ABSTRACT The application of GaInNAs to heterojunction bipolar transistors (HBTs) enables re-engineering the band structure of these devices to provide a variety of benefits. Conventional GaAs-based HBTs have emitters of GaInP (or AlGaAs), base and collector regions of GaAs and are grown on a GaAs substrate. GaInNAs is used to replace the GaAs in the base region of the transistors. The lower band gap of GaInNAs leads to devices which have lower input turn-on voltages (VBE) than those of conventional devices, which is highly beneficial for battery-operated power-efficient circuits. The difference in band gap between the GaInNAs base and GaAs collector also contributes to reducing the offset voltage (Voffset) and lowering the knee voltage of the transistors, which leads to enhanced efficiency in microwave power amplifiers. Although the material transport characteristics of GaInNAs are not as favorable as those of GaAs due to lower electron and hole mobilities, and higher recombination rates, it has been shown that the HBT dc current gain, current gain cutoff frequency (fT), and maximum frequency of oscillation (fmax) can be as good as (or better than) those of conventional devices by proper engineering–for example, by grading of the composition of the GaInNAs material across the base region. It is also critical to compensate for the discontinuities of the conduction band at the base-emitter and base-collector junctions. This chapter outlines the key strategies to improve HBT characteristics, and describes device design and fabrication with GaInNAs base regions. It also summarizes reported HBT characteristics and circuit performance with these devices.