Thermally activated sub-threshold transport has been investigated in undoped triple gate MOSFETs. The evolution of the barrier height and of the active cross-section area of the channel as a function of gate voltage has been determined. The results of our experiments and of the Tight Binding simulations we have developed are both in good agreement with previous analytical calculations, confirming the validity of thermionic approach to investigate transport in FETs. This method provides an important tool for the improvement of devices characteristics.
We report on DC and microwave electrical transport measurements in silicon-on-insulator CMOS nano-transistors at low and room temperature. At low source-drain voltage, the DC current and RF response show signs of conductance quantization. We attribute this to Coulomb blockade resulting from barriers formed at the spacer-gate interfaces. We show that at high bias transport occurs thermionically over the highest barrier: Transconductance traces obtained from microwave scattering-parameter measurements at liquid helium and room temperature is accurately fitted by a thermionic model. From the fits…
This work is the first step towards understanding thermionic transport properties of graphene/phosphorene/graphene van der Waals heterostructures in contact with gold electrodes by using density functional theory based first principles calculations combined with real space Green's function formalism. We show that for monolayer phosphorene in the heterostructure, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling dominated transport to thermionic dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing …
Vertical heterostructures of van der Waals materials enable new pathways to tune charge and energy transport characteristics in nanoscale systems. We propose that graphene Schottky junctions can host a special kind of photoresponse which is characterized by strongly coupled heat and charge flows that run vertically out of the graphene plane. This regime can be accessed when vertical energy transport mediated by thermionic emission of hot carriers overwhelms electron-lattice cooling as well as lateral diffusive energy transport. As such, the power pumped into the system is efficiently extracted…
Transport in photoactive graphene heterostructures, originating from the dynamics of photogenerated hot carriers, is governed by the processes of thermionic emission, electron-lattice thermal imbalance and cooling. These processes give rise to interesting photoresponse effects, in particular negative differential resistance (NDR) arising in the hot-carrier regime. The NDR effect stems from a strong dependence of electron-lattice cooling on the carrier density, which results in the carrier temperature dropping precipitously upon increasing bias. The ON-OFF switching between the NDR regime and t…
This paper is devoted to photocarrier transport across a two-dimensional graphene-semiconductor Schottky junction. We study linear response to monochromatic light with excitation energy well below the semiconductor band gap. The operation mechanism relies on both photoelectric and thermionic emission from graphene to a two-dimensional semiconductor under continuous illumination and zero bias. Due to the thermalization bottleneck for low-energy carriers in graphene, the photoelectric contribution is found to dominate the photoresponse at near-infrared excitation frequencies and below. The exten…
Over the past years, thermionic energy conversion (TEC) with a reduced inter-electrode vacuum gap has been studied as an effective way to mitigate a large potential barrier due to space charge accumulation. However, existing theoretical models do not fully consider the fundamental aspects of thermionic emission when the inter-electrode gap shrinks to the nanoscale, which results in underestimation of thermionic power generation for such small gaps. The present work addresses this challenge by comprehensively modeling charge and thermal transport processes with specific consideration of nanosca…
We report a study of transport blockade features in a quantum dot single-electron transistor, based on an undoped AlGaAs/GaAs heterostructure. We observe suppression of transport through the ground state of the dot, as well as negative differential conductance at finite source-drain bias. The temperature and magnetic field dependence of these features indicate the couplings between the leads and the quantum dot states are suppressed. We attribute this to two possible mechanisms: spin effects which determine whether a particular charge transition is allowed based on the change in total spin, an…
