Harsh Bais, PhD



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  • Michael Lekas, Sunwoo Lee, Wujoon Cha, James Hone, Member, IEEE, and Kenneth Shepard, Fellow, IEEE Noise Modeling of Graphene Resonant Channel Transistors, IEEE Transactions on Electron Devices (advanced on-line)

    In this paper, we present a compact model for graphene resonant channel transistors (G-RCTs) that uses extracted electrical and mechanical parameters to provide an accurate simulation of dc, RF, noise, and frequency-tuning characteristics of the device. The model is validated with measurements on fabricated G-RCTs, which include what we believe to be the first noise measurements conducted on any resonant transistor. The noise model, which considers both electrical and mechanical sources, is used to demonstrate the fundamental differences in the noise behavior of active and passive resonator technologies, and to show how optimization of device parameters can be used to improve the noise performance of RCTs.

  • Michael Lekas, Sunwoo Lee, Wujoon Cha, James Hone, and Kenneth Shepard Third-order intermodulation distortion in graphene resonant channel transistors, Applied Physics Letters 106, 073504 (2015); doi: 10.1063/1.4913462

    Third-order intermodulation distortion (IM3) is an important metric for electromechanical resonators used in radio frequency signal processing applications since it characterizes the nonlinearity of the device, and the amount of in-band interference it generates when subject to unwanted, out-of-band signals. In this letter, we measure and model IM3 in a strain-engineered graphene mechanical resonator operated as a graphene resonant channel transistor (G-RCT). The device analyzed in this work has a voltage third-order intercept point (VIIP3) of 69.5 dBm V at a gate-to-source DC bias (Vgs) of 2.5 V, which drops to 52.1 dBm V at Vgs=4.5V when driven with two out-of-band input tones spaced 5 and 10MHz from the resonant frequency. The decrease in the VIIP3 with Vgs coincides with an increase in the transmission response (S21) of the device, illustrating a trade-off between transduction efficiency and linearity. In addition, we find that conventional micro-electro-mechanical systems theory for IM3 calculation does not accurately describe our measurement data. To resolve this discrepancy, we develop a model for IM3 in G-RCTs that takes into account all of the output current terms present in the embedded transistor structure, as well as an effective Duffing parameter (αeff).

  • Matthew L. Johnston, Erik F. Young, Kenneth L. Shepard Whole-blood immunoassay for γH2AX as a radiation biodosimetry assay with minimal sample preparation, J Radiation and Environmental Biophysics (2015); doi: 10.1007/s00411-015-0595-4

    The current state of the art in high-throughput minimally invasive radiation biodosimetry involves the collection of samples in the field and analysis at a centralized facility. We have developed a simple biological immunoassay for radiation exposure that could extend this analysis out of the laboratory into the field. Such a forward placed assay would facilitate triage of a potentially exposed population. The phosphorylation and localization of the histone H2AX at double-stranded DNA breaks has already been proven to be an adequate surrogate assay for reporting DNA damage proportional to radiation dose. Here, we develop an assay for phosphorylated H2AX directed against minimally processed sample lysates. We conduct preliminary verification of H2AX phosphorylation using irradiated mouse embryo fibroblast cultures. Additional dosimetry is performed using human blood samples irradiated ex vivo. The assay reports H2AX phosphorylation in human blood samples in response to ionizing radiation over a range of 0–5 Gy in a linear fashion, without requiring filtering, enrichment, or purification of the blood sample.

  • Kevin J. Emmett, Jacob K. Rosenstein, Jan-Willem van de Meent, Ken L. Shepard, Chris H. Wiggins Statistical Inference for Nanopore Sequencing with a Biased Random Walk Model , Biophysical Journal Volume 108, Issue 8, p1852–1855, 21 April 2015

    Nanopore sequencing promises long read-lengths and single-molecule resolution, but the stochastic motion of the DNA molecule inside the pore is, as of this writing, a barrier to high accuracy reads. We develop a method of statistical inference that explicitly accounts for this error, and demonstrate that high accuracy (>99%) sequence inference is feasible even under highly diffusive motion by using a hidden Markov model to jointly analyze multiple stochastic reads. Using this model, we place bounds on achievable inference accuracy under a range of experimental parameters.

  • Kevin Tien, Noah Sturcken, Naigang Wang, Jae-woong Nah, Bing Dang, Eugene O’Sullivan, Paul Andry, Michele Petracca, Luca P. Carloni, William Gallagher, Kenneth Shepard "An 82%-Efficient Multiphase Voltage-Regulator 3D Interposer with On-Chip Magnetic Inductors," VLSI Circuits Digest of Technical Papers, 2015 Symposium on, vol., no., pp.1,2, 16-19 June 2015.

    This paper presents a three-dimensional (3D) fully integrated high-speed multiphase voltage regulator. A complete switched-inductor regulator is integrated with a four-plane NoC in a two-high chip stack combining integrated magnetics, through-silicon vias (TSVs), and 45-nm SOI CMOS devices. Quasi-V2 hysteretic control is implemented over eight injection-locked fixed-frequency phases to achieve fast response, steady-state regulation, and fixed switching frequency. Peak efficiency of 82% for conversion from 1.66 V to 0.83 V is observed at a 150 MHz per-phase switching frequency. This is the first demonstration of high-speed voltage regulation using on-chip magnetic-core inductors in a 3D stack and achieves sub-μs dynamic supply voltage scaling for high-density embedded processing applications.

