Improvement of High-Frequency Characteristics of InGaAsSb-Base Double Heterojunction Bipolar Transistors by Inserting a Highly Doped GaAsSb Base Contact Layer

This letter presents InP/GaAsSb/InGaAsSb/InP double heterojunction bipolar transistors (DHBTs) with a highly doped base contact layer. In order to reduce the base contact resistivity, a 3-nm-thick highly doped GaAsSb contact layer is inserted between the InP emitter and 17-nm-thick composition- and doping-graded InGaAsSb base. Fabricated DHBTs with a 0.25-μm emitter show a current gain of 32 and a high open-base breakdown voltage BVCEO of 5.2 V. The DHBTs also exhibit fT/f_max = 513/637 GHz at a collector current density of 9.5 mA/μm² and VCE = 1 V. The f_max is higher by 124 GHz than that for InP/InGaAsSb DHBTs without the GaAsSb contact layer. These results indicate that the use of the GaAsSb/InGaAsSb base structure is very effective in improving f_max.