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/sub x/Ga/sub 1-x/As (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/sub x/Ga/sub 1-x/As base (x:0 /spl rarr/ ;0.1 and x:0 /spl rarr/ 0.05) are measured to investigate optimum-grading profiles. The measured average current gains, /spl beta/s 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/sub sh/ of /spl sim/200 /spl Omega//sq establishing a new benchmark for InGaP/GaAs HBTs. Furthermore, these compositionally graded-base HBTs show higher unity current/gain cutoff frequency, f/sub T/ and maximum oscillation frequency, f/sub max/. Compared to the control sample with the same base thickness, the base transit time /spl tau//sub B/ of the graded sample is reduced by /spl sim/15% to /spl sim/20% by the induced built-in potential, resulting in an increase of f/sub max/ from 16 to 18.5 GHz in a device with an emitter size of 10/spl times/10 /spl mu/m/sup 2/. Additionally, for the 5% graded-base sample, with a 5/spl times/5 /spl mu/m/sup 2/ emitter region, f/sub T/ and f/sub max/ are 16.3 and 33.8 GHz, respectively, under low-level collector current. These results demonstrate that InGaP/GaAs HBTs with In/sub x/Ga/sub 1-x/As graded-base layers (x:0 /spl rarr/ 0.05) have the potential for high-speed analogue to digital converters.