IEEE Transactions on Electron Devices

/IEEE Transactions on Electron Devices

Damage-Free ALD Blocking Oxide Layer on Functionalized Graphene Nanosheets as Nonvolatile Memories

2019-01-18T00:34:52+00:00January 17th, 2019|Categories: Publications|Tags: |

In this brief, nonvolatile memory (NVM) devices, based on the capacitor structure with the graphene nanosheets (GNSs) as the discrete charge storage media and atomic-layer-deposited (ALD) Al2O3 dielectric as the blocking oxide (BO) layer, have been demonstrated. The GNSs are formed by using the gold (Au) nanoparticles as the self-aligned mask and functionalized by using the NH3 plasma to enrich the charge trapping centers. In addition, the ALD Al2O3 layer is adopted as the BO layer of the GNS NVMs, resulting in a damage-free deposition on GNSs, compared to the conventional chemical-vapor-deposited SiO2 layer. The GNS NVM devices with a 15-nm-thick ALD Al2O3 BO layer can exhibit an excellent endurance of more than 105 cycles and superior data retention of lower than 30% charge loss for 10 years, suitable for future NVM applications.

Published in: "IEEE Transactions on Electron Devices".

Carrier Transport and Thermoelectric Properties of Differently Shaped Germanene (Ge) and Silicene (Si) Nanoribbon Interconnects

2018-12-25T00:33:42+00:00December 24th, 2018|Categories: Publications|Tags: |

Here, we report a study of carrier transport and thermoelectric properties of silicene/germanene nanoribbon (SiNR/GeNR) interconnects of varying shapes (namely, “V,” “L,” “U,” and “S” shaped) with empirical tight binding—nonequilibrium Green’s function approach. Our simulation shows that a significant reduction in both electron and phonon transmission in such differently shaped NRs compared to perfect ones. In terms of thermoelectric properties, Seebeck coefficient ($mathbb {S}$ ) variation in the range of −72.3 to $3.52~mu text{V}$ /K at 300 K for SiNR and −176 to $-textsf {95.5},,mu text{V}$ /K at 300 K for GeNR is observed in different pattern conditions. For Peltier coefficient ($pi $ ), these ranges are −0.23 to 0.001 V for SiNR and −0.053 to −0.011 V for GeNR at 300 K. Simulation also shows significant change in thermoelectric figure of merit ($textit {ZT}$ ) for a different pattern and temperature with peak $textit {ZT}~0.09$ at 800 K for “U”-shaped SiNR and 0.80 at 400 K for “L”-shaped GeNR.

Published in: "IEEE Transactions on Electron Devices".

On the ESD Behavior of Large-Area CVD Graphene Transistors: Physical Insights and Technology Implications

2018-12-25T00:33:39+00:00December 24th, 2018|Categories: Publications|Tags: |

In this paper, for the first time, the record high-performance top-gated graphene technology platform is used for electrostatic discharge (ESD) physics exploration while investigating the implications of various design and technology options. Impact of diffusive versus ballistic carrier transport on the failure mechanism in top-gate as well as back-gate graphene FET (GFET) is investigated. A unique contact limited failure in graphene transistors is reported. Physical insights on current saturation in GFET and unique step-by-step failure in dielectric capped transistors are presented for the first time. Moreover, device degradation under ESD timescales and its implications on current saturation are revealed. Finally, the influence of various top-gate designs on the ESD performance is reported. New physical insights and matured GFET technology has eventually enabled record high ESD robustness.

Published in: "IEEE Transactions on Electron Devices".

Explicit Model of Channel Charge, Backscattering, and Mobility for Graphene FET in Quasi-Ballistic Regime

2018-11-23T00:38:52+00:00November 22nd, 2018|Categories: Publications|Tags: |

Ballistic (collision free) and drift-diffusive (collision dominated) transport mechanisms are both present in graphene, and they together contribute in the current conduction in a graphene FET (GFET). In this paper, we propose an analytical drain current model based on ballistic (${n}_{B}$ ) and drift-diffusive (${n}_{D}$ ) charge densities, backscattering coefficient (${R}$ ), and quasi-ballistic mobility ($mu _{text {eff}}$ ). ${n}_{B}$ is calculated using the McKelvey flux theory and ${n}_{D}$ using the surface potential approach. A closed-form analytical expression is derived for the backscattering coefficient, which is valid under both low and high electric field conditions. The effective quasi-ballistic mobility is obtained by considering both scattering-dominated and scattering free mobilities. The proposed model is well aligned with experimental data, in all regions of operation, for single- and double-gate GFETs.

