14.2 Nano-scale Type-II InP/GaAsSb DHBTs to Reach THz Cutoff Frequencies

We demonstrate vertically and laterally scaled GaAsSb/InP type-II DHBTs with fT = 670 GHz at 10.3 mA/μm emitter current density and off-state collector-emitter breakdown voltage BVCEO = 3.2 V. Small-signal modeling is used to extract delay terms and to identify material design and device fabrication requirements for next-generation devices with > 1 THz cutoff frequencies. INTRODUCTION InP based heterojunction bipolar transistors (HBTs) have the highest current gain cutoff frequencies of any transistor technology but the limits of the material for this application have not yet been reached. The reported breakdown voltages versus current gain cutoff frequencies of modern SiGe HBT, InP pHEMT, and InP HBT transistors are plotted in Fig. 1, revealing scaling trends for each technology. As illustrated in Fig. 1, InP HBTs maintain a breakdown advantage over SiGe HBTs and InP HEMT devices as the structures of each are scaled to achieve higher cutoff frequencies at the expense of breakdown voltage. Double heterojunction HBT (DHBT) designs using wide-gap InP collector layers improve the breakdown voltage and power handling of vertically scaled devices. Type-I DHBTs using InGaAs base layers have a current-blocking conduction band discontinuity at the basecollector junction that is alleviated through the use of setback and superlattice or step-graded layers in the collector. These devices benefit from ballistic electron injection and high electron mobility through the InGaAs base. Because type-I DHBTs must incorporate narrow band gap material at the base-collector interface they possess a limit to vertical scaling that results in breakdown voltages approaching those of the SHBT for vertically scaled designs with fT greater than 500 GHz. The type-II InP DHBT with GaAsSb as the base layer is an alternative having a base-collector junction energy band alignment that naturally favors electron collection—allowing for an all-InP collector layer to be used to achieve higher breakdown voltage and thermal dissipation in scaled material structures. Fig. 2 plots reported transistor breakdown voltage versus collector layer thickness of several high performance InP SHBT, type-I DHBT, and type-II DHBT devices where this trend and the potential type-II advantage becomes apparent below 1000 Å. DESIGN AND FABRICATION Low electron mobility in the type-II GaAsSb base layer typically limits the fT of these devices, despite electron velocities exceeding 4.5 x 10 in the InP collector [2]. To enhance electron transport and lower total transit delay, we incorporate an InGaAsSb compositional grading—creating a built-in potential difference across the base layer of approximately 55 meV. Device structures were grown at the University of Illinois using a gas-source molecular beam epitaxy system. The epitaxial structures of the DHBT consisted of a 350 nm InP subcollector doped to n = 3 × 10 cm, a 25 nm InGaAs subcollector doped to n = 4 × 10 cm, an InP collector doped to n = 3 × 10 cm, a carbon-doped GaAsSb-InGaAsSb graded base with p = 8 × 10 cm, a 40 nm Si-doped InP emitter, and a 40 nm graded InGaAs emitter cap. Electron beam lithography and wet chemical etching were used to fabricate devices with emitter widths as small as 250 nm and lengths ranging from fT [GHz] 10