Recent improvements in the fabrication technology of InGaAs/InP heterobipolar transistors have enabled highly scaled transistors with power gain bandwidths above 1 THz. Limitations of the conventional fabrication process that reduce RF bandwidth have been identified and mitigated, among which are high resistivity base ohmic contacts, resistive base electrodes, excessive emitter end undercut, and insufficient undercut of largediameter base posts. A novel two-step deposition process for self-aligned metallization of sub-20-nm bases has been developed and demonstrated. In the first step, a metal stack is directly evaporated onto the base semiconductor without any lithographic processing so as to minimize contamination from resist/developer chemistry. The composite metal stack exploits an ultrathin layer of platinum that controllably reacts with base, yielding low contact resistance, as well as a thick refractory diffusion barrier, which permits stable operation at high current densities and elevated temperatures. Further reduction of overall base access resistance is achieved by passivating base and emitter semiconductor surfaces in a combined atomic layer deposition Al 2 O 3 and plasma-enchanced chemical vapor depositon SiN x sidewall process. This technology enables the deposition of low-sheetresistivity base electrodes, further improving overall base access resistance and f max bandwidth. Additional process enhancements include the significant reduction of device parasitics by scaling base posts and controlling emitter end and base postundercut.

The DC current gain in In0.53Ga0.47As/InP double-heterojunction bipolar transistors is computed based on a drift-diffusion model, and is compared with experimental data. Even in the absence of other scaling effects, lateral diffusion of electrons to the base Ohmic contacts causes a rapid reduction in DC current gain as the emitter junction width and emitter-base contact spacing are reduced. The simulation and experimental data are compared in order to examine the effect of carrier lateral diffusion on current gain. The impact on current gain due to device scaling and approaches to increase current gain are discussed.

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Planar ultrathin InAs-channel MOSFETs were demonstrated on Si substrates with gate lengths (L g ) as small as 20 nm. The III-V epitaxial buffer layers were grown on 300 mm Si substrates by metal-organic chemical vapor deposition (MOCVD) and the subsequent InAlAs bottom barriers and InAs channel were grown by molecular beam epitaxy (MBE). The devices at 20 nm L g show high transconductance, ~2.0 mS/Pm at V DS =0.5V.

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We report an InP/InGaAs/InP double heterojunction bipolar transistor fabricated in a triplemesa structure, exhibiting simultaneous 404 GHz f τ and 901 GHz f max . The emitter and base contacts were defined by electron beam lithography with better than 10 nm resolution and smaller than 20 nm alignment error. The base-collector junction has been passivated by depositing a SiN x layer prior to benzocyclobutene planarization, improving the open-base breakdown voltage BV CEO from 3.7 to 4.3 V.INDEX TERMS HBT, InGaAs/InP DHBT, THz device.

As the dimensions of In0.53Ga0.47As/InP double-heterojunction bipolar transistors (DHBTs) scale for terahertz applications, the DC current (β) decreases. To improve the DC performance in such scaled devices, we analyze three modified HBT geometries: a HBT with a surface pulse-doped layer in the base, a HBT having this pulse-doped layer under the emitter junction and under the base contact, but with it removed by etching in the region between the base and emitter contacts, and a device, necessarily fabricated by regrowth, in which the pulsed doped layer is present under only the base contacts. Based on a drift-diffusion/recombination model, carrier transport in the DHBT base is simulated and the corresponding β is computed using TCAD software. The structures with a pulse doped layer can attain β = 31 ∼ 39 at 100 nm emitter width. The structures with a trench between the base contact and emitter show β = 39 ∼ 54 at 100 nm emitter width. Finally, the structure with recessed base-emitter junction and regrown emitter demonstrate β = 62–119 at 100 nm emitter width.