Paolo Lugli

The Monte Carlo method for semiconductor device simulation

1 Introduction.- References.- 2 Charge Transport in Semiconductors.- 2.1 Electron Dynamics.- 2.2 Energy Bands.- 2.2.1 Relationship of Energy to Wavevector.- 2.2.2 Effective Masses.- 2.2.3 Nonparabolicity.- 2.2.4 Herring and Vogt Transformation.- 2.2.5 Actual Bands of Real Semiconductors.- 2.3 Scattering Mechanisms.- 2.3.1 Classification and Physical Discussion.- 2.3.2 Fundamentals of Scattering.- 2.4 Scattering Probabilities.- 2.4.1 Phonon Scattering, Deformation-Potential Interaction.- 2.4.2 Phonon Scattering, Electrostatic Interaction.- 2.4.3 Ionized Impurity Scattering.- 2.4.4 Carrier-Carrier Scattering.- 2.5 Transport Equation.- 2.6 Linear Response and the Relaxation Time Approximation.- 2.6.1 Relaxation Times for the Various Scattering Mechanisms.- 2.6.2 Carrier Mobilities in Various Materials.- 2.7 Diffusion, Noise, and Velocity Autocorrelation Function.- 2.7.1 Basic Macroscopic Equations of Diffusion.- 2.7.2 Diffusion, Autocorrelation Function, and Noise.- 2.7.3 Electron Lifetime and Diffusion Length.- 2.8 Hot Electrons.- 2.9 Transient Transport.- 2.10 The Two-dimensional Electron Gas.- 2.10.1 Subband Levels and Wavefunctions.- 2.10.2 Scattering Rates.- References.- 3 The Monte Carlo Simulation.- 3.1 Fundamentals.- 3.2 Definition of the Physical System.- 3.3 Initial Conditions.- 3.4 The Free Flight, Self Scattering.- 3.5 The Scattering Process.- 3.6 The Choice of the State After Scattering.- 3.6.1 Phonon Scattering, Deformation-Potential Interaction.- 3.6.2 Phonon Scattering, Electrostatic Interaction.- 3.6.3 Ionized Impurity Scattering.- 3.6.4 Carrier-Carrier Scattering.- 3.7 Collection of Results for Steady-State Phenomena.- 3.7.1 Time Averages.- 3.7.2 Synchronous Ensemble.- 3.7.3 Statistical Uncertainty.- 3.8 The Ensemble Monte Carlo (EMC).- 3.9 Many Particle Effects.- 3.9.1 Carrier-Carrier Scattering.- 3.9.2 Molecular Dynamics and Monte Carlo Method.- 3.9.3 Degeneracy in Monte Carlo Calculations.- 3.10 Monte Carlo Simulation of the 2DEG.- 3.11 Special Topics.- 3.11.1 Periodic Fields.- 3.11.2 Diffusion, Autocorrelation Function, and Noise.- 3.11.3 Ohmic Mobility.- 3.11.4 Impact Ionization.- 3.11.5 Magnetic Fields.- 3.11.6 Optical Excitation.- 3.11.7 Quantum Mechanical Corrections.- 3.12 Variance-reducing Techniques.- 3.12.1 Variance Due to Thermal Fluctuations.- 3.12.2 Variance Due to Valley Repopulation.- 3.12.3 Variance Related to Improbable Electron States.- 3.13 Comparison with Other Techniques.- 3.13.1 Analytical Techniques.- 3.13.2 The Iterative Technique.- 3.13.3 Comparison of the Different Techniques.- References.- 4 Review of Semiconductor Devices.- 4.1 Introduction.- 4.2 Historical Evolution of Semiconductor Devices.- 4.2.1 Evolution of Si Devices.- 4.2.2 Evolution of GaAs Devices.- 4.2.3 Technological Features.- 4.2.4 Scaling and Miniaturization.- 4.3 Physical Basis of Semiconductor Devices.- 4.3.1 p-n Junction.- 4.3.2 Bipolar Transistors.- 4.3.3 Heterojunction Bipolar Transistor.- 4.3.4 Metal-Semiconductor Contacts.- 4.3.5 Metal-Semiconductor Field-Effect Transistor.- 4.3.6 Metal-Oxide-Semiconductor Field-Effect Transistor.- 4.3.7 High Electron Mobility Transistor.- 4.3.8 Hot Electron Transistors.- 4.3.9 Permeable Base Transistor.- 4.4 Comparison of Semiconductor Devices.- 4.4.1 Device Parameters.- 4.4.2 Comparison of Semiconductor Devices.- References.- 5 Monte Carlo Simulation of Semiconductor Devices.- 5.1 Introduction.- 5.2 Geometry of the System.- 5.2.1 Boundary Conditions.- 5.2.2 Grid Definition.- 5.2.3 Superparticles.- 5.3 Particle-Mesh Force Calculation.- 5.3.1 Particle-Mesh Calculation in One Dimension.- 5.3.2 Charge Assignment Schemes in Two Dimensions.- 5.4 Poisson Solver and Field Distribution.- 5.4.1 Finite Difference Scheme.- 5.4.2 Matrix Methods.- 5.4.3 Rapid Elliptic Solvers (RES).- 5.4.4 Iterative Methods.- 5.4.5 Calculation of the Electric Field.- 5.4.6 The Collocation Method.- 5.5 The Monte Carlo Simulation of Semiconductor Devices.- 5.5.1 Initial Conditions.- 5.5.2 Time Cycles.- 5.5.3 Free Flight.- 5.5.4 Scattering.- 5.5.5 Carrier-Carrier Scattering.- 5.5.6 Degenerate Statistics.- 5.5.7 Statistics.- 5.5.8 Static Characteristics.- 5.5.9 A.C. Characteristics.- 5.5.10 Noise.- References.- 6 Applications.- 6.1 Introduction.- 6.2 Diodes.- 6.2.1 n+-n-n+ Diodes.- 6.2.2 Schottky Diode.- 6.3 MESFET.- 6.3.1 Short Channel Effects.- 6.3.2 Geometry Effects.- 6.3.3 Space-Charge Injection FET.- 6.3.4 Conclusions.- 6.4 HEMT and Heterojunction Real Space Transfer Devices.- 6.4.1 HEMT.- 6.4.2 Real-Space Transfer Devices.- 6.4.3 Velocity-Modulation Field Effect Transistor.- 6.5 Bipolar Transistor.- 6.6 HBT.- 6.7 MOSFET and MISFET.- 6.7.1 MOSFET.- 6.7.2 GaAs Injection-modulated MISFET.- 6.7.3 Conclusions.- 6.8 Hot Electron Transistors.- 6.8.1 The THETA Device.- 6.8.2 GaAs FET with Hot-Electron Injection Structure.- 6.8.3 Planar-doped-Barrier Transistors.- 6.9 Permeable Base Transistor.- 6.10 Comparison with Traditional Simulators.- References.- Appendix A. Numerical Evaluation of Some Integrals of Interest.- References.- Appendix B. Generation of Random Numbers.- References.

