An analytical model for the heterostructure-emitter bipolar transistors

Heterojunction bipolar transistors (KBTs) have been the subject of much attention for high gain and high speed operation, and high fan-out capabifity. However, from the device operation point of view, an undesirable characteristic of HBTs in switching application has been the presence of large collector-emitter (c-e) offset voltage. The higher offset voltage increases the c-e saturation voltage, which results in unnecessary consumption in circuit applications. The higher tum-on voltage of the e-b heterojunction is attributed to the conduction band spike at the heterojunction interface. Different schemes for obtaining a graded bandgap base or a compositional grading in the emitter have been proposed to suppress the conduction band spike appearing at the e-b heterojunction. Unfortunately, recent work has suggested that grading techniques may not be completely effective in eliminating barriers to electron conduction at high current densityfq. Moreover, from the point view of manufacturability, the fkbrication of graded bandgap base or compositional grading e-b heterojunction HBTs is far more complicated than that for conventional BJTs.