Velocity Overshoot Research Articles

Monte Carloanalysis of dynamic and noise performance of submicron MOSFETs at RF and microwave frequencies

In this paper, an ensemble 2D bipolar Monte Carlosimulator is employed for the study of static characteristics,high-frequency response and noise behaviour in a 0.3 µmgate-length n-MOSFET in common source configuration.Short-channel effects, such as velocity overshoot in the pinch-off region, together with the appearance of hot electrons at thedrain end of the channel are observed in the staticcharacteristics. Admittance parameters and the small-signalequivalent circuit have been calculated in order tocharacterize the dynamic response of the device. The use of abipolar simulator allows one to study the dynamics of both types ofcarriers simultaneously. While the static results are dominatedby the electron transport, the contribution of holes mainly affects the drain-substrate capacitive coupling. The noisebehaviour of the simulated MOSFET is also studied (up to40 GHz) by means of different parameters, such as the spectral densities of the current fluctuations at the drain and gateterminals (and their cross-correlation), normalized α,β and C parameters and NFmin . In the saturationregime, due to the presence of hot carriers, an increase indrain and gate noise with respect to the long-channelprediction has been found. Moreover, a stronger correlationbetween drain and gate noise is observed, especially at lowdrain current. Induced gate noise is found to play a crucialrole in the determination of NFmin at high draincurrents.

Recent critical issues in Si/Si 1− xGe x/Si heterostructure FET devices

In this paper we present a summary of the most important critical issues in Si/Si 1− x Ge x p-type and n-type heterostructure field-effect transistors. The controversial issue of alloy scattering and the phenomenon of velocity overshoot are reviewed and discussed. Achievements and problems associated with channel engineering and the use of alternative gate electrodes and high- κ dielectric materials are also addressed.

An exact solution of visco-capillary flow in an inclined channel

An exact solution of visco-capillary flow in an inclined channel is presented. Assuming that the flow is steady and unidirectional the surface shape and the velocity profile are obtained as exact solutions of the Navier-Stokes equations valid for arbitrary Reynolds numbers.¶The contributions of the determining parameters of the problem to the surface shape and to the velocity profile are discussed. A structural change in the velocity profile, the 'velocity overshoot', is found for cases with meniscus elevation higher than the apex height of the flow. The theoretical findings are compared to experimental results.

Analytic modelling for current–voltage characteristics of InGaP/InGaAs/GaAs pseudomorphic doped-channel field-effect transistors

A two-dimensional analytic model is proposed for characterizing the InGaP/InGaAs/GaAs metal–insulator–semiconductor (MIS) like pseudomorphic doped-channel field-effect transistor (PDCFET). The velocity overshoot effects, associated with the low effective mass in the In 0.15Ga0.85 As channel, have been included to solve the 2D Poisson equation. The theoretical simulation provides a convenient and efficient way to describe the device properties of PDCFET’s. The calculated results demonstrate in excellent agreement with the experimental current–voltage characteristics. Device performances with respect to calculations of various structural dimensions have also been extended and investigated.

Design and analysis of a new self-aligned asymmetric structure for deep sub-micrometer MOSFET

A new self-aligned asymmetric structure (SAAS) is proposed for deep sub-micrometer MOSFET and its device characteristics are analyzed. The proposed structure enables the source, drain and channel to be designed independently without additional lithography steps. SAAS with lateral asymmetric channel and highly doped source extension improves driving capability and short channel behavior without sacrificing hot carrier reliability. Based on the results of hydrodynamic device simulation over a wide range of process conditions, it is shown that highly doped asymmetric halo provides enhanced velocity overshoot and suppressed drain-induced barrier lowering. By employing asymmetric highly doped source extension, the degradation of driving capability is suppressed that can be caused by the increased parasitic resistance in highly doped asymmetric halo.

Time-resolved electroabsorption measurement of the transient electron velocity overshoot in GaN

A femtosecond time-resolved electroabsorption technique employing an AlGaN/GaN heterojunction p–i–n diode with a p-type AlGaN window layer and a semitransparent p contact has been used to measure the transient electron velocity overshoot in GaN. A peak transient electron velocity of 7.25×107 cm/s within the first 200 fs after photoexcitation has been observed at a field of 320 kV/cm. The increase in electron transit time across the device with increasing field beyond 320 kV/cm provides experimental evidence for a negative differential resistivity region of the steady-state velocity-field characteristic in this high field range.

Gate-length dependence of negative differential resistance in ridge-type InGaAs/InAlAs quantum wire field-effect transistor

We investigate the gate-length dependence of negative differential resistance (NDR) in a ridge-type InGaAs/InAlAs quantum wire field-effect transistor with a gate varying from 50 nm to 1 μm in length. The gate leakage current is detectable only with the onset of NDR and increases with larger gate bias. The NDR could be attributed to the real space transfer of channel carrier into the barrier layer by gate field-assisted tunneling. The NDR characteristics are dependent strongly on gate length. The improvement in NDR characteristic in terms of peak-to-valley current ratio and onset voltage (VNDR) is observed clearly with the reduction of gate length. The QWIR-FETs with shorter gate length show the NDR even at room temperature. The improvement in NDR characteristic is attributed to a velocity overshoot enhanced by the suppression of scattering probability.

