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We present a first-principles model to study tunnel transistors based on van der Waals heterojunctions of 2D materials in the presence of dissipative mechanisms due to the electron-phonon interaction. To this purpose we employed a reduced basis set composed of unit-cell restricted Bloch functions computed with a plane wave ab-initio solver and performed self-consistent quantum transport simulations within the non-equilibrium Green’s functions formalism. Phonon scattering was included with specific self-energies making use of the deformation potential approximation for the electron-phonon coupling. Our simulations identify the van der Waals tunnel FET as a promising option to attain high on-state currents at low supply voltages, but also show a strong impact of the phonon scattering on the transport properties of such device in the sub-threshold regime.

International audience

We present an efficient simulation method for electronic transport and avalanche in single-photon avalanche diodes (SPAD). Carrier transport is simulated in the real space using a particle Monte Carlo approach based on the Fokker–Planck point of view on an advection-diffusion equation, that enables us to reproduce mobility models, including electric fields and doping dependencies. The avalanche process is computed thanks to impact ionization rates implemented using a modified Random Path Length algorithm. Both transport and impact ionization mechanisms are computed concurrently from a statistical point of view, which allows us to achieve a full multi-particle simulation. This method provides accurate simulation of transport and avalanche process suitable for realistic three-dimensional SPADs, including all relevant stochastic aspects of these devices, together with a huge reduction of the computational time required, compared to standard Monte Carlo methods for charge carrier transport. The efficiency of our method empowers the possibility to precisely evaluate SPADs figures of merit and to explore new features that were untrackable by conventional methods. An extensive series of comparisons with experimental data on state-of-the art SPADs shows a very good accuracy of the proposed approach.

International audience; The statistical behavior of silicon-based single-photon-avalanche-diodes (SPADs) is investigated by using self-consistent 3-D Monte Carlo simulations. The coupling of Poisson and Boltzmann transport equations allows us to go beyond the analysis of avalanche breakdown and its timing and to extend the investigation to the quenching of the photodetector circuit. We find out that the quenching of SPADs is probabilistic and strongly depends on the surrounding circuit, in particular on the socalled quenching resistance. Independently of the SPAD deadtime, it appears that the extinction time needed to suppress any avalanche event may vary over a very large range.

Single Photon Avalanche Diodes (SPAD) are key optoelectronic detectors for medical imaging, camera ranging and automotive laser imaging detection and ranging (LiDAR) applications. Today, most of SPADs in the Time Of Flight (TOF) market are composed of a micrometric Silicon PN junction associated to a proximity CMOS electronics biasing the system above the breakdown voltage. These devices present low noise, high pixel-matrix integration capabilities, but their Photon Detection Efficiency (PDE) is relatively modest in the near infrared operating region (achieving typically only few percent at a wavelength of 940 nm). A considerable optimisation of the SPAD design is currently on going at an industrial level with a view of increasing the PDE without compromising the timing statistic response to avalanche (Jitter) and its quench probability. Within these perspectives, standard commercial Technology-Computer-Assisted-Design (TCAD) simulations can only provide a limited guidance for technological and design splits optimisation since they are based on deterministic solvers that are unable to capture these stochastic figures of merit. Since the seminal work from Spinelli [1] clearly showing that Monte Carlo simulations predictions can compare favourably with experimental measurements of PDE and timing resolution, the Monte Carlo method can be considered to be a useful one for the design of improved structures. It has been recently applied to optimize the PDE and Jitter of Silicon [3] and InGaAs SPADs [4],[5]. In this abstract we report a rigorous comparison between Monte Carlo predictions and measurements of PDE and Jitter. We also discuss in detail, the quench probability of these diodes once in avalanche. This latter point has rarely been discussed in literature and to the best of our knowledge never addressed within a Monte Carlo perspective.

International audience

A Full Band Monte Carlo simulatorhas been developed to considerphonon transmission across interfaces disposedperpendicularlyto the heat flux. This solver of the Boltzmann transport equation does not require any assumption on the shape the phonon distribution and can naturally consider all phonon transport regimes from the diffusive to the fully ballistic regime. This simulatoris used to study single and double Si/Ge heterostructures from the micrometer scale downto the nanometer scale,i.e. in all phonon transport regime from fully diffusive toballistic.A methodology to determine the thermal conductivity atthermal interfaces is presented.

