A proposed structure for collector transit-time reduction in AlGaAs/GaAs bipolar transistors

The impact of velocity overshoot in the collector space-charge region on carrier transport is explored. The effects of overshoot on transit time for conventional collector structures are found to be minor. A new collector structure which exhibits extended velocity overshoot is proposed. This structure promises both simple fabrication and significant improvements (≃ 25 percent) in carrier transit time over conventional collector structures as demonstrated by Monte Carlo simulation.     

Monte Carlo modeling of electron transport in repeated overshootstructures

Repeated velocity overshoot has been proposed as a way of obtaining high average velocities over significant distances in semiconductor devices. The potential of this concept is examined using a fully-self-consistent particle-field Monte Carlo simulation. Numerical results are presented for realistic periodic overshoot structures for a range of bias conditions and operating temperatures of 77 and 300 K. Local velocity overshoot peaks are observed in the simulated structures, but the average carrier velocity and current at each bias point are in all cases less than those associated with transport in bulk material at the same bias point. The physical mechanisms underlying this result are analyzed. It is found that ensemble (diffusion) effects, which were neglected in the original proposal of the repeated overshoot concept, strongly influence the results that are achievable in practice     

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     

Analytic accounting for carrier velocity overshoot in advancedBJT's for circuit simulation

An analytic model for electron velocity overshoot in advanced silicon-based bipolar junction transistors (BJTs) is presented. The model, which characterizes an effective saturated drift velocity in the collector space-charge regions, is intended for circuit simulation and has been implemented in MMSPICE. The model is based on a nonlocal augmented drift-velocity formalism that involves a length coefficient derived from Monte Carlo simulations. A phenomenological representation of the associated velocity relaxation is defined to be consistent with the overshoot analysis. Demonstrative MMSPICE device and circuit simulations show that effects of velocity overshoot in contemporary silicon BJTs produce only small performance enhancements, but can be exploited to optimize design tradeoffs in scaled technologies     

亚微米栅GaAs MESFET物理特性研究

本文基于对过冲效应的分析、计算,说明了亚微米栅GaAs MESFET的特性。建立了简明的考虑了过冲的漂移速度模型,并结合求解泊松方程、电流连续性方程进行了器件的二维计算机模拟。得到的结果与实际接近。     

A physically based base pushout model for submicrometer BJTs in thepresence of velocity overshoot

By using a two-dimensional relaxation time approximation device simulator, base pushout phenomena for submicrometer bipolar junction transistors (BJTs) are analyzed. From the numerical analysis, it was clarified that, under the base pushout condition, the electron velocity exceeds the saturation velocity in most of the epi-collector region. Considering this velocity overshoot effect with two-dimensional carrier behavior, a base pushout model was developed. This model is applicable to the BJT equivalent circuit model. The model utility was verified for a 0.8 μm emitter-width BIT, and excellent agreement with measured I-V characteristics was obtained over wide injection conditions. Scaling effects on the velocity overshoot are also calculated, based on the constant current scaling. It is shown that the base pushout is suppressed due to the increased velocity overshoot level as the device sizes are scaled down     

A new set of semiconductor equations for computer simulation of submicron devices

A new set of semiconductor equations is proposed here which will be suitable for the numerical study of the carrier transport effects in submicron devices. With simplified models and comprehensive numerical techniques, the relative importance (on the device behavior) of several physical effects, such as velocity overshoot, intracollisional field effect, avalanche breakdown, carrier generation and temperature effect can be determined. The program that is capable of solving this set of equations will become an indispensible CAD tool if the current trend of the decreasing of the device dimensions continues.     

Two-dimensional hot-electron models for short-gate-length GaAs MESFET's

A detailed hot-electron device model suitable for modeling short-gate-length GaAs MESFET's is described. A two-dimensional numerical simulation is used to solve a set of semiclassical carrier transport equations, including a full rigorous solution of the energy conservation equation. The importance of the hot-electron effects is demonstrated and in particular the role of the electron temperature gradient in addition to velocity overshoot is emphasized. The influence of doping and mobility profiles are investigated and found to have a very significant effect on the device characteristics. The model is applied to a range of submicrometer-gate-length devices and is shown to be useful for characterizing devices with gate lengths down to less than 0.1 µm. The dependence of saturated drain current on gate length is quantified.     

Monte Carlo simulation of bipolar transistors

A new approach is proposed to investigate, the limits of validity of the conventional drift-diffusion equation analysis for modeling bipolar transistor structures containing submicrometer dimensions. The single-particle Monte Carlo method is used for the solution of the Boltzmann equation. An electron velocity overshoot of 1.8 times the static saturation velocity has been found for electrons near the base-collector junction of a silicon device. The effect of this velocity overshoot was calculated to enhance the output collector current and reduce the electron transit time by 5 percent for the device structure considered in this work.     

Two-dimensional analysis of velocity overshoot effects in ultrashort-channel Si MOSFET's

A new two-dimensional device simulator is developed to investigate the effects of velocity overshoot on Si MOSFET's. An electron temperature-dependent mobility model, in which mobility is determined as a function of electron-gas temperature, is used in the simulator. Marked velocity overshoot occurs in the vicinity of the drain edge of MOSFET's and makes the potential barrier height at the source edge lower for ultrashort-channel MOSFET's. Therefore, velocity overshoot effects appear not only as degradation of electron transit time but also as increased drain current as compared with the case in which drift velocity does not overshoot. The increase in drain current depends strongly upon low-field mobility and bias conditions and appears for channel lengths shorter than 1000 nm. When low-field mobility is higher than 500 cm²/V. s and channel length is 100 nm, the increase in drain current is more than 1.5 times for bias conditions of strong inversion and a lateral electric field of more than 10⁵V/cm in the vicinity of the drain edge.     

