Field-effect transistors (FETs) were grown on both GaN and AlGaN buffers. X-ray reciprocal space mapping and ω-2θ scans showed that the AlGaN barriers grown on these two buffers had different Al compositions and growth rates, which was attributed to the compositional pulling effect. AlGaN/GaN/AlGaN double heterojunction FETs exhibited lower output conductance and better pinch-off due to the improved electron confinement resulting from the increase in the effective back-side barrier height. Thus, this device is promising for highly scaled transistors. This device also demonstrated a state-of-the-art power added efficiency of 53.5% and an associated power gain of 9.1 dB at a drain bias of 20 V at 30 GHz.