Design and performance of tunnel collector HBTs for microwave power amplifiers

AlGaAs/GaAs/GaAs and GaInP/GaAs/GaAs n-p-n heterojunction bipolar transistors (HBTs) are now in widespread use in microwave power amplifiers. In this paper, improved HBT structures are presented to address issues currently problematic for these devices: high offset and knee voltages and saturation charge storage. Reduced HBT offset and knee voltages (V_CE,os and V_k) are important to improve the power amplifier efficiency. Reduced saturation charge storage is desirable to increase gain under conditions when the transistor saturates (such as in over-driven Class AB amplifiers and switching mode amplifiers). It is shown in this paper that HBT structures using a 100-Å-thick layer of GaInP between the GaAs base, and collector layers are effective in reducing V_CE,os to 30 mV and V_k measured at a collector current density of 2×10⁴ A/cm² to 0.3 V (while for conventional HBTs V_CE,os=0.2 V and V_k=0.5 V are typical). A five-fold reduction in saturation charge storage is simultaneously obtained.