Publications – 2017

Y. Zhang, M. Sun, D. Piedra, J. Hu, Z. Liu, Y. Lin, X. Gao, K. Shepard, and T. Palacios. 1200 V GaN Vertical Fin Power Field-Effect Transistors. Proceedings of the International Electron Devices Meeting, 2016.

We demonstrate record performance in a novel normally-off GaN vertical transistor with submicron finshaped channels. This vertical fin transistor only needs nGaN layers, with no requirement for epitaxial regrowth or pGaN layers. A specific on-resistance of 0.2 mΩ·cm2 and a breakdown voltage over 1200 V have been demonstrated with extremely high ON current (over 25 kA/cm2) and low OFF current at 1200 V (below 10-4 A/cm2), rendering an excellent Baliga’s figure of merit up to 7.2 GW/cm2. A threshold voltage of 1 V was achieved and was stable up to 150 oC.Large devices with high current up to 10 A and breakdown voltage over 800 V were also demonstrated. These results show the great potential of GaN vertical fin transistors for high-current and high-voltage power applications.

David Tsai, Rafael Yuste, and Kenneth L. Shepard. Statistically Reconstructed Multiplexing for Very Dense, High-Channel-Count Acquisition Systems. IEEE Transactions on Biomedical Circuits and Systems12(1), 13-23.

Multiplexing is an important strategy in multichannel acquisition systems. The per-channel antialiasing filters needed in the traditional multiplexing architecture limit its scalability for applications requiring high channel density, high channel count, and low noise. A particularly challenging example is multielectrode arrays for recording from neural systems. We show that conventional approaches must tradeoff recording density and noise performance, at a scale far from the ideal goal of one-to-one mapping between neurons and sensors. We present a multiplexing architecture without per-channel antialiasing filters. The sparsely sampled data are recovered through a compressed sensing strategy, involving statistical reconstruction and removal of the undersampled thermal noise. In doing so, we replace large analog components with digital signal processing blocks, which are much more amenable to scaled CMOS implementation. The resulting statistically reconstructed multiplexing architecture recovers input signals at significantly improved signal-to-noise ratios when compared to conventional multiplexing with antialiasing filters at the same per-channel area. We implement the new architecture in a 65 536-channel neural recording system and show that it is able to recover signals with performance comparable to conventional high-performance, single-channel systems, despite a more than four-orders-of-magnitude increase in channel density.

Oliver Rauh, Ulf-Peter Hansen, Sebastian Mach1, Andreas J.W. Hartel, Kenneth L. Shepard, Gerhard Thiel and Indra Schroeder. Extended beta distributions open the access to fast gating in bilayer experiments—assigning the voltage-dependent gating to the selectivity filter. FEBS Letters. DOI: 10.1002/1873-3468.12898. Volume 591, Issue 23, December 2017, Pages 3850–3860.

Lipid bilayers provide many benefits for ion channel recordings, such as control of membrane composition and transport molecules. However, they suffer from high membrane capacitance limiting the bandwidth and impeding analysis of fast gating. This can be overcome by fitting the deviations of the open-channel noise from the baseline noise by extended beta distributions. We demonstrate this analysis step-by-step by applying it to the example of viral K+ channels (Kcv), from the choice of the gating model through the fitting process, validation of the results, and what kinds of results can be obtained. These voltage sensor-less channels show profoundly voltage-dependent gating with dwell times in the closed state of about 50 μs. Mutations assign it to the selectivity filter.

David Tsai, Daniel Sawyer, Adrian Bradd, Rafael Yuste & Kenneth L. Shepard. A very large-scale microelectrode array for cellular- resolution electrophysiology. Nature Communications 8, Article number: 1802 (2017) | DOI: 10.1038/s41467-017-02009-x

In traditional electrophysiology, spatially inefficient electronics and the need for tissue-toelectrode proximity defy non-invasive interfaces at scales of more than a thousand low noise, simultaneously recording channels. Using compressed sensing concepts and silicon complementary metal-oxide-semiconductors (CMOS), we demonstrate a platform with 65,536 simultaneously recording and stimulating electrodes in which the per-electrode electronics consume an area of 25.5 μm by 25.5 μm. Application of this platform to mouse retinal studies is achieved with a high-performance processing pipeline with a 1 GB/s data rate. The platform records from 65,536 electrodes concurrently with a ~10 µV r.m.s. noise;senses spikes from more than 34,000 electrodes when recording across the entire retina; automatically sorts and classifies greater than 1700 neurons following visual stimulation; and stimulates individual neurons using any number of the 65,536 electrodes while observing spikes over the entire retina. The approaches developed here are applicable to other electrophysiological systems and electrode configurations.

Sefi Vernick, Scott M. Trocchia, Steven B. Warren, Erik F. Young, Delphine Bouilly, Ruben L. Gonzalez, Colin Nuckolls & Kenneth L. Shepard. Electrostatic melting in a single-molecule field-effect transistor with applications in genomic identification. Nature Communications 8, Article number: 15450 (2017) | DOI: 10.1038/ncomms15450.

