Impact of Al composition on RF noise figure of AlGaAs/GaAsheterojunction bipolar transistors

The influence of Al content on the RF noise characteristics of Al _xGa_1-xAs/GaAs heterojunction bipolar transistors (HBT's) is presented. It is shown that the minimum noise figure (F_min) at 2 GHz is reduced by increasing the Al mole fraction (x). This observed improvement in noise figure is directly correlated to the differences in dc current gain. The lowest measured F_min(2 GHz) of HBT's with emitter dimensions 2×(3.5×30) μm², were 1.3, 1.61, and 2.1 dB for x=0.35, 0.30, and 0.25 devices, respectively at I_c=3 mA. The measured results were found to agree well with calculated values over a wide range of collector currents