International audience; A two-valley formulation of 1D drift-diffusion transport is presented that takes the coupling between the valleys into account via a new approximation for the non-local electric field. The proposed formulation is suitable for the simulation of III-V heterojunction bipolar transistors as opposed to formulations that employ the single electron gas approximation with a modified velocity-field model, which also causes convergence problems. Based on Boltzmann transport equation simulations, model parameters of the proposed two-valley formulation are given for GaAs, InP, InAs and GaSb at room temperature. Applications of the new formulation are also demonstrated.

International audience; This work presents the performance degradation of a 32-bit DSP fabricated with 28 nm FDSOI technology subjected to different levels of gamma radiation. A new mixed hardening methodology based on electrical compensation and thermal regeneration is applied to the irradiated devices to recover the degraded electrical characteristics induced by the total ionizing dose. The application of this new methodology seeks to extend the lifetime of FDSOI integrated circuits under high radiation conditions while keeping good reliability.

The availability of transistors capable of operating at low supply voltage is essential to improve the key performance metric of computing circuits, i.e., the number of operations per unit energy. In this paper, we propose a device concept for energy-efficient, steep-slope transistors based on heterojunctions of two-dimensional (2D) materials. We show that by injecting electrons from an isolated and weakly dispersive band into a strongly dispersive one, subthermionic subthreshold swings can be obtained, as a result of a cold-source effect and of a reduced thermalization of carriers. This mechanism is implemented by integrating in a metal-oxide-semiconductor field-effect transistor (MOSFET) architecture two different monolayer materials coupled through a van der Waals heterojunction, combining the subthermionic behavior of tunnel field-effect transistors (FETs) with the robustness of a MOSFET architecture against performance-degrading factors, such as traps, band tails, and roughness. A further advantage with respect to tunnel FETs is that only an n-type or p-type doping is required to fabricate the device. In order to demonstrate the device concept and to discuss the underlying physics and the design options, we study through ab initio and full-quantum transport simulations a possible implementation that exploits two recently reported 2D materials., Physical Review Research, 2 (4), ISSN:2643-1564

International audience; Our home made Full-Band particle Monte Carlo is used to investigate the thermal interface conductance at Silicon/Germanium heterojunctions.

We present a Verilog-A model accounting for the temporal avalanche buildup and its statistics in Single-Photon Avalanche Diodes (SPADs). This physics-based approach is compared to TCAD mixed-mode analyzing predictions, as well as measurements. The buildup that can be in the order of hundreds picoseconds, affects the statistical pulse width distribution, which is experimentally verified. Furthermore, we address in detail the voltage swing across the device during avalanche and its quenching, studying its impact on power consumption. This model can help a chip designer to optimize circuits for quenching the SPAD photodiode.

Gaussian deformation in graphene structures exhibits an interesting effect in which flower-shaped confinement states are observed in the deformed region [Carrillo-Bastos et al., Phys. Rev. B 90 041411 (2014)]. To exploit such a deformation for various applications, tunable electronic features including a bandgap opening for semi-metallic structures are expected. Besides, the effects of disorders and external excitations also need to be considered. In this work, we present a systematic study on quantum transport of graphene ribbons with Gaussian deformation. Different levels of deformation are explored to find a universal behavior of the electron transmission. Using a tight-binding model in combination with Non-Equilibrium Green Functions formalism, we show that Gaussian deformation influences strongly the electronic properties of ribbons in which the electron transmission decreases remarkably in high energy regions even if small deformations are considered. Interestingly, it unveils that the first plateau of the transmission of semi-metallic armchair ribbons is just weakly affected in the case of small deformations. However, significant large Gaussian bumps can induce a strong drop of this plateau and a transport gap is formed. The transmission at the zero energy is found to decrease exponentially with increasing the size of the Gaussian bump. Moreover, the gap of semi-conducting ribbons is enlarged with large deformations. The opening or the widening of the transport gap in large deformed armchair structures is interpreted by a formation of a three-zone behavior along the transport direction of the hopping profile. On the other hand, a transport gap is not observed in zigzag ribbons regardless of the size of Gaussian bumps. This behavior is due to the strong localization of edge states at the energy point E = 0..., 25 pages, 12 figures

