Optimization of the base electrode for InGaAs/InP DHBT structures with a buried emitter-base junction

초록

We have optimized the base electrode for InGaAs/InP based double heterojunction bipolar transistors with a buried emitter-base junction. For the buried emitter-base structure, the base metal is diffused through a thin graded quaternary region, which is doped lightly n-type, to make ohmic contact to the p(+)InGaAs base region. The metal diffusion depth must be controlled, or contact will also be made to the collector region. Several metal schemes were evaluated. An alloy of Pd/Pt/Au was the best choice for the base metal, since it had the lowest contact resistance and a sufficient diffusion depth after annealing. The Pd diffusion depth was easily controlled by limiting the thickness to 50 Angstrom, and using ample Pt, at least 350 Angstrom, as a barrier metal to the top layer of Au. Devices with a 500 Angstrom base region show no degradation in de characteristics after operation at an emitter current density of 90 kA/cm(2) and a collector bias, V-CE, of 2V at room temperature for over 500 h. Typical common emitter current gain was 120. An f(t) of 95 GHz and f(max), of 131 GHz were achieved for 2 x 4 mu m(2) emitter size devices.

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