Device characteristics of InP/In 0.34 Al 0.66 As 0.85 Sb 0.15 /GaAs metamorphic high electron mobility transistors with a dual-compositiongraded In x Ga 1Àx As 1Ày Sb y (x ¼ 0.53 ! 0.63 ! 0.53, y ¼ 0.01 ! 0.02 ! 0.01) channel are comprehensively investigated by forming different Schottky-barrier gate structures of Pt/Au-In 0.34 Al 0.66 As 0.85 Sb 0.15 , Ni/Au-In 0.34 Al 0.66 As 0.85 Sb 0.15 , or Pt/Au-InP, respectively. Superior device gain, current drive, power, noise, and high-frequency characteristics have been achieved due to relieved kink effects and decreased gate leakages, which are caused by the decreased electron concentration in the channel and suppressed thermionic emission by forming high Schottky-barrier gate structures. High-temperature device characteristics at 300-450 K are also studied. Excellent thermal threshold stability is achieved due to the combinational effects of both barrier-lowering and carrier-carrier scattering mechanisms at high temperatures.