Critical discussion on unified 1/f noise models for MOSFETs

Recently, unified noise models, like BSIM3, have been proposed in literature to describe the 1/f noise of n- and p-type MOSFETs in all operating regimes. These models combine carrier number fluctuations and correlated mobility fluctuations. The latter are induced by the Coulomb scattering of free carriers at trapped interface charge. The unified 1/f noise models assume implicitly that the mobility, limited by Coulomb scattering, does not depend on the inversion carrier density. However, this assumption is not correct in view of theoretical calculations and recent experimental results. In this paper, we show that the correlated mobility fluctuations are negligible, if the correct dependence on inversion carrier density is taken into account for the Coulomb scattering limited mobility. Consequently, the unified 1/f noise models cannot predict the 1/f noise observed experimentally in p-type MOSFETs, except if nonphysical fitting parameters are used. This paper serves as a critical discussion on the unified 1/f noise models for MOSFETs. Here it is not our intention to propose a new 1/f noise model     

Modeling path duration distributions in MANETs and their impact on reactive routing protocols

We develop a detailed approach to study how mobility impacts the performance of reactive mobile ad hoc network routing protocols. In particular, we examine how the statistics of path durations including probability density functions vary with the parameters such as the mobility model, relative speed, number of hops, and radio range. We find that at low speeds, certain mobility models may induce multimodal distributions that reflect the characteristics of the spatial map, mobility constraints and the communicating traffic pattern. However, this paper suggests that at moderate and high velocities the exponential distribution with appropriate parameterizations is a good approximation of the path duration distribution for a range of mobility models. Analytically, we show that the reciprocal of the average path duration has a strong linear relationship with the throughput and overhead of dynamic source routing (DSR), which is also confirmed by simulation results. In addition, we show how the mathematical expression obtained for the path duration distribution can also be used to prove that the nonpropagating cache hit ratio in DSR is independent of velocity for the freeway mobility model. These two case studies illustrate how various aspects of protocol performance can be analyzed with respect to a number of significant parameters including the statistics of link and path durations.     

Compact Channel Noise Models for Deep-Submicron MOSFETs

In this paper, compact channel noise models valid in all regions of operation for deep-submicron MOSFETs have been developed and experimentally verified. The physics-based expressions for thermal noise and flicker noise and corner frequency constitute compact channel noise models. The carrier heating, channel-length modulation, and mobility degradation due to the lateral electric field have been incorporated in the models. The effect of the mobility and carrier number fluctuations on the flicker noise, as well as the dependence of the mobility limited by Coulomb scattering on the inversion carrier density, have been considered in the flicker noise model. The measurement results validate the proposed models.      

A comprehensive model for inversion layer hole mobility forsimulation of submicrometer MOSFET's

A comprehensive model of effective (average) mobility and local-field mobility for holes in MOSFET inversion layers is presented. The semiempirical equation for effective mobility, coupled with the new local-field mobility model, permits accurate two-dimensional simulation of source-to-drain current in MOSFETs. The model accounts for the dependence of mobility on transverse and longitudinal electric fields, channel doping concentration, fixed interface charge density, and temperature. It accounts not only for the scattering by fixed interface charges, and bulk and surface acoustic phonons, but it also correctly describes screened Coulomb scattering at low effective transverse fields (near threshold) and surface roughness scattering at high effective transverse fields. The model is therefore applicable over a much wider range of conditions compared to earlier reported inversion layer hole mobility models while maintaining a physically based character     

The random waypoint mobility model with uniform node spatial distribution

In this paper, we tackle the problem of designing a random mobility model generating a target node spatial distribution. More specifically, we solve a long standing open problem by presenting two versions of the well-known random waypoint (RWP) mobility model in bounded regions generating a uniform steady-state node spatial distribution. In the first version, named temporal-RWP, we exploit the temporal dimension of node mobility and achieve uniformity by continuously changing the speed of a mobile node as a function of its location and of the density function of trajectories in the movement region R. In the second version, named spatial-RWP, we instead exploit the spatial dimension and achieve uniformity by selecting waypoints according to a suitably defined mix of probability density functions. Both proposed models can be easily incorporated in wireless network simulators, and are thus of practical use. The RWP models presented in this paper allow for the first time completely removing the well-known border effect causing possible inaccuracies in mobile network simulation, thus completing the picture of a “perfect” simulation methodology drawn in existing literature.     

