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 InxGa1-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 InxGa1-xAs base (x: 0 → 0.1 and x: 0 → 0.05) are measured to investigate optimum-grading profiles. The measured average current gains, ßs 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/Rsh 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, fT and maximum oscillation frequency, fmax. Compared to the control sample with the same base thickness, the base transit time τB of the graded sample is reduced by ∼15% to ∼20% by the induced built-in potential, resulting in an increase of fmax 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, fT and fmax are 16.3 and 33.8 GHz, respectively, under low-level collector current. These results demonstrate that InGaP/GaAs HBTs with InxGa1-xAs graded-base layers (x: 0 → 0.05) have the potential for high-speed analogue to digital converters. © 2005 IEEE.