High current density emitter-down InGaAlAs heterojunction bipolar transistors

The authors discuss the use of deep O/sup +/ ion implantation, in conjunction with multiple self-aligned device geometries, to more efficiently block parasitic injection through the extrinsic base diode and allow much higher current density operation of inverted HBTs (heterojunction bipolar transistors) than would be achieved by standard techniques. In GaAlAs on InP, inverted HBTs were fabricated using several self-aligned as well as standard processes, with the final goal a novel four-mask, triple self-aligned process using semi-insulating substrates. The collector current density for maximum current gain (I/sub max/) has been measured for inverted HBTs of varying emitter stripe width and exhibits a clear inverse proportionality to the stripe width. While maximum current densities of approximately=150 A/cm/sup 2/ were measured for devices of 60- mu m stripe width, values in excess of 1000 A/cm/sup 2/ were measured for a device with a 6- mu m stripe width.