The IMPORTANT framework for analyzing the Impact of Mobility on Performance Of RouTing protocols for Adhoc NeTworks

A Mobile Ad hoc Network (MANET) is a collection of wireless mobile nodes forming a temporary network without using any existing infrastructure. Since not many MANETs are currently deployed, research in this area is mostly simulation based. Random Waypoint is the commonly used mobility model in these simulations. Random Waypoint is a simple model that may be applicable to some scenarios. However, we believe that it is not sufficient to capture some important mobility characteristics of scenarios in which MANETs may be deployed. Our framework aims to evaluate the impact of different mobility models on the performance of MANET routing protocols. We propose various protocol independent metrics to capture interesting mobility characteristics, including spatial and temporal dependence and geographic restrictions. In addition, a rich set of parameterized mobility models is introduced including Random Waypoint, Group Mobility, Freeway and Manhattan models. Based on these models several ‘test-suite’ scenarios are chosen carefully to span the metric space. We demonstrate the utility of our test-suite by evaluating various MANET routing protocols, including DSR, AODV and DSDV. Our results show that the protocol performance may vary drastically across mobility models and performance rankings of protocols may vary with the mobility models used. This effect can be explained by the interaction of the mobility characteristics with the connectivity graph properties. Finally, we attempt to decompose the reactive routing protocols into mechanistic “building blocks” to gain a deeper insight into the performance variations across protocols in the face of mobility.     

Stochastic Properties of Mobility Models in Mobile Ad Hoc Networks

The stochastic model assumed to govern the mobility of nodes in a mobile ad hoc network has been shown to significantly affect the network's coverage, maximum throughput, and achievable throughput-delay trade-offs. In this paper, we compare several mobility models, including the random walk, random waypoint, and Manhattan models on the basis of the number of states visited in a fixed time, the time to visit every state in a region, and the effect of the number of wandering nodes on the time to first enter a set of states. These metrics for a mobility model are useful for assessing the achievable event detection rates in surveillance applications where wireless-sensor-equipped vehicles are used to detect events of interest in a city. We also consider mobility models based on Correlated Random Walks, which can account for time dependency, geographical restrictions, and nonzero drift. We demonstrate that these models are analytically tractable by using a matrix-analytic approach to derive new, closed-form results in both the time and transform-domains for the probability that a node is at any location at any time for both semi-infinite and finite 1D lattices. We also derive first entrance time distributions for these walks. We find that a correlated random walk 1) covers more ground in a given amount of time and takes a smaller amount of time to cover an area completely than a random walk with the same average transition rate, 2) has a smaller first entrance time to small sets of states than the random waypoint and random walk models, and 3) leads to a uniform distribution of nodes (except at the boundaries) in steady state.     

Modeling the transition from ohmic to space charge limited current in organic semiconductors

This paper presents a model for charge transport in organic and polymeric diodes that provides a physical explanation of the transition from ohmic to space-charge limited current (SCLC) regimes. The proposed model is based on two established models: a unified model for the injection and transport of charge in organic diodes, including a proper boundary condition for the free charge density at the metal–organic interface; and a temperature and electric-field dependent mobility model. The model reproduces published experimental current–voltage characteristics (for different temperatures and device lengths). The modeling results highlight the importance of the boundary condition at the interface that is used to explain different trends and their transitions in the current–voltage characteristics: linear, quadratic and a higher than quadratic trend at high electric fields. The model uses a finite charge at the interface to eliminate any dependence of the parameters of the mobility model with the length of the organic layer. Importantly, this result differs from others that consider an infinite charge at the interface, and make use of a length-dependent mobility, or other more complex models that incorporate a dependence with the free charge density in order to avoid such a dependence with the device length.     

Connectivity Probability of Wireless Ad Hoc Networks: Definition, Evaluation, Comparison

The paper presents a new approach investigating mobile ad hoc network connectivity. It is shown how to define and evaluate the connectivity probability of a mobile network where the position of the nodes and the link quality changes over time. The connectivity probability is a measure that can capture the impact of the node movement on the network connectivity. A number of mobility models is considered ranging from the classical Random Direction model to the Virtual World model based on the mobility measurements of a multi–player game. We introduce an Attractor model as a simple way to model non–homogeneous node distribution by incorporating viscosity regions in the simulation area. Methods of ergodic theory are used to show the correctness of the approach and to reduce the computational time. Simulation results show how the node density distribution affects the network connectivity.     

