Dependence of DC current gain and f/sub max/ of AlInAs/GaInAs HBTs on base sheet resistance

To investigate the effect of base sheet resistance on DC current gain and maximum frequency of oscillation (f/sub max/) values, AlInAs/GaInAs HBT wafers were grown with base doping levels in the range of 2*10/sup 19/ to 1.6*10/sup 20/ cm/sup -3/. To confine the p-dopant within a nominal base thickness of 0.05 mu m, the molecular beam epitaxial (MBE) growth was performed over a substrate temperature range of 430 degrees C to 280 degrees C. The resulting base sheet resistances of 11 wafers were in the range of 900 down to 141 Omega /sq. DC current gains of greater than 20 were obtained with a base sheet resistance of 250 Omega /sq, and the current gain dropped to five for a sheet resistance of 141 Omega /sq. The f/sub max/ did not scale with the square root of the base resistances as predicted by the classical expression.<
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