Impact of the insertion of AlGaN back barrier on crystal quality, $\boldsymbol{N}_{\mathbf{S}}$, and mobility of GaN-channel HEMTs with high-Al-content AlGaN top barrier grown on high-resistivity Si substrate

We investigate the impact of inserting an AlGaN back barrier on the crystal quality and electrical characteristics of GaN-channel HEMTs with a thin AlGaN top barrier grown on high-resistivity Si substrate. AFM and XRD measurements revealed that the AlGaN back barrier does not significantly affect the crystal quality of the HEMT layers. However, the channel electron mobility decreases as the thickness of the GaN channel (an AlGaN back barrier) decreases (increases), which is probably due to interface roughness and alloy scattering. For further enhancement of the high-frequency characteristics of scaled GaN-channel HEMTs on Si, it is important to consider these influences of AlGaN back barrier insertion in designing HEMT layer structures.