EFFECTS OF MACROSCOPIC POLARIZATION IN III-V NITRIDE MULTIPLE QUANTUM WELLS

Huge built-in electric fields have been predicted to exist in wurtzite III-V nitrides thin films and multilayers. Such fields originate from heterointerface discontinuities of the macroscopic bulk polarization of the nitrides. Here we discuss the background theory, the role of spontaneous polarization in this context, and the practical implications of built-in polarization fields in nitride nanostructures. To support our arguments, we present detailed self-consistent tight-binding simulations of typical nitride quantum well structures in which polarization effects are dominant.

Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures

The free-carrier screening of macroscopic polarization fields in wurtzite GaN/InGaN quantum well lasers is investigated via a self-consistent tight-binding approach. We show that the high carrier concentrations found experimentally in nitride laser structures effectively screen the built-in spontaneous and piezoelectricpolarization fields, thus inducing a “field-free” band profile. Our results explain some heretofore puzzling experimental data on nitride lasers, such as the unusually high lasing excitation thresholds and emission blue shifts for increasing excitation levels.

Fully Spray Coated Organic Photodiodes

Solution-processed organic diodes based on bulk heterojunctions are attractive for large area photodetection. We report a general approach for fully spray-coated organic photodiodes with outstanding characteristics in comparison to bladed or spin-coated devices. Despite the high surface roughness and the less defined morphology of the spray-deposited organic layers, we observe organic photodetectors with responsivities of 0.36 A/W and noise equivalent powers of 0.2 pW/H(1/2) in the visible spectrum at high reverse biases of -5 V. Furthermore, we demonstrate device lifetimes beyond 1 year as well as superior yield and reproducibilties for the dark current and photocurrent densities.