The study of biomolecular interactions at the single-molecule level holds great potential for both basic science and biotechnology applications. Single-molecule studies often rely on fluorescence-based reporting, with signal levels limited by photon emission from single optical reporters. The point-functionalized carbon nanotube transistor, known as the single-molecule field-effect transistor, is a bioelectronics alternative based on intrinsic molecular charge that offers significantly higher signal levels for detection. Such devices are effective for characterizing DNA hybridization kinetics and thermodynamics and enabling emerging applications in genomic identification. In this work, we show that hybridization kinetics can be directly controlled by electrostatic bias applied between the device and the surrounding electrolyte. We perform the first single-molecule experiments demonstrating the use of electrostatics to control molecular binding. Using bias as a proxy for temperature, we demonstrate the feasibility of detecting various concentrations of 20-nt target sequences from the Ebolavirus nucleoprotein gene in a constant-temperature environment

Ko-Tao Lee, Can Bayram, Daniel Piedra, Edmund Sprogis, Hariklia Deligianni, Balakrishnan Krishnan, George Papasouliotis, Ajit Paranjpe, Eyal Aklimi, Ken Shepard, Tomás Palacios, and Devendra Sadana. GaN Devices on a 200 mm Si Platform Targeting Heterogeneous IntegrationIEEE Electron Device Letters (Volume: 38, Issue: 8, Aug. 2017 ).

GaN-based high electron mobility transistors (HEMTs) were fabricated on 200-mm silicon-oninsulator (SOI) substrates possessing multiple crystal orientations. These SOI substrates have the Si (100)-SiO2- Si (111) structure, which allows Si (111) to be exposed below the buried oxide to enable GaN epitaxial growth adjacent to Si (100). The current collapse in GaN HEMTs of < 150 × 150 µm2 patterns is 2%–6%, which is remarkably lower than the devices on blanket materials. We believe that stress relaxation resulting from substrate patterning contributes to the reduction of current collapse. By creating small GaN patterns on a larger diameter Si wafer, co-integration of GaN with Si technology may be possible.

Hassan M. Edrees, Aida R. Colón-Berrios, Daniel de Godoy Peixoto, Kenneth L. Shepard, Peter R. Kinget, and Ioannis Kymissis. Monolithically Integrated CMOS-SMR Oscillator in 65 nm CMOS Using Custom MPW Die-Level Fabrication Process, Journal of Microelectromechanical Systems. Journal of Microelectromechanical Systems ( Volume: 26, Issue: 4, Aug. 2017 )

Acoustic resonators, such as thin-film solidly mounted resonators (SMRs) and silicon microelectromechanical systems have been used widely in commercial and research RF communication circuits to implement high-Q oscillators and highly selective filters. Monolithic integration is a promising solution to address the growing demand for such components while continuing the aggressive miniaturization of radios. In this paper, we demonstrate successful monolithic SMR-CMOS co-integration by building a high-Q SMR atop a standard 65-nm CMOS substrate using a custom die-level fabrication process. The approach takes advantage of features in the back-end-of-line to deliver the surface smoothness required for fully supported mechanical resonators, which was not possible using traditional process approaches. This paper marks the first demonstration of SMR integration on 65-nm CMOS. The CMOS die used is more than 400% smaller in area than those in the previous die-level demonstrations of monolithically integrated piezoelectric resonators on CMOS. [2016-0192]

Hasti Amiri, Kenneth L. Shepard, Colin Nuckolls, and Raúl Hernández Sánchez Single-Walled Carbon Nanotubes: Mimics of Biological Ion Channels. Nano Lett., 2017, 17 (2), pp 1204–1211, DOI: 10.1021/acs.nanolett.6b04967.

Here we report on the ion conductance through individual, small diameter single-walled carbon nanotubes. We find that they are mimics of ion channels found in natural systems. We explore the factors governing the ion selectivity and permeation through single-walled carbon nanotubes by considering an electrostatic mechanism built around a simplified version of the Gouy−Chapman theory. We find that the single-walled carbon nanotubes preferentially transported cations and that the cation permeability is size-dependent. The ionic conductance increases as the absolute hydration enthalpy decreases for monovalent cations with similar solid-state radii, hydrated radii, and bulk mobility. Charge screening experiments using either the addition of cationic or anionic polymers, divalent metal cations, or changes in pH reveal the enormous impact of the negatively charged carboxylates at the entrance of the single-walled carbon nanotubes. These observations were modeled in the low-to-medium concentration range (0.1−2.0 M) by an electrostatic mechanism that mimics the behavior observed in many biological ion channel-forming proteins. Moreover, multi-ion conduction in the high concentration range (>2.0 M) further reinforces the similarity between single-walled carbon nanotubes and protein ion channels.

Eyal Aklimi, Student Member, IEEE, Daniel Piedra, Student Member, IEEE, Kevin Tien, Student Member, IEEE, Tomás Palacios, Member, IEEE, and Kenneth L. Shepard, Fellow, IEEE Hybrid CMOS/GaN 40-MHz Maximum 20-V Input DC–DC Multiphase Buck Converter. IEEE Journal of Solid-State Circuits, Volume: 52, Issue: 6, June 2017.

This paper presents a 40-MHz hybrid CMOS/GaN integrated multiphase dc–dc switched-inductor buck converter with a maximum 20-V input voltage. The half-bridge switches are realized using lateral AlGaN/GaN HEMTs, while the drivers and other circuitry are implemented in standard 180-nm CMOS. The interface between the CMOS and GaN dice is achieved through face-to-face bonding, reducing inductive parasitics for the connection to less than 15 pH. A capacitively coupled level shifter provides the gate drive for the high-side GaN switch using 5-V CMOS devices. The converter demonstrates 76% efficiency for 8:1 V conversion and over 60% efficiency for conversion ratios up to 16:1.

Jordan Thimot and Kenneth L. Shepard. Wirelessly powered implants. Nature Biomedical Engineering 1, Article number: 0051 (2017), DOI:10.1038/s41551-017-005.

Phased-array antennas that conform to body surfaces efficiently transfer electromagnetic energy to miniaturized semiconductor devices implanted in pigs.