Engineering the Electron-Hole Bilayer Tunneling Field-Effect Transistor

The electron-hole (EH) bilayer tunneling field-effect transistor promises to eliminate heavy-doping band tails enabling a smaller subthreshold swing voltage. Nevertheless, the electrostatics of a thin structure must be optimized for gate efficiency. We analyze the tradeoff between gate efficiency versus ON-state conductance to find the optimal device design. Once the EH bilayer is optimized for a given ON-state conductance, Si, Ge, and InAs all have similar gate efficiency, around 40%-50%. Unlike Si and Ge, only the InAs case allows a manageable work function difference for EH bilayer transistor operation.National Science Foundation (U.S.). Center for Energy Efficient Electronics Science (Award 0939514

Investigation of hole mobility in gate-all-around Si nanowire p-MOSFETs with high-k/metal-gate: Effects of hydrogen thermal annealing and nanowire shape

A detailed study of hole mobility is presented for gate-all-around Si nanowire p-MOSFETs with conformal high-κ/MG and various high-temperature hydrogen annealing processes. Hole mobility enhancement relative to planar SOI devices and universal (100) is observed for 15 nm-diameter circular Si nanowires, due to an optimized anneal process which smoothes and reshapes the suspended nanowires. Increasing hole mobility is experimentally observed with decreasing nanowire width down to 12 nm. The measured inversion capacitance-voltage characteristics are in excellent agreement with quantum mechanical simulations. In addition, a method to extract areal inversion charge density in Si nanowires is introduced and its impact on the mobility of Si nanowires with various shapes is explored.Semiconductor Research Corporation. Center for Materials, Structures and Device

Extraction of large valence-band energy offsets and comparison to theoretical values for strained-Si/strained-Ge type-II heterostructures on relaxed SiGe substrates

Metal-oxide-semiconductor capacitors were fabricated on type-II staggered gap strained-Si/strained-Ge heterostructures epitaxially grown on relaxed SiGe substrates of various Ge fractions. Quasistatic quantum-mechanical capacitance-voltage (CV) simulations were fit to experimental CV measurements to extract the band alignment of the strained layers. The valence-band offset of the strained-Si/strained-Ge heterostructure was found to be 770, 760, and 670 meV for 35, 42, and 52% Ge in the relaxed SiGe substrate, respectively. These values are approximately 100 meV larger than the usually recommended band offsets for modeling Si/Ge structures. It is shown that the larger valence-band offsets found here are consistent with an 800-meV average valence-band offset between Si and Ge, which also explains the type-II band alignment observed in strained-Si[subscript 1−x]Ge[subscript x] on unstrained-Si heterostructures.United States. Defense Advanced Research Projects Agency (Contract No. FA8650-08-C-7835)National Science Foundation (U.S.) (Contract No. ECCS-0939514)National Defense Science and Engineering Graduate Fellowshi

Risk of COVID-19 after natural infection or vaccinationResearch in context

Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health

Risk of COVID-19 after natural infection or vaccinationResearch in context

Summary: Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health