AlN Passivation Over AlGaN/GaN HFETs for Surface Heat Spreading

Reduction of thermal resistance in AlGaN/GaN heterojunction field-effect transistors (HFETs) is critical for further increase in their output power to be handled in these promising material systems. In this paper, we present a new technique to reduce it using polycrystalline AlN passivation deposited by dc sputtering as a surface heat spreader over AlGaN/GaN HFETs. The AlN is deposited by dc sputtering, and it is well oriented to the c-axis. The measured thermal resistances of AlGaN/GaN HFETs with AlN passivation are lower than those with SiN passivation, which well agree with the results of the thermal simulation. The most notable change of the characteristics by AlN passivation is that the drain current increases by 30%, and the on-state resistance is reduced by 66% by the passivation; moreover, the current collapse suppresses successfully. As a result, the radio frequency output power is higher than that with conventional SiN passivation and is increased by increasing the thickness of AlN passivation. Thus, reduction of the thermal resistance and higher output power by AlN passivation is experimentally achieved.