Transient Monte Carlo analysis and application to heterojunctionbipolar transistor switching

A self-consistent simulation using the Monte Carlo ensemble particle technique for analysis of heterojunction bipolar transistor (HBT) transient behavior, such as switching performance, is presented. The transient Monte Carlo method has been applied to a self-consistent simulation of two HBT designs with improved collector structures for high-speed and high-frequency applications, and the results are compared with the characteristics of conventional HBTs. The simulation results indicate that the two new collector structures, the inverted field collector and the undoped collector, have better switching performance than the conventional HBT. The study of the switching characteristics' dependence on collector-base bias voltage and collector current suggests that the inverted field HBT is the best approach in terms of switching properties. The results are supported and explained by examining electron transport properties such as overshoot velocity and energy valley occupation, as well as band bending in the collector space-charge region at different current levels     

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.     

Semiconductor structures for repeated velocity overshoot

We discuss conditions necessary for obtaining repeated velocity overshoot in semiconductors. Two classes of structures which provide these conditions are considered. These structures offer the potential for achieving average drift velocities well in excess of the maximum steady-state velocity over distances ranging from submicron to tens of microns.     

短沟道MOSFET渡越时间物理模型

叙述了一个考虑包括速度过冲等短沟道效应的ＭＯＳＦＥＴ渡越时间解析模型，计算结果与二维数值模拟符合较好。基于该模型，探讨了在线性工作区和饱和工作区渡越时间对栅偏压依赖关系的不同，并作了物理解释。模型还表明由速度过冲带来渡越时间的缩短对沟道长度大于０．２５μｍ的ＭＯＳＦＥＴ不超过１０％     

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.     

Transient transport in central-valley-dominated ternary III–V alloys

This paper describes results of an ensemble Monte Carlo study of ballistic transport and velocity overshoot in Al_0.25In_0.75As and Ga_0.4In_0.6As. Velocity overshoot in these materials is limited primarily by transport properties of the central valley. The transfer of carriers into higher-lying energy valleys occurs only after velocity overshoot has subsided. Instantaneous conduction-band occupancies, carrier positions, and carrier velocities are given as functions of time for electric field intensities of 10 and 40 kV/cm. These curves show that transient velocities exceed steady-state values by as much as 27 to one, that these transients persist over distances ranging from 0.22 to 0.79 μm, and that average velocities during velocity overshoot are as large as 5.6 × 10⁷ cm/sec. These results have important implications for submicron-device applications of these materials.     

Influence of electron velocity overshoot on collector transit timesof HBTs

Collector transit times of heterojunction bipolar transistors with a pronounced electron velocity overshoot effect are investigated using a simple analytical model. The effective carrier velocity, ν_eff , which is a measure for determining the transit time, is defined as π_C=W_C/2ν_eff. It is found that ν_eff is much different from the average velocity, ν_av, that is given by the traveling time through the whole collector depletion layer and the depletion width. With a higher overshoot peak velocity, the collector transit time is shorter than that estimated simply from the average velocity ν_av     

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-DIMENSION NUMERICAL SIMULATION OF SUBMICRON-SCALE GaAs MESFET

This paper deals with 2-D simulation of GaAs MESFET,which includes velocityovershoot effects by using energy transport model suitable for submicron devices.Computationtime is greatly reduced by simplifying the model and using fast convergence algorithms,e.g.Gummel interaction and ICCG method.The program shows good stability and convergence.Several types of GaAs MESFET structures,e.g.epitaxial,ion-implanted and buried P-layer,have been simulated.The results show that buried P-layer can decrease carrier's injections intosubstrate and improve device performance.The results are also used to study carefully thevelocity overshoot effects.By comparison of results of energy transport model and drift-diffusionmodel,the limitation of drift-diffusion-model is derived.     

New evidence for velocity overshoot in a 200 nm pseudomorphic HEMT

It is believed that significant velocity overshoot effects are responsible for the high performance of pseudomorphic HEMTs (PsHEMTs). The overshoot is associated with the low effective mass in the InGaAs channel and the large Γ-L separation. Average channel electron velocities well in excess of 3.0 × 10⁷ cm/s have been predicted in Monte-Carlo PsHEMT simulations. However, average electron velocities extracted from transconductance measurements of such devices are much lower, typically in the range 1.5–2.0 × 10⁷ cm/s. In this paper we analyse real device measurements by using Monte-Carlo and drift diffusion simulations. We show clear evidence that the average velocity in the channel of a 200 nm PsHEMT fabricated in the Nano-electronics Research Centre of Glasgow University exceeds 3.0 × 10⁷ cm/s.     

Numerical simulation of GaAs MESFETS including velocity overshoot

An effective procedure considering the velocity overshoot effect in numerical simulation of GaAs MESFETs is presented. The results of the corresponding routine SEMICO II are compared with results of Monte-Carlo simulations and with results of the routine SEMICO I without overshoot.Implanted MESFETs have been simulated with and without including the overshoot effect, and from these data equivalent circuits have been derived. From circuit calculations of E/D inverter ring oscillators it is concluded that the velocity overshoot effect leads to a twofold decrease of the gate propagation delay time for 0.25 μm gate length.     