Impact of Strained-Si Channel on Complementary Metal Oxide Semiconductor Circuit Performance under the Sub-100 nm Regime

The impact of the enhanced mobility of a strained-Si channel on the performance of sub-100 nm complementary metal oxide semiconductor (CMOS) circuits is investigated by the combination of device and circuit simulations, considering both velocity saturation and velocity overshoot effects. It is found that higher mobilities of strained Si become more advantageous with reducing the channel length. It is also pointed out that the increase in energy relaxation time in strained Si, in addition to higher mobility, is effective for the enhancement of the circuit performance. The evaluated performance of 50 nm strained-Si CMOS amounts to around 1.7 times that of bulk-Si CMOS.

Nonstationary electron/hole transport in sub-0.1 μm MOS devices: correlation with mobility and low-power CMOS application

We have experimentally studied the high-lateral-field carrier velocity near the source edge in sub-0.1 /spl mu/m MOSFETs. It is demonstrated that the high-field electron velocity and hole velocity have universal low-field mobility dependence. This shows that the hole velocity is lower than the electron velocity due to the hole's lower mobility. Moreover, we have investigated the low-power CMOS operation using the velocity overshoot. It is verified that there is a most suitable supply voltage for improving the CMOS operation using velocity overshoot. The most suitable supply voltage is shown to be about 1 V. Therefore, the velocity overshoot will be very useful for low voltage CMOS operation in the future.

Comparative study of electron transit times evaluated by DD, HD, and MC device simulation for a SiGe HBT

Transit times of a silicon/germanium heterojunction bipolar transistor (HBT) with a base width of 24 nm are investigated in the quasi-stationary limit for the first time by consistent drift-diffusion (DD), hydrodynamic (HD), and fullband Monte Carlo (MC) simulations. The quasi-ballistic transport in the base and collector leading to a strong velocity overshoot is well described by the HD model and corresponding transit times are in good agreement with the MC results. On the other hand, the DD model fails in this region and substantially overestimates the base transit time bearing the possibility of wrong guidelines for transistor design optimization. However, since the base transit time is no longer dominating the cutoff frequency of high-speed HBTs, the failure of the DD model leads to an underestimation of the peak cutoff frequency by only 10%. Close to high injection differences in the emitter transit times of the HD and MC model are observed which are mainly related to small differences in the Gummel plot.

Nonequilibrium electron transport in HBTs

Nonequilibrium electron transport in heterojunction bipolar transistors (HBTs) becomes clearly observable as their vertical dimensions are reduced, This is the reason for the superior tradeoff relationships between the device parameters and the resultant high-speed performance of HBTs fabricated with III-V semiconductor materials. This paper reviews the hot carrier effect in the base and the velocity overshoot effect in the collector and discusses the roles these effects play in reducing carrier traveling times and improving device performance.

Physical compact modeling and analysis of velocity overshoot in extremely scaled CMOS devices and circuits

A compact physics-based velocity-overshoot model is developed, implemented in metal oxide semiconductor field-effect transistor (MOSFET) circuit models, verified based on measured current-voltage data and Monte Carlo-simulation results, and demonstrated in performance projections for 25 nm bulk-Si complementary metal-oxide-semiconductor (CMOS). The demonstration, involving predicted current-voltage characteristics and ring-oscillator propagation delays, reveals a significant benefit of velocity overshoot, or quasi-ballistic transport, in extremely scaled nMOS and even pMOS devices, although the on-state currents are well below the ballistic limits. Physical insight afforded by the model reveals why the ballistic limits are not being reached in scaled bulk-Si CMOS technologies.

Electron transport in AlN under high electric fields

AbstractThe energy distribution of electrons transported through an intrinsic AlN film was directly measured as a function of the applied field, and AlN film thickness. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through films thicker than 95 nm and applied field between 200 kV/cm -350 kV/cm occurred as steady-state hot electron transport represented by a Maxwellian energy distribution, with a corresponding carrier temperature. At higher fields (470 kV/cm), intervalley scattering was evidenced by a multi-component energy distribution with a second peak at the energy position of the first satellite valley. Electron transport through films thinner than 95 nm demonstrated velocity overshoot under fields greater than 550 kV/cm. This was evidenced by a symmetric energy distribution centered at an energy above the conduction band minimum. This indicated that the drift component of the electron velocity was on the order of the “thermal” component. A transient length of less than 80 nm was deduced from these observations.

Modification of drift-diffusion model for shortbase transport

A modified drift-diffusion model for the simulation of quasi-ballistic transport in short-base bipolar transistors has been proposed. It includes the effects of base end velocity overshoot and reduction in the minority-carrier diffusion coefficient. Numerical results show good agreement with those obtained by Monte Carlo simulation.

The Velocity Overshoot in Semiconductors According to a Transport Model Derived from the Boltzmann Equation

ABSTRACT A kinetic model derived from the Boltzmann equation, by using the spherical harmonic expansion method, is considered. This describes the evolution of an electron gas inside a bulk semiconductor in thermal equilibrium. The problem of velocity overshoot is analyzed in detail. A numerical scheme is proposed and the numerical results are shown.

