High-performance InGaP/GaAs HBTs with compositionally graded bases grown by solid-source MBE

N-p-n InGaP/GaAs heterojunction bipolar transistors (HBTs) with compositionally graded In_xGa_1-xAs (Be doped) bases have been successfully grown by solid-source molecular beam Epitaxy (SSMBE) using a gallium phosphide (GaP) decomposition source. In this paper, the dc and RF characteristics of HBTs with different indium mole fractions in the graded In_xGa_1-xAs base (x:0 → ;0.1 and x:0 → 0.05) are measured to investigate optimum-grading profiles. The measured average current gains, βs of a control sample, a 10% graded-base sample and a 5% graded-base sample, are 162, 397 and 362, respectively. To our knowledge, these current gains are the highest values ever reported in compositionally graded-base InGaP/GaAs HBTs with a base sheet resistance R_sh of ∼200 Ω/sq establishing a new benchmark for InGaP/GaAs HBTs. Furthermore, these compositionally graded-base HBTs show higher unity current/gain cutoff frequency, f_T and maximum oscillation frequency, f_max. Compared to the control sample with the same base thickness, the base transit time τ_B of the graded sample is reduced by ∼15% to ∼20% by the induced built-in potential, resulting in an increase of f_max from 16 to 18.5 GHz in a device with an emitter size of 10×10 μm². Additionally, for the 5% graded-base sample, with a 5×5 μm² emitter region, f_T and f_max are 16.3 and 33.8 GHz, respectively, under low-level collector current. These results demonstrate that InGaP/GaAs HBTs with In_xGa_1-xAs graded-base layers (x:0 → 0.05) have the potential for high-speed analogue to digital converters.