Published in: "IEEE Transactions on Electron Devices".

Selective Reduction of Oxygen Functional Groups to Improve the Response Characteristics of Graphene Oxide-Based Formaldehyde Sensor Device: A First Principle Study

2018-10-23T00:32:45+00:00October 23rd, 2018|Categories: Publications|Tags: , |

This paper presents a density functional theory-based first principle study to investigate the atomic-scale interactions of formaldehyde (H2CO) molecules with different oxygen containing functional groups of graphene oxide to identify which particular functional group possesses better adsorption capability toward H2CO molecule. The detailed study on formaldehyde adsorption has been conducted in terms of changes in structural, energetic, electronic, and transport properties of graphene oxides modeled with sp3 hybridized hydroxyl (-OH) and epoxy (C-O-C) groups on the carbon basal plane. Our computational results suggest that the graphene oxidized with only -OH group shows the highest affinity toward formaldehyde as compared with epoxy oxidized graphene and graphene oxide containing both the functional groups. The influence of vacancy defect on improving the sensing response of graphene oxide has also been studied. The results of current–voltage (${I}$ –${V}$ ) characteristics reveal that graphene oxidized with only hydroxyl group can achieve an improvement in sensitivity by almost two times and five times as compared with graphene oxide containing both the functional groups and the pristine graphene sheet, respectively. Moreover, the selectivity test for some common indoor air pollutants was also carried out and the test results suggest that H2CO molecule is highly selective toward the -OH group of graphene oxide as compared with the epoxy group.

Published in: "IEEE Transactions on Electron Devices".

3-D Interfacial Stress Decoupling Method Based on Graphene Foam

2018-10-23T00:32:39+00:00October 23rd, 2018|Categories: Publications|Tags: |

Measuring the interfacial stress between robots and objects is a prerequisite for robots to finish complicated works. In general, the interfacial stress is the 3-D interfacial stress which not only couple with the vertical normal interfacial stress but also the parallel shear interfacial stress. It is very important to develop a method to decouple the 3-D interfacial stress. This paper presents a measurement method that was able to decouple the measurement of the 3-D interfacial stress components. To implement the method, a 3-D interfacial stress sensor was fabricated based on graphene foams and superelastic materials. A high-resolution multichannel resistance measurement circuit was developed, and experiments were carried out with 3-D stress simulation equipment. The results showed that the sensor was capable of measuring z-direction normal stress at a range of 0–21 kPa with a sensitivity of 0.029 kPa−1 and x- and y-direction shear stresses at a range of 0–12.5 kPa with sensitivities of 0.020, 0.019, 0.018, and 0.019 kPa−1, and the measurement circuit was capable of measuring a range of $100,,Omega -100,,textsf {M}Omega $ with a resolution of 1%. This method can be utilized in robots to decouple the measurement of the 3-D interfacial stress components.

Published in: "IEEE Transactions on Electron Devices".

High-Quality Reconfigurable Black Phosphorus p-n Junctions

2018-10-23T00:32:37+00:00October 23rd, 2018|Categories: Publications|Tags: , , , , |

The p-n junction is the fundamental building block for electronics and photonics applications. In a conventional p-n junction, the performance parameters cannot be tuned due to the fixed doping level after ion implantation. In this paper, reconfigurable p-n or n-p junctions were built based on the ambipolar black phosphorus (BP) and other 2-D materials, namely, graphene and hexagonal boron nitride (h-BN). High-quality graphene/h-BN/BP/h-BN sandwich device was created by the dry transfer method in an inert environment. Graphene serves as the local back gate, which can tune BP partially into either n-type or p-type material. The rest of the BP channel can be controlled by the back gate. The BP encapsulated device can enable the high performance of the ideality factor down to 1.08 (p-n junction) and 1.18 (n-p junction). The device developed in this paper has a more balanced ideality factor than previously reported reconfigurable WSe2 and BP devices. By tuning the back gate and graphene gate, the BP device can be tuned into four operational quadrants, namely, n-n, n-p, p-n, and p-p junctions. Moreover, the hole mobility of BP is up to 404 cm2/$text {V}cdot text {s}$ at room temperature. Our work shows that the BP heterostructure is very promising for high-speed reconfigurable logic functions and circuits.