Experimental observation of velocity overshoot in n-channel AlGaAs/InGaAs/GaAs enhancement mode MODFETs

Velocity overshoot phenomena in n-channel Al-GaAs/InGaAs/GaAs enhancement mode MODFETs have been investigated for gate lengths ranging from 1 to 0.5 /spl mu/m. The study is based on Motorola's established CGaAs/sup TM/ technology. The observed average electron velocity /spl upsi//sub a/spl upsi// under the gate is 1.05, 1.34, 1.48, and 1.71/spl times/10/sup 7/ cm/s for a gate length L/sub G/ of 1, 0.7, 0.6, and 0.5 /spl mu/m, respectively. The presence of velocity overshoot in InGaAs channels is clearly proven with average electron velocities exceeding the steady-state saturation velocity of /spl cong/1/spl times/10/sup 7/ cm/s for L/sub G//spl les/0.7 /spl mu/m, and with the significant increase of /spl upsi//sub a/spl upsi// with shorter gate length.

Gate-Length Dependence of Negative Differential Resistance in InGaAs/InAlAs Quantum Well Field-Effect Transistor

We have investigated the gate-length dependence of negative differential resistance (NDR) in an InGaAs/InAlAs quantum well field-effect transistor (QW-FET) by varying the gate length from 50 nm to 0.8 µm. A pronounced N-shaped NDR is clearly observed in these QW-FETs. The gate leakage current is detectable only by the onset of NDR. We believe that the NDR should be attributed to the real space transfer of channel electrons into the barrier layer with low mobility underneath the gate by gate field-assisted tunneling (RSTT). Both peak-to-valley current ratio (PVR) and onset voltage (VNDR) are found to be strongly dependent on the gate length. The NDR characteristics in terms of PVR and VNDR are improved dramatically by reducing the gate length, and originate from the velocity overshoot due to the suppression of scattering probability.

Field observations of the wave bottom boundary layer

This paper presents a comprehensive set of velocity and suspended sediment observations in the nearshore wave bottom boundary layer, collected during the Duck94 field experiment on the Outer Banks of the North Carolina coast. Cross‐shore velocity measurements in the wave bottom boundary layer were made using five hot film anemometers, nominally spaced from 1 to 5 cm above the bed in 2 m of water depth. The time‐varying location of the seabed was estimated to roughly 1 cm with a stacked set of bed‐penetrating fiber‐optic backscatter sensors. The instrument array was intermittently located in the surf zone on the crest of a bar. The location of the bottom varied several centimeters over a 34 min data run. Even over 4 min segments of quasi‐steady statistics, occasional large waves caused short erosion and redeposition events, complicating the definition of bottom location and causing the root‐mean‐square velocity statistics to be nonzero below the mean bed location. This leads to obvious difficulties in comparisons with two, one‐dimensional time‐dependent, eddy viscosity wave bottom boundary layer models. For example, bed shears based on rms amplitude decay were lower than predicted. The observations show some evidence for a velocity overshoot region within the wave bottom boundary layer. The observations were compared with two linear eddy viscosity models. Larger estimates of a constant eddy viscosity and smaller than predicted phase leads are indicative of more rapid mixing of momentum than predicted by the models. The phase and amplitude frequency response estimated with frequency domain empirical orthogonal functions shows a nonlinear response of the wave bottom boundary layer over the incident band. These observations are among the first coherent looks at the wave bottom boundary layer under conditions of significant sediment response. They highlight the added complexity of the dynamics in natural environments.

Indication of velocity overshoot in strained Si0.8Ge0.2p-channel MOSFETs

A velocity-field study of several Si0.8Ge0.2/Si p-channel MOSFETs with self-aligned poly-Si gates,thick gate oxides and effective channel lengths ranging from 1.5 to8.5 µm, was carried out at room temperature. Comprehensivetwo-dimensional simulations of devices using drift-diffusion (DD), andbulk Monte Carlo calibrated hydrodynamic (HD) and energy transport (ET)models have revealed enhanced high-field hole transport in strained-channelMOSFETs. A close agreement is obtained between higher-level (HD/ET) modelsand DD model with calibrated high-field mobility parameters. It is foundthat the relatively low value of extracted saturation velocity inlong-channel Si0.8Ge0.2 p-MOSFETs increasesconsiderably as the gate length is decreased. The increase in short-channelsamples is attributed to non-equilibrium transport effects in the regionnear the source, resulting from higher mobility and longer relaxation timesof holes in the strained SiGe layer. Our results not only confirm theexpected advantage of strained SiGe p-MOSFETs in low-field transport,but also indicate that this is accompanied by an early onset of velocityovershoot, which may be beneficial in aggressively scaled devices.

Deep submicrometer SOI MOSFET drain current model including series resistance, self-heating and velocity overshoot effects

We have developed a new analytical ultrashort channel SOI MOSFET for circuit simulation where the effects of series resistance, self-heating and velocity overshoot are included. We have reproduced experimental measurements validating our model. Its simplicity allowed us to study the contribution of each effect separately in an easy way.