A simple and easy-to-handle semi-analytical model able to describe heat transport in heterostructures of length varying from the nano to the microscale is presented. It consists in redefining three intrinsic parameters: the ballistic thermal conductance, the effective thermal conductivity, and the interface thermal conductance by using two temperatures T + and T − distinguishing the phonon populations according to the direction of their velocities instead of the standard pseudo-temperature T. The resulting model agrees well with the thermal conductance and temperature profiles predicted by advanced Monte Carlo simulation in all phonon transport regimes, i.e. diffusive, ballistic, and quasi-ballistic regimes, even in the presence of multiple interfaces. It is able to provide new insights into the interpretation of thermal properties of complex nanostructures.

International audience; We study the effects of isotope doping and vacancies on the electronic, phononic and thermoelectric properties of graphene nano-ribbons by means of atomistic simulation based on semi-empirical Tight Binding and Force Constant models combined with the Non-Equilibrium Green's Function formalism of transport. We demonstrate that from the phononic perspective, the impact of isotope doping on transmission is very dependent on the phonon frequency and particularly strong at high frequency, while vacancies wield their influence on the whole frequency range. From the electronic point of view, if placed at dimer line positions m = 3i + 1, 3i + 2, vacancies induce a strong reduction of the electrical conductance while vacancies at positions m = 3i act as isotopes, i.e. they are transparent to electrons and do not affect the electronic properties. The thermoelectric study is conducted by combining both electronic and phononic properties. It demonstrates the possibility of a remarkable enhancement of figure of merit ZT. In the presence of both types of defects, ZT can be enhanced significantly from 0.26 in pristine ribbons to values larger than 2.5 for ribbons of width M = 5.

Single charge electronics offer a way for disruptive technology in nanoelectronics. Coulomb blockade is a realistic way for controlling the electric current through a device with the accuracy of one electron. In such devices the current exhibits a step-like increase upon bias which reflects the discrete nature of the fundamental charge. We have assembled a double tunnel junction on an oxide-free silicon substrate that exhibits Coulomb staircase characteristics using gold nanoparticles (AuNPs) as Coulomb islands. The first tunnel junction is an insulating layer made of a grafted organic monolayer (GOM) developed for this purpose. The GOM also serves for attaching AuNPs covalently. The second tunnel junction is made by the tip of an STM. We show that this device exhibits reproducible Coulomb blockade I-V curves at 40 K in vacuum. We also show that depending on the doping of the silicon substrate, the whole Coulomb staircase can be adjusted. We have developed a simulation approach based on the orthodox theory that was completed by calculating the bias dependent tunnel barriers and by including an accurate calculation of the band bending. This model accounts for the experimental data and the doping dependence of Coulomb oscillations. This study opens new perspectives toward designing new kind of single electron transistors (SET) based on this dependence of the Coulomb staircase with the charge carrier concentration.

In-plane heterojunction tunnel field effect transistors based on monolayer transition metal dichalcogenides are studied by means of self-consistent non-equilibrium Green's functions simulations and an atomistic tight-binding Hamiltonian. We start by comparing several heterojunctions before focusing on the most promising ones, i.e WTe2-MoS2 and MoTe2-MoS2. The scalability of those devices as a function of channel length is studied, and the influence of backgate voltages on device performance is analysed. Our results indicate that, by fine-tuning the design parameters, those devices can yield extremely low sub-threshold swings (below 5mV/decade) and Ion/Ioff ratios higher than 1e8 at a supply voltage of 0.3V, making them ideal for ultra-low power consumption., Comment: 10 pages

