We report a new InP/GaAsSb double heterojunction bipolar transistor (DHBT) emitter fin architecture with a record f MAX = 1.2 THz, a simultaneous f T = 475 GHz, and BV CEO = 5.4 V. The resulting BV CEO × f MAX = 6.48 THz-V is unparalleled in semiconductor technology. Devices were realized with a 20-nm-thick compositionally and impurity graded GaAsSb-base and a 125-nm InP collector. The performance arises because the process allows: 1) a tunable base-emitter access distance down to 10 nm; 2) the use of thicker base contact metals; and 3) the minimization of parasitic capacitances and resistances via precise lateral wet etching of the base-collector (B/C) mesa. Perhaps more significantly, InP/GaAsSb DHBTs with f MAX ≥ 1 THz are demonstrated with emitter lengths as long as 9.4 μm and areas as high as 1.645 μm 2 . Such an area is >6× larger than previously reported terahertz (THz) DHBTs, representing a breakthrough in THz transistor scalability. This attractive performance level is achieved with a very low dissipated power density which makes InP/GaAsSb DHBTs well-suited for high-efficiency millimeter-and submillimeter-wave applications. Furthermore, we provide the first large-signal characterization of a THz transistor with 94 GHz load-pull measurements showing a peak power-added-efficiency (PAE) of 32.5% (40% collector efficiency) and a maximum saturated power of 6.67 mW/μm 2 or 1.17 mW/μm of emitter length in a common-emitter configuration. Devices operate stably under large-signal conditions, with voltages nearly twice higher than those for peak small-signal performance.

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GaInAs/InAs composite channels in InP-based pHEMTs enable wideband and/or low-noise performances because of their superior carrier transport properties. To date, the influence of the InAs inset design details on transistor performance has not been parametrized in the literature. We present a systematic study of the effects of the InAs channel inset thickness on transistor characteristics and cutoff frequencies versus temperature, and on the noise performance at 300 K. The epitaxial layer structures considered here incorporate 2 to 5-nm InAs insets in a fixed total composite channel thickness. All layers exhibit excellent electron mobilities (from 40 200 to 54 800 cm 2 /Vs at 77 K). Thicker InAs insets improve both the current gain cutoff frequency (f T) and the maximum oscillation frequency (f MAX). However, they also result in higher gate leakage currents and increased channel impact ionization. 50-nm gate length pHEMTs with a 5-nm InAs inset feature the highest simultaneous f T /f MAX ≥ 390/675 (455/800) GHz at 300 (15) K for a low-noise bias but exhibit the poorest minimum noise figure NF MIN. Whereas higher f T (and/or f MAX) values have traditionally been associated with improved noise performances, this is no longer the case.

Double heterojunction bipolar transistors (DHBTs) are intended to extend the breakdown voltage beyond what is possible in single heterojunction bipolar transistors, ideally without sacrificing frequency performance. InP/GaAsSb DHBTs offer the most favorable cutoff frequency versus breakdown voltage tradeoff among all bipolar transistors. It has been shown that the addition of Indium to a GaAsSb base further increases current gain cutoff frequencies fT. In the present article, we compare ternary (GaAsSb) and quaternary (GaInAsSb) graded base DHBTs fabricated side-by-side to shed light on the physical mechanism responsible for the increase in cutoff frequencies. Ternary and quaternary base DHBTs show markedly distinct RF behaviors when measured at reduced temperatures—we use these differences to infer that the improved cutoff frequencies with GaInAsSb base layers arise because of a reduced electron population of the L-valley with increasing In-content in the base. A quantum transport calculation based on the non-equilibrium Green’s function formalism and the empirical tight-binding method for electron transport through the base and collector regions with different Γ-L separations for different temperatures reproduces the main experimentally observed features.

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