Thermal stability of gold free fully Cu/Ge metalized GaAs pHEMT

The thermal stability performance of the fully Cu/Ge metalized GaAs pHEMT and pHEMT with Cu/Ge ohmic contacts and Ti/Mo/Cu based T-gate were investigated. The CuGe compound was formed by the atomic hydrogen treatment of Cu/Ge/GaAs two layer system. It was found that such processing in the atomic hydrogen flow with density of 10¹⁵ at. cm² s^-1 at room temperature for 5 min leads to the solid state interdiffusion of Cu and Ge thin films and polycrystalline CuGe compound formation with the vertically oriented grains. The fully Cu/Ge metalized pHEMT with 170 nm T-gate had a saturation drain current of I_dss = 660 mA/mm, off-state gate-drain breakdown voltage of BV_gd = 7 V and a transconductance peak S_m = 380 mS/mm at F_ds= 3 V. The maximum stable gain of the fully Cu/Ge based pHEMT was about 17.5 dB at the frequency of 10 GHz and the current gain cut-off frequency was about 80 GHz at U_ds = 3 V and I_ds = 1/4I_dss. The transistor with Cu/Ge ohmic contacts and Ti/Mo/Cu based 170 nm T-gate had worse DC and RF parameters. The thermal stability test of both GaAs pHEMTs was performed in a nitrogen environment at the temperature of T = 250 °C for a period of 5-120 min. The gold free fully Cu/Ge-metalized pHEMT fabricated with an atomic hydrogen treatment demonstrated more superior thermal stability performance than the pHEMT with Cu/Ge ohmic contacts and Ti/Mo/Cu T-gate fabricated without atomic hydrogen treatment. The experimental results allow considering of CuGe compounds as a perspective gold replacement in the GaAs MMIC production.