Improvement of CBE grown InGaAs/InP HBT´s using a carbon doped and compositionally graded base

InGaAs/lnP heterojunction bipolar transistors (HBT) with a carbon doped base are shown to have lower gain than those with Be doped base, partly because of lower electron lifetime. In order to keep advantage of the low diffusivity of carbon dopant, a composition graduality is introduced in the base, which significantly improves the current gain of HBT´s. This graduality also leads to some advantageous side effects such as higher doping levels, low base sheet resistance and constant gain over a four-decade current range, without degrading the lifetime of the device. Such structures were processed as single HBT devices with high frequency performances (f_t=168 GHz), digital circuits (2:1 MUX) operating at 46 Gbit/s and phototransistors with 35 dB optical gain and 62 GHz optical cut-off frequency