Gallium arsenide-based epitaxial structures for heterojunction bipolar transistors with increased efficiency

AlGaAs/GaAs/GaAs and GaInP/GaAs/GaAs Npn heterojunction bipolar transistors (HBTs) are now in widespread use in microwave power amplifiers. In this Dissertation, improved HBT structures are presented to address issues currently problematic for these devices: high turn-on voltage, high offset and knee voltages, and saturation charge storage.; For a variety of applications, particularly operation with low power supply voltage and reduced power dissipation, it would be desirable to have a smaller value of turn-on voltage (V_BE). In this Dissertation, the design of reduced V_BE GaAs-based HBTs was investigated using GaInNAs in the base region. In order to gain understanding of the effect of nitrogen on these devices, GaInNAs-base HBTs with varying amounts of nitrogen, were fabricated and characterized which yielded devices with varying reductions in V_BE. In devices with a large reduction in V_BE of 0.4 V the minority carrier diffusion length (L_diff) was reduced to 0.1 μm, which is considerably lower than that of GaAs (L_diff = 0.5 μm). This resulted in lower current gain and longer base transit time. Devices with a smaller reduction of V_BE of 90 mV also had a reduced L_diff of 0.4 μm. However, with graded composition in the base, the minority carrier transport of GaInNAs-base HBTs was significantly improved. The resulting HBT had an 80 mV V_BE with larger current gain and comparable f_T to the GaAs device.; The second part of this Dissertation deals with the reduction of the offset (V_CE,sat) and knee (V_k) voltages as well as saturation charge storage in HBTs fabricated with the GaInP/GaAs material system. Reduced HBT offset and knee voltages are important to improve the power amplifier efficiency. Switching mode power amplifiers are particularly attractive for high efficiency applications, but to improve their performance it is necessary to reduce the HBT saturation charge storage. It is shown in this Dissertation that HBT structures using a 100 Å thick layer of GaInP between the GaAs base and collector layers are effective in reducing V_CE,sat to 30 mV and V_k to 0.3 V (while for conventional HBTs V_CE,sat = 0.2 V and V_k = 0.5 V are typical). A 5-fold reduction in saturation charge storage is simultaneously obtained.