GaAs/AlGaAs heterojunction bipolar transistors on Si substrates

We have investigated the properties of GaAs/AlGaAs heterojunction bipolar transistors grown on Si substrates. Common emitter current gains of have been obtained for a structure with a m thick base and is not mainly limited by recombination in the neutral base region but by other base current components. Current densities of 100kA/cm²before the onset of current gain fall-off were obtained in these devices. The base resistance is reduced through optimization of p-type ohmic contacts to GaAs and by the use of a new self aligned process which can produce gaps between base contact and active base of less than 0.25µm. Specific contact resistances of as low as 1.3×10^-7Ω-cm²have been obtained to p=5×10¹⁸cm^-3GaAs. Finally, preliminary microwave measurements have been made on devices with relatively large geometries (10×40µm²emitter area) and fmaxvalues of 7 GHz have been obtained with fTvalues of 11 GHz at 15kA/cm².