International audience; I. Abstract The Boltzmann transport equation is one of the most relevant framework to study the heat transport at the nanoscale, beyond the diffusive regime and up to the micrometer-scale. In the general case of three-dimensional devices, the particle Monte Carlo approach of phonon transport is particularly powerful and convenient, and requires reasonable computational resources. In this work, we propose an original and versatile particle Monte Carlo approach parametrized by using ab-initio data. Both the phonon dispersion and the phonon-phonon scattering rates have been computed by DFT calculation in the entire 3D Brillouin zone. To treat the phonon transport at rough interfaces, a combination of specular and diffuse reflections has been implemented in phase space. Thermal transport has been investigated in nanowires and thin films made of cubic and hexagonal Silicon, including edge roughness, in terms of effective thermal conductivity, phonon band contributions and heat flux orientation. It is shown that the effective thermal conductivity in quasi-ballistic regime obtained from our Monte Carlo simulation cannot be accurately fitted by simple semi-analytical Matthiessen-like models and that spectral approaches are mandatory to get good results. Our Full Band approach shows that some phonon branches exhibiting a negative group velocity in some parts of the Brillouin zone may contribute negatively to the total thermal flux. Besides, the thermal flux clearly appears to be oriented along directions of high density of states. The resulting anisotropy of the heat flux is discussed together with the influence of rough interfaces. II. Introduction The optimization of the thermoelectric conversion is an active subject of research. The main applications are related to energy harvesting for power supply autonomous systems as well as heat management in CPU cooling. Yet, common and efficient thermoelectric materials such as Bismuth Telluride, Lead Telluride, etc., often rely on unfortunately rare and toxic compounds. Their replacement by Silicon and Germanium which are more abundant and widely used in microelectronics would be appealing if their naturally poor thermoelectric properties could be significantly improved, especially close to room temperature. The thermoelectric efficiency of a material is characterized by a unitless thermoelectric figure of merit í µí±í µí±‡ = í µí±† 2 í µí¼Ží µí±‡/í µí¼ that depends on the electrical conductivity í µí¼Ž, the Seebeck coefficient S and the thermal conductivity í µí¼. To improve the conversion efficiency, í µí±í µí±‡ has to be increased. Consequently, í µí¼ must be reduced while í µí¼Ž must be preserved as far as possible. As these two parameters are strongly interdependent in common bulk materials, ZT optimization remained very limited for decades[1]. However, nanotechnologies provide new routes to optimize the thermoelectric conversion[2] For instance, nanostructures with a characteristic width í µí±Š both larger than the mean free path of electrons í µí±™ í µí±ší µí±“í µí± í µí±’ and smaller than the mean free path of phonons í µí±™ í µí±ší µí±“í µí± í µí±ℎ (í µí±™ í µí±ší µí±“í µí± í µí±’ < í µí±Š < í µí±™ í µí±ší µí±“í µí± í µí±ℎ) can be used to specifically control and limit the effective phonon mean free path. This way, since heat transfer in non-degenerate semiconductors is mainly due to phonons, higher ratios í µí¼Ž/í µí¼ can be achieved and finally ZT can be significantly higher in such nanomaterials than in their bulk counterpart. For instance, experimental measurements in Silicon nanowires of appropriate diameters[3] i.e. on the order of 100 nm, have demonstrated a large reduction of the effective thermal

International audience; A Radiation hardened temperature controller intended to regenerate CMOS circuits has been designed using commercial off-the-shelf components. Tested with $^{60}$Co gamma rays, it has shown a radiation hardness greater than 65 kGy. The hardening methodology used to design the temperature controller is validated through these results.

By means of advanced numerical simulation, the thermoelectric properties of a Si-quantum dot-based single-electron transistor working in sequential regime are investigated using our homemade simulator that self-consistently couples the 3D Poisson, Schrodinger and Master equations. At high voltage bias, the multi-level effects are shown to induce non-linear characteristics of the heat current. Additionally, the single-electron transistor operating in generator regime is shown to exhibit very good efficiency at maximum power.

