DEPENDENCE OF DC CURRENT GAIN AND FMAX 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(max)) values, AlInAs/GaInAs HBT wafers were grown with base Bc doping levels in the range of 2 x 10(19) to 1.6 x 10(20) cm-3. To confine the p-dopant within a nominal base thickness of 0.05 mum, the MBE growth was performed over a substrate temperature range of 430 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 while the current gain dropped to 5 for a sheet resistance of 141 OMEGA/sq. The f(max) did not scale with the square root of the base resistance as predicted by the classical expression.