Published in: "IEEE Transactions on Electron Devices".

Analyzing the Effect of High-k Dielectric-Mediated Doping on Contact Resistance in Top-Gated Monolayer MoS<sub>2</sub> Transistors

2018-10-09T00:33:09+00:00October 9th, 2018|Categories: Publications|Tags: |

A scalable process that can yield low-resistance contacts to transition metal dichalcogenides is crucial for realizing a viable device technology from these materials. Here, we systematically examine the effect of high-k dielectric-mediated doping on key device metrics including contact resistance and carrier mobility. Specifically, we use top-gated transistors from monolayer MoS2 as a test vehicle and vary the MoS2 doping level by adjusting the amount of oxygen vacancies in the HfOx gate dielectric. To understand the effect of doping on the contact resistance, from a fundamental standpoint, we first estimate the doping level in monolayer MoS2. The results of our device studies quantitatively show that the reduction in contact resistance with an increase in doping is due to the doping-induced lowering of the Schottky barrier height (SBH) at the metal-semiconductor interface. Furthermore, our temperature-dependent measurements reveal that a mixture of thermionic and field emissions, even at high carrier densities, dominates carrier conduction at the contact. While our study reveals the effectiveness of dielectric-induced doping in lowering SBH, it suggests that a further reduction of SBH using alternative methods is necessary for achieving an ohmic-like contact to monolayer MoS2.

Published in: "IEEE Transactions on Electron Devices".

The Role of Nonidealities in the Scaling of MoS<sub>2</sub> FETs

2018-10-09T00:33:07+00:00October 9th, 2018|Categories: Publications|Tags: |

2-D material FETs hold the promise of excellent gate control, but the impact of nonidealities on their performance remains poorly understood. This is because of the need, so far, to use computationally intensive nonequilibrium Green’s function (NEGF) simulations.Here, we therefore use a semiclassical model to investigate the role of nonidealitiesin the scaling of back-gated(BG) and top-gated (TG) monolayer MoS2 FETs. We verify the electrostatics and transport of the semiclassical model with density functional theory-based NEGF simulations and calibrate nonidealities, such as interface traps (Dit) and Schottky contact barrier height (φSB) to experimental monolayer and bilayer MoS2 FETs. We find that among the nonidealities, Dit has the strongest subthreshold swing impact with 70 mV/dec obtainable in BG devices for a Dit of 5 × 1011 cm-2eV-1, an equivalent oxide thickness (EOT) of 1 nm, and a channel length (Lch) of 5 nm. For scaled EOT, φSB only strongly impacts ION for the TG case, as the overlapping gate thins the Schottky barriers in the BG case. We show in TG devices that a spacer of only 5 nm results in a 1000-fold drop in ION because of the nonidealities. We propose positive spacer oxide charge as a solution and show that a charge density of above 1013 cm-2 is required to fully recover the device performance.

Published in: "IEEE Transactions on Electron Devices".

All CVD Boron Nitride Encapsulated Graphene FETs With CMOS Compatible Metal Edge Contacts

2018-09-25T00:33:06+00:00September 25th, 2018|Categories: Publications|Tags: , |

We report on the fabrication and characterization of field-effect transistors (FETs) based on chemical vapor deposited (CVD) graphene encapsulated between few layer CVD boron nitride (BN) sheets with complementary metal–oxide–semiconductor (CMOS) compatible nickel edge contacts. Noncontact terahertz time-domain spectroscopy (THz-TDS) of large-area BN/graphene/ BN (BN/G/BN) stacks reveals average sheet conductivity >1 mS/sq. and average mobility of 2500 cm2/V $cdot $ s. Improved output conductance is observed in dc measurements under ambient conditions, indicating the potential for radio frequency (RF) applications. Moreover, we report a maximum voltage gain of 6 dB from a low-frequency signal amplifier circuit. RF characterization of the GFETs yields an ${f}_{T}times {L}_{g}$ product of 2.64 GHz $cdot ~mu text{m}$ and an ${f}_{{textsf {Max}}}times {L}_{g}$ product of 5.88 GHz $cdot ~mu text{m}$ . This paper presents for the first time THz-TDS usage in combination with other characterization methods for device performance assessment on BN/G/BN stacks. The results serve as a step toward scalable, all CVD 2-D material-based FETs for CMOS compatible future nanoelectronic circuit architectures.