Graphite nanofibers (GNFs) have been demonstrated to be a promising material for hydrogen storage and heat management in electronic devices. Here, by means of first-principles and transport simulations, we show that GNFs can also be an excellent material for thermoelectric applications thanks to the interlayer weak van der Waals interaction that induces low thermal conductance and a step-like shape in the electronic transmission with mini-gaps, which are necessary ingredients to achieve high thermoelectric performance. This study unveils that the platelet form of GNFs in which graphite layers are perpendicular to the fiber axis can exhibit outstanding thermoelectric properties with a figure of merit ZT reaching 3.55 in a 0.5 nm diameter fiber and 1.1 for a 1.1 nm diameter one. Interestingly, by introducing 14C isotope doping, ZT can even be enhanced up to more than 5, and more than 8 if we include the effect of finite phonon mean-free path, which demonstrates the amazing thermoelectric potential of GNFs., 25 pages, 6 figures

International audience; This paper presents theoretical investigations on the interfacial thermal conductance (Kapitza conductance) in both monotype Si/Ge (cubic 3C) and polytype (cubic 3C / hexagonal 2H) Ge interfaces by using Full Band extensions of Diffusive (DMM) and Acoustic (AMM) Mismatch Models. In that aims, phonon dispersions in the Full 3D Brillouin Zone have been computed via an atomistic adiabatic bond charge model. The effects of crystal orientation are investigated and the main phonon modes involved in heat transfer are highlighted. According to our calculations, polytype interfaces without any mass mismatch but with a crystallographic phase mismatch exhibit a thermal conductance very close to that of Si/Ge interfaces with a mass mismatch but without any phase mismatch. Besides, the orientations of Ge polytype interface that have been observed experimentally in nanowires, i.e. along   115 / 5051   , exhibit the lowest interfacial conductance and thus may offer new opportunities for nanoscale thermoelectric applications.

By means of advanced numerical simulation, the thermoelectric properties of a Si-quantum dot-based single-electron transistor operating in sequential tunneling regime are investigated in terms of figure of merit, efficiency and power. By taking into account the phonon-induced collisional broadening of energy levels in the quantum dot, both heat and electrical currents are computed in a voltage range beyond the linear response. Using our homemade code consisting in a 3D Poisson-Schrödinger solver and the resolution of the Master equation, the Seebeck coefficient at low bias voltage appears to be material independent and nearly independent on the level broadening, which makes this device promising for metrology applications as a nanoscale standard of Seebeck coefficient. Besides, at higher voltage bias, the non-linear characteristics of the heat current are shown to be related to the multi-level effects. Finally, when considering only the electronic contribution to the thermal conductance, the single-electron transistor operating in generator regime is shown to exhibit very good efficiency at maximum power.

International audience; —Memristive nanodevices can feature a compact multi-level non-volatile memory function, but are prone to device variability. We propose a novel neural network-based computing paradigm, which exploits their specific physics, and which has virtual immunity to their variability. Memristive devices are used as synapses in a spiking neural network performing unsupervised learning. They learn using a simplified and customized " spike timing dependent plasticity " rule. In the network, neurons' threshold is adjusted following a homeostasis-type rule. We perform system level simulations with an experimentally verified-model of the memristive devices' behavior. They show, on the textbook case of character recognition, that performance can compare with traditional supervised networks of similar complexity. They also show that the system can retain functionality with extreme variations of various memristive devices' parameters (a relative standard dispersion of more than 50% is tolerated on all device parameters), thanks to the robustness of the scheme, its unsupervised nature, and the capability of homeostasis. Additionally the network can adjust to stimuli presented with different coding schemes, is particularly robust to read disturb effects and does not require unrealistic control on the devices' conductance. These results open the way for a novel design approach for ultra-adaptive electronic systems.

III-V Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) with a gate stack based on high-kappa dielectric appears as an appealing solution to increase the performance of either microwave or logic circuits with low supply voltage (V(DD)). The main objective of this work is to provide a theoretical model of the gate charge control in III-V MOS capacitors (MOSCAPs) using the accurate self-consistent solution of 1D and 2D Poisson-Schrödinger equations. This study allows us to identify the major mechanisms which must be included to get theoretical calculations in good agreement with experiments. Actually, our results obtained for an Al2O3/In0.53Ga0.47As MOSCAP structure are successfully compared to experimental measurements. We evaluate how III-V MOS technology is affected by the density of interface states which favors the Fermi level pinning at the Al2O3/In0.53Ga0.47As interface in both depletion and inversion regimes, which is a consequence of the poor gate control of the mobile inversion carrier density. The high energy valleys (satellite valleys) contribution observed in many theoretical calculations appears to be fully negligible in the presence of interface states. The enhancement of doping density in the channel is shown to improve the short-channel effect (SCE) immunity but to the price of higher sensitivity to the interface trap effect which manifests through a low Fermi level movement efficiency at interface in OFF-state and a low inversion carrier density in ON-state, even in the long channel case.

