Contents I. Introduction
GaAs-BASED heterojunction bipolar transistors (HBTs) have been widely used in high-performance microwave and digital communication systems. However, they have a relatively large base–emitter turn-on voltage due to the large band gap of GaAs used as the base layer, and this limits the minimum operating voltage and increases the power consumption in circuit applications. For low voltage operation, InP-based HBTs are the most suitable candidate because they have a turn-on voltage of smaller than 0.5 V. However, InP has disadvantages of brittleness, unavailability of large diameter wafers, and high substrate cost compared to GaAs. Therefore, it is important to develop novel GaAs-based HBTs with a narrow band gap base. For this reason, GaInAsN has lately attracted considerable attention as the base layer of GaAs HBTs [1]–[3]. By incorporating a proper amount of nitrogen and indium into GaAs, GaInAsN lattice-matched to GaAs can be obtained with a significant energy band-gap reduction. However, because of the large conduction-band discontinuity between InGaAsN base and GaAs collector, a collector current blocking effect would occur, giving rise to a drastic degradation of current gain at a high collector current density [1]. Although by the insertion of graded layers between the base and collector junction, the current blocking effect can be suppressed, this complicates the transistor design and fabrication.