  • Jaebin Choi, Eyal Aklimi, Chen Shi, David Tsai, Harish Krishnaswamy, Member, IEEE, and Kenneth L. Shepard, Fellow, IEEE "Matching the Power, Voltage, and Size of Biological Systems: A nW-Scale, 0.023-mm3 Pulsed 33-GHz Radio Transmitter Operating From a 5 kT/q-Supply Voltage," IEEE Transactions on Circuits and Systems Vol. 62, No. 8. August 2015

    This paper explores the extent to which a solid-state transmitter can be miniaturized, while still using RF for wireless information transfer and working with power densities and operating voltages comparable to what could be harvested from a living system. A 3.1 nJ/bit pulsed millimeter-wave transmitter, 300µm by 300µm by 250µm in size, designed in 32-nm SOI CMOS, operates on an electric potential of 130 mV and 3.1 nW of dc power. Farfield data transmission at 33 GHz is achieved by supply-switching an LC-oscillator with a duty cycle of 10-6. The time interval between pulses carries information on the amount of power harvested by the radio, supporting a data rate of 1 bps. The inductor of the oscillator also acts as an electrically small (λ/30) on-chip antenna, which, combined with millimeter-wave operation, enables the extremely small form factor.

  • D Tsai, E John, T Chari, R Yuste, K L Shepard, "High–channel–count, high–density micro- electrode array for closed–loop investigation of neuronal networks," Proceedings of the 37th Annual International Conference of the IEEE EMBS, 2015
  • Noah Sturcken, Ryan Davies, Hao Wu, Michael Lekas, Maurizio Arienzo, Kenneth Shepard, K.W. Cheng, C.C. Chen, Y.S. Su, C.Y. Tsai, K.D. Wu, J.Y. Wu, Y.C. Wang, K.C. Liu, C.C. Hsu, C.L. Chang, W.C. Hua, Alex Kalnitsky, "Magnetic Thin-Film Inductors for Monolithic Integration with CMOS," Proceedings of the International Electron Device Meeting 2015
  • Chari, T.; Meric, I.; Dean, C.; Shepard, K., Properties of Self-Aligned Short-Channel Graphene Field-Effect Transistors Based on Boron-Nitride-Dielectric Encapsulation and Edge Contacts Electron Devices, IEEE Transactions on Year: 2015 (early access)

    We present the characterization of ballistic graphene field-effect transistors (GFETs) with an effective oxide thickness of 3.5 nm. Graphene channels are fully encapsulated within hexagonal boron nitride, and self-aligned contacts are formed to the edge of the single-layer graphene. Devices of channel lengths (LG) down to 67 nm are fabricated, and a virtual-source transport model is used to model the resulting current–voltage characteristics. The mobility and sourceinjection velocity as a function of LG yields a mean-free-path, ballistic velocity, and effective mobility of 850 nm, 9.3×107 cm/s, and 13 700 cm2/Vs, respectively, which are among the highest velocities and mobilities reported for GFETs. Despite these bestin- class attributes, these devices achieve transconductance (gm) and output conductance (gds) of only 600 and 300 μS/μm, respectively, due to the fundamental limitations of graphene’s quantum capacitance and zero-bandgap. gm values, which are less than those of comparable ballistic silicon devices, benefit from the high ballistic velocity in graphene but are degraded by an effective gate capacitance reduced by the quantum capacitance. The gds values, which limit the effective power gain achievable in these devices, are significantly worse than comparable silicon devices due to the properties of the zero-bandgap graphene channel.

  • Nicholas Petrone, Tarun Chari, Inanc Meric, Lei Wang, Kenneth L. Shepard, and James Hone, Flexible Graphene Field-Effect Transistors Encapsulated in Hexagonal Boron Nitride, ACS Nano, 2015, 9 (9), pp 8953–8959.

    Flexible graphene field-effect transistors (GFETs) are fabricated with graphene channels fully encapsulated in hexagonal boron nitride (hBN) implementing a self-aligned fabrication scheme. Flexible GFETs fabricated with channel lengths of 2 μm demonstrate exceptional room-temperature carrier mobility (μFE = 10 000 cm2 V-1 s-1), strong current saturation characteristics (peak output resistance, r0 = 2000 Ω), and high mechanical flexibility (strain limits of 1%). These values of μFE and r0 are unprecedented in flexible GFETs. Flexible radio frequency FETs (RF-FETs) with channel lengths of 375 nm demonstrate μFE = 2200 cm2 V-1 s-1 and r0 = 132.5 Ω. Unitycurrent gain frequencies, fT, and unitypower gain frequencies, fmax, reach 12.0 and 10.6 GHz, respectively. The corresponding ratio of cutoff frequencies approaches unity (fmax/fT = 0.9), a record value for flexible GFETs. Intrinsic fT and fmax are 29.7 and 15.7 GHz, respectively. The outstanding electronic characteristics are attributed to the improved dielectric environment provided by full hBN encapsulation of the graphene channel in conjunction with an optimized, self-aligned device structure. These results establish hBN as a mechanically robust dielectric that can yield enhanced electronic characteristics to a diverse array of graphene-based flexible electronics.