Doping concentration and structural dependences of the thermal stability of the 2DEG in GaN-based high-electron-mobility transistor structures

The dependences of the stability of the two-dimensional electron gas (2DEG) on the doping concentration and the heterostructure in GaN-based high-electron-mobility transistors were investigated, in an annealing experiment at 500 degrees C under N-2 ambient. Both the 2DEG density and mobility decreased after annealing due to the strain relaxation in an AlGaN layer. Moreover, a decrease in 2DEG conductivity was found in samples with different doping concentrations in a Si-doped AlGaN layer. A significant decrease in 2DEG conductivity occurred in a sample with a thicker AlGaN layer due to a larger strain relaxation. We found that a GaN cap layer could enhance the stability of the 2DEG up to 75 h of aging. Relaxation suppression by the GaN cap inducing dislocation pinning is proposed to interpret this effect.