Charge-control analysis of collector transit time for (AlGa)As/GaAsHBTs under a high injection condition

Charge-control analysis of collector transit time (τ_C ) for (AlGa)As/GaAs n-p⁺-n heterojunction bipolar transistors has been carried out under the restriction of low-frequency approximation to explain the remarkable τ_C reduction at the onset of the Kirk effect. The fundamental idea of the model is that the increment of the current density must depend not only on the increment of the carrier density but also on the increment of the carrier drift velocity in the collector space-charge region. The obtained model well explains the previous Monte Carlo results, thus indicating the validity of the above idea as well as the possibility that the actual τ_C is by far smaller than that estimated from the conventional model. It also leads to a concept for the reduction of transit times, i.e. that an increasing rate of the electron overshoot velocity is as essential for the reduction of τ_C as the magnitude of the overshoot velocity itself     

High-field transport of inversion-layer electrons and holesincluding velocity overshoot

In this paper, we experimentally address the effect of a wide range of parameters on the high-field transport of inversion-layer electrons and holes. The studied parameters include substrate doping level, surface micro-roughness, vertical field strength, nitridation of the gate oxide, and device channel length. We employ special test structures built on Silicon-On-Insulator (SOI) and bulk wafers to accurately measure the high-field drift velocity of inversion-layer carriers. Our findings point to electron velocity overshoot at room temperature, dependence of electron and hole saturation velocities on nitridation of the gate oxide, dependence of the high-field drift velocity on the effective vertical field, and relative insensitivity of electron and hole mobility and saturation velocity to moderate surface roughness     

MODFET 2-D hydrodynamic energy modeling: optimization ofsubquarter-micron-gate structures

Using a two-dimensional hydrodynamic energy model incorporating nonstationary electron dynamics and nonisothermal electron transport (which characterizes submicron-gate MODFETs), the main physical phenomena that govern the device performance at 300 K are highlighted. This covers velocity overshoot effects, stationary-domain formation, and real space transfer. The model is then used systematically to predict the precise values of the small-signal parameters for different bias conditions. The potential performance improvement achieved by reducing the gate length below 0.2 μm is investigated. It is shown that improvement in transconductance is achieved through gate-length reduction if a severe restriction on the aspect ratio is respected     

考虑速度过冲效应的HEMT物理模型

本文提出了一种考虑这冲效应的ＨＥＭＴ器件静态和小信号解析物理模型。通过对栅极下面道中造近源端附近的电场采用弱强梯场近似，提出了一个半经验的速度过冲模型，在非线性电荷控制模型的基础上述导出了基于物理参数的ＨＥＭＴ器件电流－电压特性和小等效电路参数的解析表达式。实际计算结果与测得数据比较表明，本模型具有比较高的精度。     

A novel low-power transceiver topology for noncontact vital sign detection including the power management technique

In this paper, power management technique utilized in the direct down-conversion non-quadrature transceiver is presented for the low-power application of vital sign detection. The simultaneous switching noise (SSN) and overshoot and undershoot of the transient waveform distortion resulting from a pulse signal will give rise to interference with a vital sign signal. The pulse width, rise/fall time, and period of pulse bias are analyzed to mitigate the interference in this investigation. Significant issues about direct-current (DC) offset and noise confronted by the presented technique are addressed based on mathematical analysis. In radio-frequency (RF) transceiver architecture including power amplifier (PA), low-noise amplifier (LNA), and mixer, the current-reused (CRU) topology is utilized to achieve low DC power consumption. The post-layout simulation results exhibit that power consumption of the transceiver using the optimized pulse bias is reduced to 40% of the power consumption for transceiver applying the DC bias. In addition, DC offset and null detection point can be alleviated by tunable phase shifter.     

Two-dimensional transient simulation of an idealized high electron mobility transistor

We develop a model for the high electron mobility transistor (HEMT) in which we include both hot-electron effects and conduction outside the quantum subband system using hydrodynamic-like transport equations. With such a model we can assess the significance of the various physical phenomena involved in the operation of the HEMT. We calculate results with a two-dimensional numerical technique for both steady-state and transient operation. For a 3-µm device at 77 K, we determine a transconductance of 450 mS/mm, a current-switching speed of 6 ps, and a capacitive charging speed of 4 ps/fanout gate which corresponds to the performance measured by other workers. We also see that electronic heating, velocity overshoot, and conduction outside the quantum well are significant near the pinchoff point. We conclude that the advantage of HEMT is twofold. The excellent conduction in the quantum well results in a low access resistance, and the low impurity concentration in the GaAs results in optimum overshoot effects.     

亚微米GaAs MESFET二维数值模拟

本文用计入热电子效应的动量能量守恒模型讨论了亚微米GaAs MESFET二维数值模拟。为了减少计算量进行了模型简化和算法选择。文中给出并分析了三种典型器件的模用范果,根据模拟结果,研究了小尺寸器件中的速度过冲效应并得出常规的漂移扩散模型的适拟结围。     

A Carrier-Transit-Delay-Based Nonquasi-Static MOSFET Model for Circuit Simulation and Its Application to Harmonic Distortion Analysis

In this paper, a compact model of nonquasi-static (NQS) carrier-transport effects in MOSFETs is reported, which takes into account the carrier-response delay to form the channel. The NQS model, as implemented in the surface-potential-based MOSFET Hiroshima University STARC IGFET model, is verified to predict the correct transient terminal currents and to achieve a stable circuit simulation. Simulation results show that the NQS model can even reduce the circuit simulation time in some cases due to the elimination of unphysical overshoot peaks normally calculated by a QS-model. An average additional computational cost of only 3% is demonstrated for common test circuits. Furthermore, harmonic distortion characteristics are investigated using the developed NQS model. While the distortion characteristics at low drain bias and low switching frequency are determined mainly by carrier mobility, distortion characteristics at high frequency are found to be strongly influenced by channel charging/discharging.     

High field hole velocity and velocity overshoot in siliconinversion layers

We report measurements of the drift velocity of holes in silicon inversion layers. The saturation velocity of holes at 300 K is found to be strongly dependent on the effective vertical field. No hole velocity overshoot was observed down to 0.16 μm channel length at room temperature. At 77 K, hole velocity saturation is much less pronounced, and a 10% higher average velocity is observed for 0.16 μm channel length as compared to 0.36 μm channel length