Thermionic saturation of diffusion currents in transistors

Macroscopic diffusion theory is not applicable to large percentage variations in carrier concentration over distances comparable to a scattering path length. It is plausible that the transport velocity for thermionic emission over a barrier of zero height constitutes a limit on the velocity of diffusive current flow at any point in a bulk semiconductor. Not incorporating a thermionic limit, the drift-diffusion current transport equation cannot account for thermionic emission. While a full remedy to these difficulties can only lie in a complete investigation of the relevant statistical physics, we present herein an empirical modification of the current transport equation based on the elementary concept of thermionic saturation of the diffusion current. The modified equation appears capable of accounting, at least in a crude way, for thermionically limited current flow, while permitting a unified treatment of diffusion-drift-thermionic transport. For narrow base transistors (w_b 1000 Å), much larger ratios of stored base charge to collector current are predicted than from diffusion theory. A thermionic emission-diffusion equation is derived for barrier injection in floating base transistors biased into punch-through. It is found for silicon structures that transport is diffusion-drift dominated at base impurity concentrations < 10¹⁷ cm⁻³, and thermionically controlled at higher base doping. A possible small reduction in the thermionic Richardson constant by a factor e is also predicted from the diffusion saturation equation.     

A computational study of thin-body, double-gate, Schottky barrier MOSFETs

Nanoscale Schottky barrier MOSFETs (SBFETs) are explored by solving the two-dimensional Poisson equation self-consistently with a quantum transport equation. The results show that for SBFETs; with positive, effective metal-semiconductor barrier heights, the on-current is limited by tunneling through a barrier at the source. If, however, a negative metal-semiconductor barrier height could be achieved, on-current of SBFETs would approach that of a ballistic MOSFET. The reason is that the gate voltage would then modulate a thermionic barrier rather than a tunneling barrier, a process similar to ballistic MOSFETs and one that delivers more current.     

InGaAs/AlAs/InGaAsP resonant tunneling hot electron transistorsgrown by chemical beam epitaxy

InP-based resonant tunneling hot electron transistors (RHET's) were studied systematically using chemical beam epitaxy (CBE) for the first time. All the RHET's studied have a highly strained AlAs/In_0.75Ga_0.25As/AlAs resonant tunneling double barrier as a hot electron injector, and an InP collector barrier with or without InGaAsP graded layers. The highest transport ratio (α) observed is 0.98, and the highest peak-to-valley current ratios (PVR's) measured are 20 and 200 in the collector current and base current, respectively, at 80 K. A self-consistent simulation is used as a reference to optimize the hot electron injector and to explain the ballistic transport. An energy spectrometer technique was applied to the RHET's for resolving the hot electron energy distribution which showed a full width at half maximum (FWHM) of around 58 meV, indicating ballistic transport of electrons. Finally, room temperature transistor action was also observed with a β of 4 and a cutoff frequency of 31 GHz     

Npn型HBT准中性均匀基区的HHTM一维解析模型

从异质结构流体动力学模型方程（ＨＨＴＭ）出发，经适当近似，得到了Ｎｐｎ型ＨＢＴ准中性均匀基区饱和电流密度的解，定义了一个判别基区是否存在准弹道输运的判别因子─－弹道比ｒＢ，指出为了实现基区准弹道输运，应使ｒＢ《１．对（Ａｌ，Ｇａ）Ａｓ／ＧａＡｓＮｐｎ型ＨＢＴ而言，这一条件意味着基区宽度Ｗａ《１００ｎｍ。因此为了实现准弹道基区输运，应将（Ａｌ，Ｇａ）Ａｓ／ＧａＡｓＮｐｎ型ＨＢＴ的基区宽度从目前的５０～１００ｎｍ降低到３０ｎｍ以下。     

利用FN电流估计薄栅MOS结构栅氧化层的 势垒转变区的宽度

通过数值求解整个势垒的薛定谔方程，发现FN电流公式中的B因子强烈依赖势垒的转变区的宽度，而C因子则弱依赖于势垒的转变区的宽度．给出了一种利用WKB近似所得的处理电子隧穿存在转变区势垒的过程，并得到一个FN电流的分析表达式．它可用来估计薄栅MOS结构的栅氧化层的势垒转变区的宽度．在转变区的宽度小于1nm时，它与数值求解薛定谔方程的结果吻合得很好，表明该方法可以用来估计势垒转变区的宽度．实验的结果表明B因子随温度有较大的变化，这个结果验证了该方法的部分预测结果．     

Numerical Analysis on Current Transport Characteristics in Single Layer Organic Electroluminescent Devices

A new model to describe I-V characteristics of organic light-emitting devices (OLEDs) is developed based on experimental results. The dependence of I-V characteristics on energy barrier, trap density and carrier mobility is analyzed. The result shows that this model combines the Fowler-Nordheim tunnel theory and the trap charge limited current theory with exponential trap distribution (TCL), and it describes the current transport characteristics of OLEDs more comprehensively. The I-V characteristics follow Fowler-Nordheim theory when the energy barrier is high, the trap density is small and the carrier mobility is large.In other cases they follow the TCL theory.     