Tools for evaluation of social relations in mobility models

In MANET-DTN, the main idea is to detect the social relations between nodes in mobility models because the wireless mobile devices are carried by humans and the network uses the social relations to transport messages between isolated islands of the mobile terminals in order to increase the network performance. Today there are a lot of social and non-social mobility models. The problem is how to use these models in MANET-DTN. Therefore, proper evaluation method is needed, that is able to reveal the social aspect of investigated mobility model. Since there are a lot of methods that do not directly exhibit the social aspect of mobility models, in this paper the new evaluation method was proposed based on Louvain method for community detection and the other network graph parameter (average weighted degree). Simulations of evaluation method were made as a comparison between two random mobility models and one social based mobility model in order to point out differences between social and non-social mobility models. All models were evaluated by proposed method and other existing protocol dependent and independent methods. The main idea of the simulations was to analyse how the mobility models with social and non-social mobility models can affect the network performance and provide new and reliable tool, which enables analysis of the mobility models from social behaviour point of view.     

Charge‐Carrier Density Independent Mobility in Amorphous Fluorene‐Triarylamine Copolymers

A charge‐carrier density dependent mobility has been predicted for amorphous, glassy energetically disordered semiconducting polymers, which would have considerable impact on their performance in devices. However, previous observations of a density dependent mobility are complicated by the polycrystalline materials studied. Here charge transport in field‐effect transistors and diodes of two amorphous, glassy fluorene‐triarylamine copolymers is investigated, and the results explored in terms of a charge‐carrier density dependent mobility model. The nondispersive nature of the time‐of‐flight (TOF) transients and analysis of dark injection transient results and transistor transfer characteristics indicate a charge‐carrier density independent mobility in both the low‐density diode and the high‐density transistor regimes. The mobility values for optimized transistors are in good agreement with the TOF values at the same field, and both have the same temperature dependency. The measured transistor mobility falls two to three orders of magnitude below that predicted from the charge‐carrier density dependent model, and does not follow the expected power‐law relationship. The experimental results for these two amorphous polymers are therefore consistent with a charge‐carrier density independent mobility, and this is discussed in terms of polaron‐dominated hopping and interchain correlated disorder.     

Fabrication,characterization,numerical simulation and compact modeling of P3HT based organic thin film transistors

This paper presents the fabrication,characterization and numerical simulation of poly-3-hexylthiophene (P3HT)-based bottom-gate bottom-contact (BGBC) organic thin film transistors (OTFTs).The simulation is based on a drift diffusion charge transport model and density of defect states (DOS) for the traps in the band gap of the P3HT based channel.It com-bines two mobility models,a hopping mobility model and the Poole-Frenkel mobility model.It also describes the defect dens-ity of states (DOS) for both tail and deep states.The model takes into account all the operating regions of the OTFT and in-cludes sub-threshold and above threshold characteristics of OTFTs.The model has been verified by comparing the numerically simulated results with the experimental results.This model is also used to simulate different structure in four configurations of OTFT e.g.bottom-gate bottom-contact (BGBC),bottom-gate top-contact (BGTC),top-gate bottom-contact (TGBC) and top-gate top-contact (TGTC) configurations of the OTFTs.We also present the compact modeling and model parameter extraction of the P3HT-based OTFTs.The extracted compact model has been further applied in a p-channel OTFT-based inverter and three stage ring oscillator circuit simulation.     

Oxide thickness dependent compact model of channel noise for E-mode AlGaN/GaN MOS-HEMT

In this paper, a compact channel noise model for gate recessed enhancement mode GaN based MOS-HEMT which is valid for all regions of operation is proposed. The compact noise model consists of high frequency thermal noise and low frequency flicker noise. The drain current, which is one of the most important parameters for compact noise model is developed by incorporating interface and oxide traps, mobility degradation due to vertical electric field, velocity saturation effect and self-heating effect. The flicker noise model is derived by considering mobility and carrier fluctuation due to traps present in both oxide and interface layer. The thermal noise and flicker noise models are validated by comparing the results with TCAD simulation and experimental results from literature respectively. Effect of thermal and flicker noise power spectral density (PSD) variation with different oxide thickness has also been analyzed.     

