Copper-Metalized GaAs pHEMT with Cu/Ge Ohmic Contacts

The fully Cu-metalized GaAs pHEMT using developed Cu/Ge-based ohmic contacts and T-gate Ti/Mo/Cu with length of the 150 nm has been successfully fabricated for the high-frequency applications. The fabricated Cu-metalized pHEMT has a maximum drain current of 360 mA/mm, an off-state gate-drain breakdown of 7 V, and a transconductance peak of 320 mS/mm at ds=3 V. The maximum stable gain value was about 15 dB at frequency 10 GHz. The current gain cutoff frequency of the copper-metalized device is about 60 GHz at ds=3 V, and maximum frequency of oscillations is beyond 100 GHz. This work investigated in detail the formation of Cu/Ge ohmic contacts to n-GaAs with an atomic hydrogen preannealing step. It was shown that after the first preliminary annealing is carried out in a flow of atomic hydrogen with a flow density of atoms of 1013/1016 at. cm2 s？1 a reduction in specific contact resistance by 2/2.5 times is observed. The reduction in specific contact resistance is apparently caused by the action of the hydrogen atoms which minimise the rate of the oxidizing reactions and activate solid phase reactions forming the ohmic contact during the thermal treatment process.