ABSTRACT
This chapter deals with the collector current which peculiarities can arise due to heavy-doping effects in the layers and the double heterojunction at both base-emitter and base-collector diode. It then reviews theories describing DC- and AC-behaviour of the heterojunction bipolar transistor. Due to channeling effects during the phosphorous-implant of the emitter, the latter found a strong reduction of the collector current due to a parasitic barrier at the base edge of the base-collector junction. King mentioned in his pioneering paper of 1989 that, based on the temperature dependence of the ratio of the base current in the HBT to the collector current of the reference homojunction transistor, an important portion of the base current was caused by neutral base recombination. Surface passivation and more specifically perimeter passivation is not a difficult problem in classical Si-technology, since the thermal oxide of Si has excellent passivation properties.
