Microtransfer-Printed InGaAs/InP HBTs Utilizing a Vertical Metal Sub-Collector Contact

InGaAs/InP heterojunction bipolar transistors (HBTs) have enabled record results in RF, mm-wave, and high-speed mixed signal circuit designs, due to their simultaneously high $\mathrm{f}_{\mathrm{t}}$ and $\mathrm{f}_{\max}$ cutoff frequencies [1]. For millimeter-wave power amplification, InP HBTs offer wide fractional bandwidths and high efficiencies compared to GaN HEMT technologies, while typically operating at lower power densities (1–2 W/mm normalized to emitter length) and output power levels $( . To increase the RF power density of InP HBTs, we wish to increase the utilization of the active device area in a fixed footprint, while managing device self-heating. Decoupling the HBT from the InP substrate allows for reduced device thermal resistance by removing high thermal resistivity ternary materials, and the InP substrate itself [2], [3]. Microtransfer printing (MTP) enables devices to be removed from their native substrate, and placed accurately (~1 micron placement error) on another surface [4]. Here, we report results on microtransfer printed InP HBTs placed directly on a metallic sub-collector ohmic contact deposited on a high-thermal conductivity SiC substrate. The HBTs utilize a fine-pitch array of emitter fingers on a single base-collector mesa to increase the HBT active-area utilization while maintaining RF performance. Load-pull measurements at 30 GHz show a $> 4\mathrm{x}$ increase in RF power density compared to conventional mesa-HBTs with a similar device footprint.