Published in: "IEEE Transactions on Electron Devices".

Atomically Thin CBRAM Enabled by 2-D Materials: Scaling Behaviors and Performance Limits

2018-09-21T00:34:31+00:00September 21st, 2018|Categories: Publications|Tags: |

Reducing the energy and power dissipation of conductive bridge random access memory (CBRAM) cells is of critical importance for their applications in future Internet of Things (IoT) device and neuromorphic computing platforms. Atomically thin CBRAMs enabled by 2-D materials are studied theoretically by using 3-D kinetic Monte Carlo simulations together with experimental characterization. The results indicate the performance potential of attoJoule energy dissipation for intrinsic filament formation and a filament size of a single atomistic chain in such a CBRAM cell. The atomically thin CBRAM cells also show qualitatively different features from conventional CBRAM cells, including complete rupture of the filament in the reset stage and comparable forming and set voltages. The scaling and variability of the CBRAM cells down to sub-nanometer size of the switching layer as realized in the experiment are systematically studied, which indicates performance improvement and increased relative variability as the switching layer scales down. The results establish the ultimate limits of the size and energy scaling for CBRAM cells and illustrate the unique application of 2-D materials in ultralow power memory devices.

Published in: "IEEE Transactions on Electron Devices".

Modeling of Electron Devices Based on 2-D Materials

2018-09-21T00:34:29+00:00September 21st, 2018|Categories: Publications|Tags: , |

The advent of graphene and related 2-D materials has attracted the interest of the electron device research community in the past 14 years. The possibility to boost the transistor performance and the prospects to build novel device concepts with 2-D materials and their heterostructures has awakened a strong experimental interest that requires continuous support from modeling. In this paper, we review the state of the art in the simulation of electron devices based on 2-D materials. We outline the main methods to model the electronic bandstructure and to study electron transport, classifying them in terms of accuracy and computational cost. We briefly discuss, how they can be combined in a multiscale approach to provide a quantitative understanding of the mechanisms determining the operation of electron devices, and we examine the application of these methods to different families of 2-D materials. Finally, we shortly analyze the main open challenges of modeling 2-D-based electron devices.

Published in: "IEEE Transactions on Electron Devices".

<italic>Ab Initio</italic> Simulation of Band-to-Band Tunneling FETs With Single- and Few-Layer 2-D Materials as Channels

2018-09-21T00:34:28+00:00September 21st, 2018|Categories: Publications|

Full-band atomistic quantum transport simulations based on first principles are employed to assess the potential of band-to-band tunneling FETs (TFETs) with a 2-D channel material as future electronic circuit components. We demonstrate that single-layer (SL) transition metal dichalcogenides are not well suited for TFET applications. There might, however, exist a great variety of 2-D semiconductors that have not even been exfoliated yet; this paper pinpoints some of the most promising candidates among them to realize highly efficient TFETs. SL SnTe, As, TiNBr, and Bi are all found to ideally deliver ON-currents larger than 100 $mu text{A}/mu text{m}$ at 0.5-V supply voltage and 0.1 nA/$mu text{m}$ OFF-current value. We show that going from single to multiple layers can boost the TFET performance as long as the gain from a narrowing bandgap exceeds the loss from the deteriorating gate control. Finally, a 2-D van der Waals heterojunction TFET is revealed to perform almost as well as the best SL homojunction, paving the way for research in optimal 2-D material combinations.

Published in: "IEEE Transactions on Electron Devices".

