|M.Sc Thesis||Department of Electrical Engineering|
|Supervisor:||Prof. Ritter Dan|
Heterojunction bipolar transistors (HBTs) are bipolar transistors in which the bandgap of the emitter layer is larger than that of the base layer. This reduces back injection of holes from the base to the emitter. As a result, high values of current gain can be obtained for any base doping level, by contrast to homojunction transistors, in which the doping level in the base must be lower than in the emitter. Indium phosphide base HBTs are faster than silicon based HBTs due to their higher electron velocity, and are therefore leading competitors for analog circuits operating at frequencies of 40 GHz and higher. With InP emitter, GaInAs base, and GaInAs collector, npn single heterojunction bipolar transistors (SHBTs) demonstrate excellent potential for high-frequency applications. However, low breakdown voltage and low output resistance limit SHBTs performance. The collector to emitter breakdown in SHBTs is caused by impact ionization at high electric field in the collector region. The relatively low breakdown voltage is a result of the high impact ionization rate in the narrow gap GaInAs collector. To increase the breakdown voltage, a larger bandgap collector layer is required. In the material system studied in this work, the best option is to replace the GaInAs collector by an InP layer, obtaining a double heterojunction bipolar transistor (DHBT). The larger bandgap reduces impact ionization rate and thus enhances the breakdown voltage and the power handling capabilities of the device for high power microwave applications.
The introduction of a wide gap material complicates the collector design, because of current blocking resulting from the conduction band discontinuity between the narrow bandgap base and wide bandgap collector. To overcome this problem, several solutions have been suggested. The most elegant method is using a GaAsSb layer as the base. This eliminated the potential barrier because the conduction band of GaAsSb lays 0.10-0.15 eV above the conduction band of the InP collector. Other methods use various doping and compositional grading procedures to reduce the barrier height between the GaInAs base and InP collector. In this research, a continuous compositional grading method was investigated.