IMPLANT ISOLATION OF GAAS-ALGAAS HETEROJUNCTION BIPOLAR-TRANSISTOR STRUCTURES

The formation of high-resistivity (>107Ω/□) regions in GaAs-AlGaAs heterojunction bipolar transistor (HBT) structures by oxygen and hydrogen ion implantation has been investigated as a function of ion dose and subsequent annealing temperature (400-700 °C). Isolation leakage currents as low as 8 μA mm-1 at 6 V can be achieved between 100-μm-wide ohmic contacts separated by a 16 μm spacing. The isolation of these 1.8-μm-thick heterojunctions requires up to six different energy oxygen implants (40-400 keV) and three different energy proton implants (100-200 keV) with doses in the mid 1012 cm-2 range for O+ and 5×1014 cm-2 for H+ ions. Similar results can be achieved by substituting a MeV energy oxygen implant for the proton implants. The optimum post-implant annealing temperature depends on the ion dose but is in the range 500-600 °C. The evolution of the sheet resistance of the implanted GaAs-AlGaAs material with annealing is consistent with a reduction in tunneling probabilities of trapped carriers between deep level states for temperatures up to ∼600 °C, followed by significant annealing of these deep levels. Small geometry (2×9 μm2) HBTs exhibiting current gain of 44 and cutoff frequency fT as high as 45 GHz are demonstrated using implant isolation.