Channel Thickness Optimization for Ultrathin and 2-D Chemically Doped TFETs

2018-09-21T00:34:26+00:00September 21st, 2018|Categories: Publications|Tags: , |

The 2-D material-based TFETs are among the most promising candidates for low-power electronics applications since they offer ultimate gate control and high-current drives that are achievable through small tunneling distances ($Lambda $ ) during the device operation. The ideal device is characterized by a minimized $Lambda $ . However, devices with the thinnest possible body do not necessarily provide the best performance. For example, reducing the channel thickness (${T}_{text {ch}}$ ) increases the depletion width in the source, which can be a significant part of the total $Lambda $ . Hence, it is important to determine the optimum ${T}_{text {ch}}$ for each channel material individually. In this paper, we study the optimum ${T}_{text {ch}}$ for three channel materials: WSe2, black phosphorus, and InAs using full-band self-consistent quantum transport simulations. To identify the ideal ${T}_{text {ch}}$ for each material at a specific doping density, a new analytic model is proposed and benchmarked against the numerical simulations.

Published in: "IEEE Transactions on Electron Devices".

The Understanding of SiNR and GNR TFETs for Analog and RF Application With Variation of Drain-Doping Molar Fraction

2018-09-21T00:34:24+00:00September 21st, 2018|Categories: Publications|Tags: , , |

The 1-D nanoribbon (NR) of monolayer materials has gained immense interest due to their unique properties qualitatively distinct from their bulk properties and the demand for nanoscale applications. In this paper, the quantum transport properties of two most prominent 2-D materials, i.e., silicene NR (SiNR) and a graphene NR (GNR) tunnel field-effect transistor (TFET) with the effect of different dopant molar fractions in the drain region are studied numerically using nonequilibrium Green’s function formalism. In SiNR TFET, higher on-state current (${I}_{ mathrm{scriptscriptstyle ON}}$ ) is observed due to wider tunneling energy window and high transmission probability of carriers. In order to observe the effect of variation of doping density in the drain region, we have studied analog figures of merit such as the transconductance (${g}_{m}$ ), output resistance (${r}_{o}$ ), transconductance generation factor (${g}_{m}/{I}_{D}$ ), and the intrinsic gain (${g}_{m}{r}_{o}$ ) for different molar fractions. Similarly, we have evaluated the RF performance of the SiNR and GNR TFETs as a function of cutoff frequency (${f}_{T}$ ), gate capacitance (${C}_{G}$ ), and transport delay ($tau$ ).

Published in: "IEEE Transactions on Electron Devices".

An Intuitive Equivalent Circuit Model for Multilayer Van Der Waals Heterostructures

2018-09-21T00:34:22+00:00September 21st, 2018|Categories: Publications|Tags: , , |

Stacks of 2-D materials, known as van der Waals (vdW) heterostructures, have gained vast attention due to their interesting electrical and optoelectronic properties. This paper presents an intuitive circuit model for the out-of-plane electrostatics of a vdW heterostructure composed of an arbitrary number of layers of 2-D semiconductors, graphene, or metal. We explain the mapping between the out-of-plane energy band diagram and the elements of the equivalent circuit. Although the direct solution of the circuit model for an n-layer structure is not possible due to the variable, nonlinear quantum capacitance of each layer, this paper uses the intuition gained from the energy band picture to provide an efficient solution in terms of a single variable. Our method employs Fermi–Dirac statistics while properly modeling the finite density of states (DOS) of 2-D semiconductors and graphene. The approach circumvents difficulties that arise in commercial TCAD tools, such as the proper handling of the 2-D DOS and the simulation boundary conditions when the structure terminates with a nonmetallic material. Based on this methodology, we have developed 2dmatstacks, an open-source tool freely available on Overall, this paper equips researchers to analyze, understand, and predict experimental results in complicated vdW stacks.

Published in: "IEEE Transactions on Electron Devices".