We demonstrate the detection of cyclotron resonance in graphene by using a photo-induced thermionic emission mechanism at the graphene/MoS2 van der Waals (vdW) Schottky junction. At cyclotron resonance in Landau-quantized graphene, the infrared light is absorbed and an electron-hole pair is generated. When the energy of a photoexcited electron exceeds the band offset energy at the graphene/MoS2 interface, the electron transfer occurs from graphene to the conduction band of MoS2, and the hole remains in graphene. This creates an electron-hole separation at the graphene/MoS2 interface at cyclotr…
Layer interdependence of transport in an undoped electron-hole bilayer
研究了非掺杂电子空穴双层（uEHBL）器件中载流子输运的层间依赖性与载流子密度、层间电场和温度的关系。uEHBL器件由密度可调的、独立接触的二维电子气（2DEG）和二维空穴气（2DHG）组成，所述二维电子气和二维空穴气通过由30 nm Al $_{0.9}$ Ga $_{0.1}$ As势垒分隔的不同GaAs量子阱中的场效应诱导。运输测量同时进行的每一层上使用范德堡方法。观察到随着2DEG密度的增加2DHG迁移率增加，而2DEG迁移率对2DHG密度的依赖性可以忽略不计。降低层间电场并由此增加层间分离也增加了2DHG迁移率，而对2DEG迁移率的影响可以忽略不计。利用2DHG库仑阻力测量估计了层间电场变化时层间分离的变化。结果与各层的迁移率仅间接依赖于相邻层的密度并由背景杂质散射主导一致。温度依赖性也被确定为每个层的电阻率。
We report on the electronic transport properties of multiple-gate devices fabricated from undoped silicon nanowires. Understanding and control of the relevant transport mechanisms was achieved by means of local electrostatic gating and temperature dependent measurements. The roles of the source/drain contacts and of the silicon channel could be independently evaluated and tuned. Wrap gates surrounding the silicide-silicon contact interfaces were proved to be effective in inducing a full suppression of the contact Schottky barriers, thereby enabling carrier injection down to liquid-helium tempe…
Charge carriers in graphene are chiral quasiparticles ("massless Dirac fermions"). Graphene provides therefore an amazing opportunity to study subtle quantum relativistic effects in condensed matter experiment. Here I review a theory of one of these unusual features of graphene, a "pseudodiffusive" transport in the limit of zero charge carrier concentration, which is related to existence of zero-modes of the Dirac operator and to the Zitterbewegung of unltrarelativistic particles. A conformal mapping technique is a powerful mathematical tool to study these phenomena, as demonstrated here, usin…
This paper discusses how classical transport theories such as the thermionic emission, can be used as a powerful tool for the study and the understanding of the most complex mechanisms of transport in Fin Field Effect Transistors (FinFETs). By means of simple current and differential conductance measurements, taken at different temperatures and different gate voltages ($V_G$'s), it is possible to extrapolate the evolution of important parameters such as the spatial region of transport and the height of thermionic barrier at the centre of the channel. Furthermore, if the measurements are used i…
Several models of thermionic energy nanoconverters have been proposed to study the transport phenomena that take place in electronic devices. For example, in resonant tunneling junctions those phenomena are manifested through the thermoelectric effects. The coupling between the electron flux and the heat flux in this type of semiconductor heterostructures, not only allows to obtain transport coefficients (electrical and thermal conductivities, and a Seebeck--like and Peltier--like coefficients), but also to study its operation as a thermionic generator or as a refrigerator within the context o…
The thermalization of non-equilibrium charge carriers is at the heart of thermoelectric energy conversion. In nanoscale systems, the equilibration length can be on the order of the system size, leading to a situation where thermoelectric effects need to be considered as spatially distributed, rather than localized at junctions. The energy exchange between charge carriers and phonons is of fundamental scientific and technological interest, but their assessment poses significant experimental challenges. We addressed these challenges by imaging the temperature change induced by Peltier effects in…
Channel conductance measurements can be used as a tool to study thermally activated electron transport in the sub-threshold region of state-of-art FinFETs. Together with theoretical Tight-Binding (TB) calculations, this technique can be used to understand the evolution of source-to-channel barrier height (Eb) and of active channel area (S) with gate bias (Vgs). The quantitative difference between experimental and theoretical values that we observe can be attributed to the interface traps present in these FinFETs. Therefore, based on the difference between measured and calculated values of (i) …
