High thermal stability AlGaAs/InGaAs enhancement-mode pHEMT using iridium buried-gate technology

The dc, flicker noise, power, and temperature dependence of AlGaAs/InGaAs enhancement-mode pseudomorphic high electron mobility transistors (E-pHEMTs) were investigated using iridium (Ir) buried-gate technology. Although the conventional platinum (Pt)-buried gate has a high metal work function, which is beneficial for increasing the Schottky barrier height of the E-pHEMT, the high rate of intermixing of the Pt-GaAs interface owing to the effect of the continuous production of Pt As₂ on the device influenced the threshold voltage (V_th) and transconductance (g_m) at high temperatures or over the long-term operation. Variations in these parameters make Pt-gate E-pHEMT-related circuits impractical. Furthermore, a Pt As ₂ interlayer produced a serious gate leakage current and unstable Schottky barrier height. This study presents the Ir-GaAs Schottky contact because Ir, buried in GaAs, absorbs surface oxygen atoms, forming Ir O ₂ after annealing at 200°C. Thermally stable Ir O₂ inhibited the overdiffusion of Ir at high temperatures and simultaneously suppressed device flicker noise. The V_th of Ir/Ti/Au Schottky gate E-pHEMT was 0.238 V and this value shifted to 0.244 V after annealing at 200°C. However, the V_th shifted from 0.084 to 0.231 V after annealing of the Pt/Ti/Au Schottky gate E-pHEMT because the Pt sunk into a deeper channel. The slope of the curve of power gain cutoff frequency (f _max) as a function of temperature was -1.5× 10^-2 GHz/°C for an Ir/Ti/Au-gate E-pHEMT; it was -6.9× 10^-2 GHz/°C for a Pt/Ti/Au-gate E-pHEMT. The slight variation in the dc and radio-frequency characteristics of the Ir/Ti/Au-gate E-pHEMT at temperatures from 0 to 150°C revealed that the Ir-GaAs interface has great potential for high power transistors.