Effects of grain boundaries, field-dependent mobility, and interface trap States on the electrical Characteristics of pentacene TFT

We have fabricated pentacene-based thin film transistors and analyzed their electrical properties with the help of two-dimensional drift-diffusion simulations which favorably compare with the experimental results. We have set up a model considering the polycrystalline nature of pentacene and the presence of grains and grain boundaries. We show how this model can be applied to different devices with different grain sizes and we analyze the relationship between mobility, grain size and applied gate voltage. On the basis of the simulation results, we can introduce an effective carrier mobility, which accounts for grain-related effects. The comparison between experimental results and simulations allows us to clearly understand the differences in the mobility derived by the analysis of current-voltage curve (as done experimentally by using standard MOSFET theory) and the intrinsic mobility of the organic layer. The effect of the pentacene/oxide interface traps and fixed surface charges has also been considered. The dependence of the threshold voltage on the density and energy level of the trap states has been outlined.

Room-temperature polariton lasers based on GaN microcavities

The critical temperature for Bose condensation of exciton polaritons in an AlGaN microcavity containing 9 GaN quantum wells is calculated to be T5460 K. We have modeled the kinetics of polaritons in such a microcavity device using the two-dimensional Boltzmann equation. Room-temperature lasing is found with a threshold as small as 100 mW. The kinetic blocking of polariton relaxation that prevents formation of the Bose-condensed phase of polaritons at low temperatures disappears at high temperatures, especially in n-doped samples. Thus, GaN microcavities are excellent candidates for realization of room-temperature polariton lasers.

Science and Engineering Beyond Moore's Law

In this paper, the historical effects and benefits of Moores law for semiconductor technologies are reviewed, and it is offered that the rapid learning curve obtained to the benefit of society by feature size scaling might be continued in several different ways. The problem is that as features approach the range of a few nanometers, electron-based devices depart radically from the ideal switch and, in fact, become very leaky in the off state. It is argued that there are some short-term solutions involving more highly parallel manufacturing, increased design efficiency, and lower cost packaging technologies that could continue the steep learning curve for cost reductions that have historically been achieved via Moores Law scaling. Another alternative might be to increase chip functionality by integrating devices that offer broadened chip functionality including, e.g., sensors, energy sources, oscillators, etc. A third alternative would be to invent an entirely new information processing state variable based on different physics, using electron spin, magnetic dipoles, photons, etc., to improve the performance and reduce switching energy for devices whose smallest features are on the order of a few nanometers. Each of these alternatives is being actively explored and an overview of each strategy and progress to date is given in the paper. A final alternative offered in the paper is to learn from information processing examples in nature, specifically in living systems. An E.coli cell of about one cubic micrometer volume is shown to be an incredibly powerful and energy-efficient information processor relative to the performance of an end-of-scaling silicon processor of the same volume. The paper concludes by pointing out some of the crucial differences between E.coli information processing and conventional approaches with the hope technologies can be invented using the hints offered by biosystems.

The Bistable Ring PUF: A new architecture for strong Physical Unclonable Functions

This paper introduces a new architecture for circuit-based Physical Unclonable Functions (PUFs) which we call the Bistable Ring PUF (BR-PUF). Based on experimental results obtained from FPGA-based implementations of the BR-PUF, the quality of this new design is discussed in different aspects, including uniqueness and reliability. On the basis of the observed complexity in the challenge-response behavior of BR-PUFs, we argue that this new PUF could be a promising candidate for Strong PUFs. Our design shows noticeable temperature sensitivity, but we discuss how this problem can be addressed by additional hardware and protocol measures.

Spontaneous and piezoelectric polarization effects on the output characteristics of AlGaN/GaN heterojunction modulation doped FETs

We report on the calculation of electrical characteristics of AlGaN/GaN heterojunction field effect transistors (HFETs). The model is based on the self-consistent solution of the Schrodinger and Poisson equations coupled to a quasi-2D model for the current flow. Both single and double heterojunction devices are analyzed for [0001] or [000-1] growth directions. The onset of a parasitic p-channel for particular growth directions and alloy concentrations is also shown.

Nanostructured interfaces in polymer solar cells

The morphology in organic photovoltaics plays a key role in determining the device efficiency. We propose a method to fabricate bilayer devices with controlled nanostructured interfaces by combining nanoimprinting and lamination techniques. This technique allows us to achieve a network structure of donor-acceptor material with a ∼80 nm periodicity and ∼40 nm width. These structures have an abrupt interface between the donor and acceptor materials and show an increased effective interfacial area and photovoltaic performance compared to bilayer solar cells. In contrast to blend films, they will allow an in depth analysis of the influence of morphology on interfacial physical processes.