Effects of Channel Electron In‐Plane Velocity on the Capacitance‐Voltage Curve of MOS Devices

The coupling between the transverse and longitudinal components of the channel electron motion in NMOS devices leads to a reduction in the barrier height. Therefore, this study theoretically investigates the effects of the in‐plane velocity of channel electrons on the capacitance‐voltage characteristics of nano NMOS devices under inversion bias. Numerical calculation via a self‐consistent solution to the coupled Schrödinger equation and Poisson equation is used in the investigation. The results demonstrate that such a coupling largely affects capacitance‐voltage characteristic when the in‐plane velocity of channel electrons is high. The ballistic transport ensures a high in‐plane momentum. It suggests that such a coupling should be considered in the quantum capacitance‐voltage modeling in ballistic transport devices.     

AlGaN/GaN异质结构中的极化工程

从自洽求解薛定谔方程和泊松方程出发,研究了AlGaN/GaN量子阱电子气密度随Al组份比、势垒层厚度和栅压的变化,比较了能带带阶和极化电荷对沟道阱能带和电子气特性的影响,研究了在势垒层和缓冲层中夹入AlN及InGaN薄层的作用,计算了AlGaN/GaN/AlGaN双异质结的能带,最后探讨了势垒层能带的优化设计,提出了剪裁势垒层能带来钝化表面的新方法。     

A comparison between Monte Carlo and extended drift-diffusionmodels for abrupt InP/InGaAs HBTs

In this paper, a comparison between the simulation results of abrupt InP/InGaAs n-p-n heterojunction bipolar transistors (HBTs) obtained from both an extended drift-diffusion (EDD) model and from a Monte Carlo (MC) HBT simulator is carried out. The EDD approach is based on Grinberg's model for the electron current through the interfacial emitter-base spike-like potential barrier. The MC procedure includes the effects of quantum transmission/reflection through the abrupt heterojunction. The transmission coefficient is obtained from a numerical solution of Schrodinger's equation (self-consistently embedded in the general MC procedure) that takes into account the nonparabolicity of the bands and the spatial change in the effective mass. The differences between both models and their physical causes are also analyzed in this paper. It Is shown that, for the HBT under consideration, the collector current is determined by the transmission of electrons through the spike, while the numerical values of the parameters associated with the transport mechanisms in the base region have a very small influence on the collector current. An alternative equation to the conventional diffusion current equation in the neutral base region has also been developed, which includes the hot electron effects     

Analytical computation of electrical parameters in GAAQWT and CNTFET with identical configuration using NEGF method

A two-dimensional quantum mechanical model is presented for calculating carrier transport in ultra-thin gate-all-around quantum wire transistor (GAAQWT) and carbon nanotube field effect transistor (CNTFET) using coupled mode space approach. Schrödinger and Poisson’s equations are self-consistently solved involving Non-Equilibrium Green’s Function (NEGF) formalism under the ballistic limit along with dissipative effects in terms of self-energy at both the source and drain ends. Effect of structural parameters on drain current, channel length modulation parameter, quantum capacitance, transconductance, subthreshold swing (SS) and drain induced barrier lowering (DIBL) are studied assuming occupancy of only a few lower sub-bands, where comparison is performed taking all other factors, biases and dimensions identical. High-k dielectric (HfO₂) independently surrounding the quantum wire (GaAs) and carbon nanotube shows higher drain current and transconductance for GAAQWT but lower quantum capacitance than that obtained for CNTFET. A smaller variation of CLM for CNFET speaks in favour of it for digital quantum circuit applications, whereas GAAQWT is suitable candidate for low-power applications. Effect of structural parameters is investigated within fabrication limit to analyse the effect on electrical characteristics under lower biasing ranges.     

