InP-based NPN and PNP heterojunction bipolar transistor design, technology, and characterization for enhanced high-frequency power amplification.

Abstract

InP-based heterojunction bipolar transistors (HBTs) have demonstrated excellent high power and low phase noise operation at microwave frequencies and have become commercially available in recent years. Research and development has almost exclusively focused on NPN HBTs due to the much higher mobility of electrons as compared to holes in materials lattice-matched to InP. However, by combining PNP HBTs together with NPNs, many complementary circuits become possible and offer decreased power consumption, increased gain, increased linearity, and smaller size circuits. Therefore, the development of microwave PNP HBTs and their integration with NPN HBTs are of significant interest for a variety of applications. For this work, a process technology was developed and optimized for minimal parasitic resistances and capacitances in InP-based HBTs. Highlights of the process include self-aligned base and collector contacts, trench mesa isolation, and lateral undercut of the base contacts. Improved capacitance-voltage and transmission-line model measurement techniques were introduced to improve the understanding and aid the optimization of the HBTs. Baseline NPN HBTs were fabricated with cut-off frequency <italic>f_T</italic> and maximum frequency of oscillation <italic>f_max</italic> of 95 GHz and 58 GHz. Other fabricated NPN HBTs had <italic>f_T</italic> as high as 100 GHz or <italic>f_max</italic> as high as 130 GHz. These results demonstrated the applicability of this HBT design and process for microwave applications. The HBTs were investigated under large-signal conditions and produced a record power level of 1.37 mW/mum<super>2</super> for InGaAs collector HBTs, which also demonstrates their suitability for power applications. InP-based PNP HBTs were fabricated and optimized for microwave operation. Prior to the experimental investigations, drift-diffusion models of PNP HBTs were simulated to determine optimum DC and high-frequency designs. The resulting devices achieved <italic>f_T</italic> up to 19 GHz and <italic> f_max</italic> up to 35 GHz, both of which are records for InP-based PNP HBTs. The power performance of the PNP HBTs was also characterized, and high power densities up to 0.49 mW/mum<super>2</super> were achieved, which are comparable to previously reported results using wider bandgap GaAs collectors. Finally, a hybrid push-pull amplifier was simulated and successfully fabricated using both the NPN and PNP HBTs. This amplifier showed better IM3 (by &sim;9 dBc) and smaller second harmonic content (by &sim;11 dBc) compared with NPN HBTs alone. In addition, the push-pull amplifier produced over 2 dBm more output power than the NPN HBT alone at 1 dB of gain compression. These results indicate the potential of PNP HBTs, and of integrated circuits employing both NPN and PNP HBTs.