Enhanced f_T and linearity performance of InGaP/GaAs HBTs using a non-uniform doping collector

In this paper, the ac and power linearity characteristics of InGaP/GaAs heterojunction bipolar transistors (HBTs) with collector doping design have been presented. The collector design is to employ a thin high-doping layer inside the low-doping collector. The thickness, doping and location of the inserted layer are systematically studied with consideration of breakdown characteristics and the Kirk effect. The Kirk effect is relieved by redistributed electric fields in the non-uniform doping collector to extend the operational current and thus to increase the cutoff frequency (f_T) and output power (P _out). The collector current at the onset of the Kirk effect is extended from 9.6 to 17.3 mA and f_T is increased from 36 to 48 GHz for a device size of 4 × 12 × 2 νm². The corresponding improvement in the saturation output power is 2.2 dB measured at 1.8 GHz. In addition, the impact of the collector doping profile on the base-collector capacitance (C_BC) and thus the linearity is simulated, discussed and measured. For an HBT with a thin high-doping layer being inserted 4000 from the base-collector junction, the experimental result on output third-order intercept point (OIP3) demonstrates the significant improvement by as large as 10 dB at a frequency of 1.8 GHz.