High thermal stability AlGaAs/InGaAs enhancement-mode pHEMT using palladium-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 palladium (Pd)-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 PtAs₂ 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 PtAs₂ interlayer caused a serious gate leakage current and unstable Schottky barrier height. This study presents the Pd-GaAs Schottky contact because Pd, an inert material with high work function of 5.12 eV. Stable Pd inhibited the less diffusion at high temperatures and simultaneously suppressed device flicker noise. The V_th of Pd/Ti/Au Schottky gate E-pHEMT was 0.183 V and this value shifted to 0.296 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 −5.76 × 10⁻² GHz/°C for a Pd/Ti/Au-gate E-pHEMT; it was −9.17 × 10⁻² GHz/°C for a Pt/Ti/Au-gate E-pHEMT. The slight variation in the dc and radio-frequency characteristics of the Pd/Ti/Au-gate E-pHEMT at temperatures from 0 to 100 °C revealed that the Pd-GaAs interface has great potential for high power transistors.