin, M. V. S. Chandrashekhar, Asif Khan
Abstract
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.