Energy oscillations in electron transport across a triangularbarrier

Carrier transport across the semiconductor space-charge region of a silicon triangular barrier diode was investigated by a Monte Carlo simulation. Oscillations of the electron mean kinetic energy are observed as a function of position along the uphill slope of the barrier under bias. At a given point on the uphill slope, the energy distribution function shows an oscillatory behavior, with a periodicity corresponding to the optical phonon energy. These oscillations are shown to be due to the nonequilibrium dynamics of the electron interaction with optical phonons in the situation when other inelastic electron scattering processes are negligible. The energy oscillations are superimposed on a smooth cooling of the distribution in the transport toward the top of the barrier, as current flows through the system. A comparison with the thermionic theory quantifies the importance of nonequilibrium effects in short-range electronic transport     

Thermal Conductivity of Diameter-Modulated Silicon Nanowires Within a Frequency-Dependent Model for Phonon Boundary Scattering

Modulated nanowires have been proposed as candidates for efficient thermoelectric applications. It has been previously shown within the low-temperature ballistic regime of phonon transport that the thermal conductivity can be significantly reduced when the width of the nanowire is modulated. Here, we report on the thermal conductivity of modulated Si nanowires calculated within a kinetic theory model. The size dependence is taken into account through the sampling of k-points in the first Brillouin zone and a frequency-dependent calculation of the boundary scattering length. It has been found that the thermal conductivity of modulated nanowires can be drastically reduced compared with that of nanowires with constant width. Interestingly, the thermal conductivity is even smaller than that of corresponding straight wires with width equal to the smallest width in the modulated nanowires. The dramatic decrease of the thermal conductivity of modulated nanowires is attributed to their small transmissivity.     

Investigation of plasmon-induced losses in quasi-ballistic transport

We present an ensemble Monte Carlo (EMC) simulation of the effect of electron-electron (e-e) and electron-plasmon (e-pl) interactions on the transient behavior of electrons under high energy injection conditions. It is shown that, in a situation that closely resembles that obtained in the base of a planar-doped barrier (PDB) transistor, the coulombic interaction severely limits the possibility of ballistic transport.     

Ballistic transport in Schottky barrier carbon nanotube FETs

Simulation of Schottky barrier carbon nanotube field effect transistors (SB-CNTFETs) is presented to yield the electrostatic potential, carrier concentration and current within the device. The simulator is based on a self-consistent solution of Poisson's equation and the carrier transport equation. The finite element method is used for solving Poisson's equation while the non-equilibrium Green's function formalism is used to model the carrier transport. The developed simulator is used to investigate the effects of device parameters on device performance.     

Cascade mechanism of nonlinear interactions between modes in a turbulent plasma

A hydrodynamical model of plasma turbulence is developed. A cascade concept is utilized to solve the problem of closure, which is characteristic of any nonlinear system. The spectra of kinetic and electrostatic energies are investigated for both the collisional and collisionless cases. In the former case, the spectrum of potential energy from the electrostatic potential fluctuations, as a function of the wave numberk, and denoted byG(k), follows the lawsG sim k^{-3/2}andk^{-9/2}in the inertial and dissipative subranges, respectively. In the latter case, the dissipative lawG sim k^{-5}is derived. The solutions of the kinetic energy spectrum are also found. The anomalous diffusion, called the Bohm diffusion, is derived in the present theory, and is found to play the role of a sink of energy flux across a potential spectrum.     

An improved energy transport model including nonparabolicity andnon-Maxwellian distribution effects

An improved energy transport model for device simulation is derived from the zeroth and second moments of the Boltzmann transport equation (BTE) and from the presumed functional form of the even part of the carrier distribution in momentum space. Energy-band nonparabolicity and non-Maxwellian distribution effects are included to first order. The model is amenable to an efficient self-consistent discretization taking advantage of the similarity between current and energy flow equations. Numerical results for ballistic diodes and MOSFETs are presented. Typical spurious velocity overshoot spikes, obtained in conventional hydrodynamic (HD) simulations of ballistic diodes, are virtually eliminated     

Effect of ballistic electron transport in metal-n-GaAs-n +-GaAs Schottky-barrier structures

The interaction of an electron with the potential formed at the Schottky-barrier metal-semiconductor contact was investigated under the assumption that the motion is ballistic. Three cases of interaction of the electron with the barrier potential were considered; i.e., strong and weak interactions and above-barrier transport. It is demonstrated that the interaction of the electron with the above-barrier region substantially affects the scattering process for all three cases, which manifests itself in the behavior of steady-state current-voltage characteristics. Based on the model suggested, it is predicted that reverse currents increase as the width of the diode’s thin (∼0.35 μm) base increases. This prediction is verified and confirmed experimentally. It is found that an increase in reverse current is related to the increasing influence of a strong electric field on the moving carriers as the width of the thin base increases. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 7, 2001, pp. 823–830. Original Russian Text Copyright ? 2001 by Torkhov.     

Diffusion effects and "Ballistic transport"

The effects of both collisional drag and diffusion effects on the operation of GaAs n⁺-n-n⁺structures are determined using a momentum transport equation which includes velocity fluctuation effects. It is shown that diffusion effects are important in practical structures, and that previous interpretations of experimental results which claim to show "ballistic" transport can be seriously in error. The general limitations of single-particle transport models when used to try to understand the results of experiments, such as diode characteristic measurements which yield only ensemble averages, are discussed.