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Carbon and Indium Codoping in GaAs for Reliable AlGaAs/GaAs Heterojunction Bipolar Transistors

This paper reports a systematic study on C and In codoping in GaAs to form an AlGaAs/GaAs heterojunction bipolar transistor (HBT) structure with minimized lattice strain. Carbon-doped layers were grown by metalorganic chemical vapor deposition. The carbon concentration in the epitaxial layer can be controlled by adjusting the CCl4 flow rate although it strongly depends on growth parameters such as substrate temperature and V/III ratio. We found the CCl4 supply disturbs In incorporation into the epitaxial layer, while the In concentration changes in proportion to trimethyl indium flow rate. The lattice mismatch of the In/C-doped layer can be explained by the strained-layer model based on Vegard's law. Indium codoping does not deteriorate the electrical characteristics of the C-doped layer or the current gains of the C-doped-base HBTs. The current gains of the In/C-doped-base HBTs were stable for more than 1×104 h at a junction temperature of 250° C.