High-fmax collector-up AlGaAs/GaAs heterojunction bipolar transistors with heavily carbon-doped base fabricated by oxygen-ion implantation

Summary form only given. The authors demonstrate the superiority of the first collector-up (C-up) HBTs (heterojunction bipolar transistors) with a heavily carbon (C)-doped base layer fabricated by oxygen-ion (O⁺) implantation to define the intrinsic emitter/base junction and zinc (Zn) diffusion to reduce base contact resistance. A maximum oscillation frequency f_max of 126 GHz was obtained; this exceeds that of the C-up HBT fabricated by proton (H⁺) implantation. A key process in fabrication of C-up structures is the formation of an external emitter/base junction to effectively suppress the parasitic base leakage current. The formation of a high-resistivity AlGaAs `barrier´ between the external base and emitter buffer layers is effective for this purpose. For microwave transistors with collector dimension of 2 μm×10 μm, a cutoff frequency f_T of 56 GHz and f_max of 125 GHz were attained