In this study, AlGaN back barriers (B.B.) with different Al mole fractions and thicknesses were used in AlGaN/GaN high electron mobility transistors (HEMTs) to improve device performance. Relative to thickness, a proper Al mole fraction (Al0.08GaN) of the B.B. more strongly affected the device’ Ion/Ioff ratio. It exhibited a low leakage current and high Ion/Ioff ratio of approximately 106. Relative to B.B. mole fraction, B.B. thickness more greatly affected the devices’ horizontal breakdown voltage (760V) and LFN characteristics. Increasing the Al mole fraction and the thickness of the B.B. more strongly affected the dynamic RON. The current gain cut-off frequency (fT) and maximum stable gain cut-off frequency (fmax) were 5.2 GHz and 10.5 GHz, respectively, for the Al0.08GaN B.B. device.