IMPACT OF BUSY LINES AND MOBILITY ON CALL BLOCKING IN A PCS NETWORK

Several analytical models have been proposed to study the blocking probability for personal communications service networks or mobile phone networks. These models cannot accurately predict the blocking probability because they do not capture two important features. First, they do not capture the busy-line effect. Even if a cell has free channels, incoming and outgoing calls must be dropped when the destination portable is already in a conversation. Second, they do not capture the mobility of individual portables. In these models, mobility is addressed by net hand-off traffic to a cell, which results in traffic with a smaller variance to a cell compared with the true situation. We propose a new analytic model which addresses both the busy-line effect and individual portable mobility. Furthermore, our model can be used to derive the portable population distribution in a cell. The model is validated against the simulation experiments. We indicate that the previously proposed models approximate a special case of our model where the number of portables in a cell is 40 times larger than the number of channels.     

Intelligent actor mobility in wireless sensor and actor networks

In wireless sensor and actor network research, the commonly used mobility models for a mobile actor are random walk model, random waypoint mobility model, or variants thereof. For a fully connected network, the choice of mobility model for the actor is not critical because, there is at least one assured path from the sensor nodes to the actor node. But, for a sparsely connected network where information cannot propagate beyond a cluster, random movement of the actor may not be the best choice to maximize event detection and subsequent action. This paper presents static and dynamic intelligent mobility models that are based on the inherent clusters’ information of a sparsely connected network. Simulation results validate the idea behind the intelligent mobility models and provide insights into the applicability of these mobility models in different application scenarios.     

基于人类真实场景的分时段的机会网络移动模型

针对现有移动模型不能有效反映出节点移动过程中的行为特性,提出了一种基于人类真实场景中的分时段的机会网络移动模型,通过建立节点日常移动模型,依据时间段划分节点子移动模型,分析了基于人类真实场景的机会网络移动模型下的节点行为特性,包括节点平均停留时间、不同时段社区节点个数以及目的变换频率等。并与已采集到的真实移动数据和其他移动模型进行仿真比较。仿真结果充分表明,在节点相遇间隔时间等方面,该模型贴近真实场景中节点所表现出的行为特性,并且优于其他节点移动模型。     

Monte Carlo simulation of electron transport in the III-nitridewurtzite phase materials system: binaries and ternaries

We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al_xGa_1-xN and In_xGa_1-xN. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides     

The Temporal Effect of Mobility on Path Length in MANET

Ad hoc network consists of a set of identical nodes that move freely and independently and communicate among themselves via wireless links. The most interesting feature of this network is that they do not require any existing infrastructure of central administration and hence is very suitable for temporary communication links in an emergency situation. This flexibility, however, is achieved at a price of communication uncertainty induced due to frequent topology changes. In this article, we have tried to identify the system dynamics using the proven concepts of time series modeling. Here, we have analyzed variation of path length between a particular source destination pair nodes over a fixed area for different mobility patterns under different routing algorithm. We have considered four different mobility models—(i) Gauss-Markov mobility model, (ii) Manhattan Grid mobility model and (iii) Random Way Point mobility model and (iv) Reference Point Group mobility model. The routing protocols under which, we carried out our experiments are (i) Ad hoc On demand Distance Vector routing (AODV), (ii) Destination Sequenced Distance Vector routing (DSDV) and (iii) Dynamic Source Routing (DSR). The path length between two particular nodes behaves as a random variable for all mobility models for all routing algorithms. The pattern of path length for every combination of mobility model and for every routing protocol can be well modeled as an autoregressive model of order p i.e. AR(p). The order p is estimated and it is found that most of them are of order unity only. We also calculate the average path length for all mobility models and for all routing algorithms.     

Study of applicability of Boltzmann-statistics and two mobility models for organic semiconductors

The organic semiconductors are treated as non-degenerate based on recent experiment on validity of classical Einstein relationship. The expression of density of holes is analytically derived by using the Boltzmann statistics. The mobility model of Pasveer et al. and the exponential model of Pai modified by Blom et al. are combined to solve drift-diffusion equations to extract information about the mobility and effective density of state. The results show that the mobility model of Pasveer et al. can not well fit the experimental J-V data for a rubrene single crystal from Krellner et al. both at low and high voltages, and some of the parameters extracted show inconsistent temperature dependence which should be constants in the theoretical framework of Pasveer et al. Whereas the exponential model gives satisfactory fit for experimental J-V data at all voltage ranges, and the extracted parameters show correct temperature dependence. The temperature dependence of parameters contained in the exponential model of mobility can be well fitted by using simple expressions proposed by Blom et al. And the temperature dependence of effective density of state derived from the non-degenerate Boltzmann statistics is verified. The distribution of potential, electric field and density of holes are calculated as analyzed.