An investigation of steady-state velocity overshoot in silicon

Electron dynamics in silicon is investigated by means of improved momentum- and energy-balance equations including particle diffusion and heat flux. The resulting system of partial differential equations is numerically solved in a variety of field configurations including strong discontinuities, in order to enhance velocity overshoot effects. It is found that diffusion, usually neglected in previous studies, plays a major role, and considerably modifies the features of the velocity vs distance curve, leading to an increase of the carrier drift velocity in the low-field region, i.e. before experiencing the effect of the strong field. In addition, it is found that, in order to take full advantage of velocity overshoot effects in MOSFET's, a structure must be designed having the strongest possible field at the source-end of the channel, where carrier density is controlled by the gate.     

Accurate modeling for submicrometer-gate Si and GaAs MESFET's using two-dimensional particle simulation

Device characteristics, including nonstationary carrier-transport effects such as velocity overshoot phenomena in submicrometergate Si and GaAs MESFET's, are presented in detail by two-dimensional full Monte Carlo particle simulation. Accurate current-voltage characteristics and transient current response are successfully obtained without relaxation time approximation. Moreover, the carrier dynamics influence on device operation is clarified in a realistic device model, compared with the conventional simulation. It can be pointed out that such nonstationary carrier transport is acutely important for accurate modeling of submicrometer-gate GaAs MESFET's, but is not as important for that of Si MESFET's.     

Current equations for velocity overshoot

A generalized current equation is presented for the operation of submicron devices by supplementing the drift and diffusion currents with gradient, rate and relaxation currents. The equation includes the most important features of velocity overshoot.     

Drain current enhancement due to velocity overshoot effects and itsanalytic modeling

The drain current enhancement due to the velocity overshoot effects is found to be due to the electron velocity enhancement at the source end. Based on this observation, a new analytic model is proposed and verified by two-dimensional (2-D) simulations and experiments. From the results of the verifications, we conclude that our model predicts the drain current enhancement due to the velocity overshoot effects reasonably well. The effects of the device parameters, such as gate oxide thickness and channel doping concentration, on the drain current enhancement ran be readily found in our model     

Simulation of a GaAs MESFET including velocity overshoot: anextended drift-diffusion formalism

Numerical simulations are described for GaAs MESFETs. They are based on an extended drift-diffusion equation formalism which allows for the inclusion of the velocity overshoot effect. The overshoot correction term which augments the drift-diffusion current equation is proportional to the gradient of the local electric field. The proportionality constant has been calculated by Monte Carlo methods as a function of the electric field. The model developed solves the extended drift-diffusion current continuity equation and Poisson's equation over a two-dimensional grid using the method of finite differences. Simulations for normally-on and normally-off devices with gate lengths of 0.2, 0.5, and 1.0 μm are compared with published Monte Carlo calculations. The calculated drain currents and average electron velocities in the device channel are in excellent agreement. The extended drift-diffusion formalism is useful and easy to implement for modeling field-effect transistors with submicron feature size     

A new analytical model for the GaAs MESFET in the saturation region

A model that provides the static characteristics and the elements of the equivalent electrical scheme is presented. It is based on an approximate quadratic form for the depleted region under the gate when the electron velocity reaches the saturation velocity. The potential in the channel is calculated using Poisson's equation and taking into account the variation of the electron density inside it. The main physical phenomena such as edge effects, overshoot velocity, and carrier injection in the buffer layer are also taken into account. Theoretical and experimental results for the I-V characteristics, transconductance, output conductance, gate-source capacitance, and gate-drain capacitance are presented for a submicrometer-gate MESFET. The results calculated using this model agree well with experimental data     

Experimental study of hot carriers in small size Si-MOSFETs

Hot carrier effects in Si-MOSFETs cause serious reliability problems in VLSI circuits. While most of them have been investigated empirically, it is also important to clarify the microscopic degradation mechanism. In addition, there are other effects which can affect the device characteristics significantly and can be used to study hot carrier transport in MOSFETs. In this paper, several experimental studies associated with hot carrier effects in MOSFETs are reviewed. In particularly, hot electron energy effects are discussed, such as velocity overshoot and photo-emission in MOSFETs, and technological efforts to overcome the degradation problem in actual devices are reviewed briefly.     

一个适用于模拟电路的深亚微米SOIMOSFET器件模型

从数值解源端和饱和点的表面电势出发 ,考虑模拟电路对 SOI MOSFET模型的一些基本要求如电荷守恒、器件源漏本征对称、各个工作区间连续并且高阶可导以及全耗尽和部分耗尽两种工作模式的转变 ,构建了一个能够满足这些要求的精确的器件模型 .同时包含了深亚微米 SOI MOSFET的一些二级效应如漏极诱生势垒降低效应 (DIBL )、速度饱和效应、自热效应等 .这个模型的参数相对较少并且精确连续 ,能够满足在模拟电路设计分析中的应用要求     

一个适用于模拟电路的深亚微米SOI MOSFET器件模型

从数值解源端和饱和点的表面电势出发,考虑模拟电路对SOI MOSFET模型的一些基本要求如电荷守恒、器件源漏本征对称、各个工作区间连续并且高阶可导以及全耗尽和部分耗尽两种工作模式的转变,构建了一个能够满足这些要求的精确的器件模型．同时包含了深亚微米SOI MOSFET的一些二级效应如漏极诱生势垒降低效应（DIBL）、速度饱和效应、自热效应等．这个模型的参数相对较少并且精确连续,能够满足在模拟电路设计分析中的应用要求．