The thermoelectric properties of defected graphene nanoribbons (GNRs) and multi-junction (MJ) GNRs coupling periodic armchair sections of different width are analyzed by means of Green's function techniques to simulate electron and phonon transport. Among the different strategies likely to enhance the thermoelectric performance, the effects of edge disorder and random vacancies are shown to be small since they lead to the concomitant degradation of the phonon thermal conductance and of the electronic conductance, which finally reduces the thermoelectric factor ZT. However, the periodic distribution of vacancies and the structuring of GNRs in MJ-GNRs both lead to the enhancement of the figure of merit ZT. In the latter case, in addition to the strong reduction of the phonon thermal conductance, an effect of resonant tunneling of electrons allows retaining high electronic conductance and enhancing significantly the thermopower. Finally, by introducing a periodic distribution of vacancies in the MJ-GNR, the maximum value ZT=0.4 is reached at room temperature.

We present a study of the giant piezoresistance effect in p-type silicon using full-band Monte Carlo simulation based on 30-band k.p calculation. This effect has been demonstrated experimentally in Si nanowires by He and Yang. By including the well-known strain effect on the band structure, and by introducing a law of variation of the surface potential according to the applied mechanical stress, we can reproduce this effect. This variation of surface potential modulates the depletion depth and then the conductivity of the structure. This modulation induces a strong variation of the total amount of carriers available for the conduction, which increases drastically this piezoresistive effect. This is probably the main origin of this effect, which may be used to achieve high performance MEMS sensors.

III-V FETs (Field Effect Transistor) with high-@k gate dielectric stack are expected to exhibit weak gate leakage and high RF performance. Hence, they could fulfil high-speed and low-power specifications for low operating power technology. Using a Poisson-Schrodinger solver and an ensemble Monte Carlo device simulator, MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) and HEMT (High Electron Mobility Transistor) structures are investigated in terms of gate charge control and both static and dynamic performance. The quasi-static RF performance estimations are confirmed by accurate Y parameter extraction from transient current analysis.

In this paper, we review and extend our recent works based on the Monte Carlo method to solve the Wigner-Boltzmann transport equation and model semiconductor nanodevices. After presenting the different possible approaches to quantum mechanical modelling, the formalism and the theoretical framework are described together with the particle Monte Carlo implementation using a technique fully compatible with semiclassical simulation. Examples are given to highlight the importance of considering both quantum and scattering effects in nanodevices operating at room temperature, such as resonant tunnelling diode (RTD), double-gate MOSFET and carbon nanotube FET. Quantum and semiclassical approaches are compared for transistor simulation. Finally, the phonon-induced electron decoherence in RTD and MOSFET is examined through the analysis of the density matrix elements computed from the Wigner function. This formalism is shown to be relevant for the quantitative analysis of devices operating in mixed quantum/semiclassical regime and to understand the transition between both regimes or between coherent and sequential tunnelling processes.

A Pearson Effective Potential model for including quantization effects in the simulation of nanoscale nMOSFETs has been developed. This model, based on a realistic description of the function representing the non zero-size of the electron wave packet, has been used in a Monte-Carlo simulator for bulk, single gate SOI and double-gate SOI devices. In the case of SOI capacitors, the electron density has been computed for a large range of effective field (between 0.1 MV/cm and 1 MV/cm) and for various silicon film thicknesses (between 5 nm and 20 nm). A good agreement with the Schroedinger-Poisson results is obtained both on the total inversion charge and on the electron density profiles. The ability of an Effective Potential approach to accurately reproduce electrostatic quantum confinement effects is clearly demonstrated.

A Multi Sub-band Monte Carlo Simulator improved to efficiently include the Pauli Exclusion Principle is presented. It is used to study the transport in highly doped and ultra-thin silicon film. Both steady state and transient regime of transport for silicon films under uniform driving field are investigated. Such approach aims to be carried out in a full device simulator to improve the modeling of the access region of nano-Double Gate MOSFETs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)