Impact of Defects on the Low-Field Electron Mobility in GaN-on-Si HEMTs

In this work, we investigate the field and temperature dependence of the electron mobility in aluminum-gallium-nitride/gallium-nitride (AlGaN/GaN) high electron mobility transistors (HEMTs) realized on GaN-on-silicon (Si) substrates. For this purpose we employ an extraction method to eliminate parasitic and fringing effects. Our results show that especially at low fields the temperature dependence of the mobility, and consequently that of the specific on-resistance, is strongly affected by stress-induced charged dislocation scattering. For explaining the mobility behaviour at low fields, the subthreshold operation regime of the HEMTs has also been analyzed. An interface trap density at the AlGaN/GaN interface (N_it) of ~ 6.9 x 10¹⁰ cm⁻² has been extracted independent of the temperature which is close to the extracted dislocation density from mobility measurements. This suggests that the relatively high dislocation density in the GaN layer, which is a consequence of the still imperfect buffer layer in the GaN-on-Si substrate that is used to accommodate the strain difference, has an impact on N_it, thus subthreshold swing, in addition to the mobility reduction.