InP double heterostructure bipolar transistors: comparison of doped and undoped InGaAsP composite collectors

In composite-collector double heterostructure bipolar transistors, the wide-gap layer in the collector is offset from the base by a spacer layer. This offset is chosen to maintain the lower turn-on voltages of the double heterostructure design and to improve the gain, but without introducing too much impact ionization in the narrow-gap spacer. Another design option is to vary the bandgap of the wide-gap layer; in InP/InGaAs devices, this can be done with the quaternary InGaAsP. Here we compare the dc performance of such quaternary devices with different doping levels in the collector. We find that at low collector doping the composite-collector structure behaves more like an ordinary double HBT than at higher doping levels