InP DHBT Linear Modulator Driver With a 3-Vppd PAM-4 Output Swing at 90 GBaud: From Enhanced Transistor Modeling to Integrated Circuit Design

In this article, we report on the modeling, design, and characterization of indium phosphide (InP) double heterojunction bipolar transistor (DHBT) devices and integrated circuits (ICs) for next-generation optical communications. Critical aspects of transistors' modeling and their influence on the IC design are detailed, as well as the design and characterization of a lumped linear modulator driver featuring a 3-Vppd four-level pulse-amplitude modulation (PAM-4) output swing at 90 GBaud (GBd). In particular, we propose an electromagnetic (EM) simulation-based parasitic extraction method of the DHBT access structures, to refine the DHBT and IC performance prediction accuracy. It is shown to provide a better estimation of a canonical cascode gain and \mu stability factor at millimeter-wave frequencies, as well as a better estimation of the driver IC gain in the 50-110 GHz frequency range. Furthermore, a high-frequency gain boosting (self-peaking) topology, based upon an emitter-degenerated paralleled-transistor cascode configuration, is analyzed using a simplified transistor model and leveraged to enhance the linear driver output-stage gain-bandwidth product with controlled amount of peaking gain. This self-peaking technique is shown to be inherent to cascode structures and can therefore be used with other technologies, with no added design complexity. The driver IC was implemented in a 0.5- \mu m InP-DHBT technology and features a bandwidth well in excess of 110 GHz, with 13 dB of peaking gain at 95 GHz. Besides, it achieves a 9.1-dBm single-ended output power at 1 dB of gain compression and a 2.7% root-mean-square total harmonic distortion (rms-THD) at a 3-Vppd output swing. The driver power consumption is 0.67 W, which is among the lowest in the state of the art and shows a 1.5-GBd driver figure of merit (FoM). To the best of our knowledge, this driver achieves the highest \geq 64 GBd PAM-4 performances reported to date, without digital signal processing (DSP) or postprocessing.