Intrinsic Performance of Germanane Schottky Barrier Field-Effect Transistors

2018-09-21T00:34:21+00:00September 21st, 2018|Categories: Publications|Tags: , |

Germanane (GeH), a hydrogenated germanium monolayer, is a new family of 2-D semiconductors, exhibiting promising potential for electronic device applications. Here, we investigate GeH Schottky barrier (SB) field-effect transistors (FETs) using atomistic quantum transport simulations. Our simulation results reveal that the ohmic-contact device with zero SB height ($Phi _{text {Bn}}$ ) exhibits ~20% lower ON-current than the metal-oxide-semiconductor (MOS) FET counterpart due to the inherent tunnel barrier at the metal–semiconductor junction. We also compare 14-nm-channel GeH and black phosphorus (BP) SBFETs with a finite SB height of $Phi _{text {Bn}} = 0.22$ eV for both devices. Our results show that GeH outperforms BP in the ON-state, but it can suffer from larger leakage current in the OFF-state. We further investigate the effect of barrier height in GeH SBFET by varying $Phi _{text {Bn}}$ from 0 eV to a half bandgap (0.78 eV). In general, as barrier height increases, both ON-current and the minimum leakage current are reduced. It is also observed that, with increasing SB height, intrinsic delay increases but the required energy per switching decreases, indicating the trade-off between the device speed and the energy dissipation. Our benchmarking of GeH SBFET against GeH and BP MOSFETs demonstrates that GeH generally outperforms BP in terms of energy-delay product (EDP). By performing careful engineering of SB height along with the device threshold voltage, we show that the minimum EDP of GeH SBFET can be as comparable as that of the MOSFET counterpart, suggesting great potential of GeH SBFETs for future switching

Published in: "IEEE Transactions on Electron Devices".

Low-Frequency Noise in Supported and Suspended MoS<sub>2</sub> Transistors

2018-09-21T00:34:18+00:00September 21st, 2018|Categories: Publications|Tags: |

Sensitivity of sensors, the phase noise of oscillators, and intrinsic device performance at the nanoscale are primarily limited by fluctuations in resistance at low frequencies, also called flicker noise. These intrinsic fluctuations in resistance become more prominent in two-dimensional materials due to their ultrathin nature. Here, we report the low-frequency noise (LFN) behavior of supported and suspended MoS2 FETs. Carrier-number fluctuation mechanism was found to be responsible for LFN in both configurations. The extracted frequency index ($gamma $ in 1/${f}^gamma $ ) indicates the different trap time constants and mechanisms responsible for LFN in MoS2 transistors. A $gamma $ value of 2 in suspended transistors indicates a tight trap time-constant distribution likely due to recombination–generation through trap states arising only from intrinsic defects and/or extrinsic adsorbates. A wide time-constant distribution ($gamma !sim textsf {1}$ to 1.4) in supported transistors indicates the presence of various kinds of traps, such as adsorbates, intrinsic defects, as well as interface and oxide traps. Furthermore, variation in S0 (characterizes the magnitude of noise) with gate voltage in both supported and suspended device configurations was validated analytically using the carrier-number fluctuation model.

Published in: "IEEE Transactions on Electron Devices".

Complementary Black Phosphorus Nanoribbons Field-Effect Transistors and Circuits

2018-09-21T00:34:11+00:00September 21st, 2018|Categories: Publications|Tags: |

This paper demonstrates a high-performance black phosphorus nanoribbons field-effect transistor (BPNR-FET) and systematically investigates methods for enhancing its anisotropic carrier transport. The BPNR-FET shows a strong dependence on crystal orientation in which the best mobility performance is achieved in armchair-oriented nanoribbons. A downscaling of nanoribbon width is shown to improve the short-channel effect owing to a better electrostatic gate control. Furthermore, hydrogenation is employed to effectively passivate the dangling bonds and heal the nanoribbon edge defects, leading to nearly hysteresis-free transfer properties. By virtue of bandgap and contact-metal workfunction engineering, n-type BPNR-FET is successfully demonstrated, which enables complementary inverter circuits to be simultaneously realized. This paper unravels the superior performance underscores a conceptually new BPNR-FET, paving the way toward the development of non-planar devices and integrated circuits based on 2-D materials platform.

Published in: "IEEE Transactions on Electron Devices".

Gate-Tuned Temperature in a Hexagonal Boron Nitride-Encapsulated 2-D Semiconductor Device

2018-09-21T00:34:07+00:00September 21st, 2018|Categories: Publications|Tags: , , |

Thermal management in 2-D electronics is critical for optimizing their performances due to the growing heat generations at the nanoscale. Here, we use Raman thermometry to study the Joule heating of a MoS2 field-effect transistor encapsulated by hexagonal boron nitride and with graphene electrodes. We show a sensitive temperature increase relevant to the heating power, which is tunable by the global back gate as well as the drain voltage. Our results provide a simple approach to characterizing the heat dissipation of microscopic devices and to controlling their temperatures.

Published in: "IEEE Transactions on Electron Devices".

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