InP DHBT technology for 100 Gbit/s applications

TECHNOLOGY In this paper, we report a manufacturable InP DHBT technology, suitable for medium scale mixed-signal and monolithic microwave integrated circuits. The MBE grown InP-based DHBTs with an emitter area of 1 × 4 μm exhibited peak cutoff frequencies (fT and fMAX) > 300 GHz, and a breakdown voltage (BVCEo) of ~ 5 V. The InP DHBT layer structures were grown on 3” semiinsulating InP substrates, in a multiwafer molecular beam epitaxy (MBE) system (GEN200). These structures feature an InP emitter, a graded InGaAs base, and a compositionally step-graded InGaAs/InGaAsP/InP collector, to minimize collector current blocking effects. Carbon and silicon were used for pand n-type doping, respectively. The growth details have been reported elsewhere [6]. The potential of this technology has been first assessed by the realization of a voltage controlled oscillator (VCO), exhibiting a high output power and a large tuning range. Subsequently, a demultiplexer (DEMUX) suitable for 100 Gbit/s fibre optical links, has been successfully fabricated and operated up to 110 Gbit/s. In contrast to most recent reports [7,8], based on aggressively scaled technologies (less than 0.5 μm), our fabrication process relies only on standard manufacturing techniques, including i-line stepper lithography and selective dry/wet etching. A detailed description of the device and IC manufacturing technology was reported in [9]. Briefly, the fabrication process is based on self-aligned base-emitter contacts and benzocyclobutene (BCB) for device passivation and planarization. The IC process is completed by NiCr resistors, MIM (metal-insulator-metal) capacitors, and three levels of Au-based interconnect metals. Figure 1 shows a top-view SEM (scanning electron microscopy) photograph of a nominal 1 × 4 μm DHBT. INTRODUCTION There has been a continuous interest in high-frequency devices for operation beyond 300 GHz. Prospective applications for these devices are high-speed analog, digital and mixed-signal integrated circuits (ICs) for signal processing and next generation communications systems. In particular, time-division-multiplexing (TDM) systems at data rates of 40 Gbit/s and beyond have been actively investigated to meet the increasing demands for high-speed and large transmission capacity. Among various material and device candidates, a considerable work related to the design and fabrication of high-bandwidth, high-speed mixed signal ICs, has been carried out using III/V based high electron mobility transistors (HEMTs) [1] or heterojunction bipolar transistors (HBTs) [2,3]. Thanks to their material properties, InP-based DHBTs provide simultaneous high operation frequency, high breakdown voltage, good uniformity and reliability. In this contribution, we report a manufacturable InP DHBT technology suitable for medium scale mixed-signal and monolithic microwave integrated circuits (MMICs). Using a previous generation technology (fT ~ 250 GHz), we already have successfully demonstrated a set of analog and digital ICs operating up to 80 Gbit/s [4,5]. In the present work, the high-frequency InP-based DHBTs exhibit current gains in the range of 90 and cutoff frequency values of over 300 GHz. The potential of this technology has been confirmed by the fabrication and testing of a VCO and a DEMUX-IC suitable for ≥ 100 Gbit/s transmission systems. Figure 1: SEM-picture of an InP-DHBT before planarization. Emitter size: 1 × 4 μm2. CS MANTECH Conference, April 14-17, 2008, Chicago, Illinois, USA