We report on transport experiments through high-mobility gate-tunable undoped InSb QWs. Due to the elimination of any Si modulation doping, the gate-defined two-dimensional electron gases in the quantum wells display a significantly increased mobility of 260,000 cm$^2$/Vs at a rather low density of $2.4\times10^{11}$ cm$^{-2}$. Using magneto-transport experiments, we characterize spin-orbit interactions by measuring weak antilocalization. Furthermore, by measuring Shubnikov-de Haas oscillations in tilted magnetic fields, we find that the g-factor agrees with $k \cdot p$ theory calculations at …
Schottky barrier inhomogeneities are expected at the metal/TMDC interface and this can impact device performance. However, it is difficult to account for the distribution of interface inhomogeneity as most techniques average over the spot-area of the analytical tool, or the entire device measured for electrical I-V measurements. Commonly used models to extract Schottky barrier heights (SBH) neglect or fail to account for such inhomogeneities, which can lead to the extraction of incorrect SBH and Richardson constants. Here, we show that a gaussian modified thermionic emission model gives the be…
Quantum transport measurements are performed in gate-defined, high-quality, two-dimensional hole and electron systems in an undoped InSb quantum well. For both polarities, the carrier systems show tunable spin-orbit interaction as extracted from weak anti-localization measurements. The effective mass of InSb holes strongly increases with carrier density as determined from the temperature dependence of Shubnikov-de Haas oscillations. Coincidence measurements in a tilted magnetic field are performed to estimate the spin susceptibility of the InSb two-dimensional hole system. The g-factor of the …
Simultaneous study of acoustic and optic phonon scattering of electrons and holes in undoped $\mathrm{GaAs}$/$\mathrm{Al_{x} Ga_{1-x} As}$ heterostructures
本研究利用非掺杂 GaAs/AlGaAs 异质结构，通过电阻率温度依赖性测量，在 1.6-92.5K 范围内同步研究了二维电子和空穴的声学与光学声子散射，提取了耦合常数和 LO 激活能，验证了该方法的有效性。
Contact resistance is a severe performance bottleneck for electronic devices based on two-dimensional layered (2D) semiconductors, whose contacts are Schottky rather than Ohmic. Although there is general consensus that the injection mechanism changes from thermionic to tunneling with gate biasing, existing models tend to oversimplify the transport problem, by neglecting the 2D transport nature and the modulation of the Schottky barrier height, the latter being of particular importance in back-gated devices. In this work, we develop a semi-analytical model based on Bardeen's transfer Hamiltonia…
In the current paper a set of experiments dedicated to investigations of local electronic transport in undoped InAs nanowires at helium temperatures in the presence of a charged atomic-force microscope tip is presented. Both nanowires without defects and with internal tunneling barriers were studied. The measurements were performed at various carrier concentrations in the systems and opacity of contact-to-wire interfaces. The regime of Coulomb blockade is investigated in detail including negative differential conductivity of the whole system. The situation with open contacts with one tunneling…
Power dissipation is a great challenge for the continuous scaling down and performance improvement of CMOS technology, due to thermionic current switching limit of conventional MOSFETs. In this work, we show that this problem can be overcome by using cold metals as the transistor's injection source, which are different from conventional metals and can filter high energy electrons to break the Boltzmann tyranny. It is proved that the subthreshold swing (SS) of thermionic current of transistor using cold metal contact can be extremely smaller than 60 mV/decade at room temperature. Specifically, …
We present a unified view of electric transport in undoped clean graphene for finite electric field. The weak field results agree with the Kubo approach. For strong electric field, the current increases non-linearly with the electric field as E^{3/2}. As the Dirac point is moved around in reciprocal space by the field, excited states are generated, in a way analogous to the generation of defects in a quench through a quantum critical point. These results are also analyzed in terms of Schwinger's pair production and Landau-Zener tunneling. An experiment for cold atoms in optical lattices is pro…
Valley-Landscape Engineering in Bilayer WSe$_2$ Gate-All-Around Transistors
Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS)（物质·材料纳米构筑研究中心，日本国立材料研究所）；Kwansei Gakuin University（关西学院大学）
