Showing papers by "Bell Labs published in 1986"
Book•
01 Jan 1986TL;DR: This book discusses the design of a Code Generator, the role of the Lexical Analyzer, and other topics related to code generation and optimization.
Abstract: 1 Introduction 1.1 Language Processors 1.2 The Structure of a Compiler 1.3 The Evolution of Programming Languages 1.4 The Science of Building a Compiler 1.5 Applications of Compiler Technology 1.6 Programming Language Basics 1.7 Summary of Chapter 1 1.8 References for Chapter 1 2 A Simple Syntax-Directed Translator 2.1 Introduction 2.2 Syntax Definition 2.3 Syntax-Directed Translation 2.4 Parsing 2.5 A Translator for Simple Expressions 2.6 Lexical Analysis 2.7 Symbol Tables 2.8 Intermediate Code Generation 2.9 Summary of Chapter 2 3 Lexical Analysis 3.1 The Role of the Lexical Analyzer 3.2 Input Buffering 3.3 Specification of Tokens 3.4 Recognition of Tokens 3.5 The Lexical-Analyzer Generator Lex 3.6 Finite Automata 3.7 From Regular Expressions to Automata 3.8 Design of a Lexical-Analyzer Generator 3.9 Optimization of DFA-Based Pattern Matchers 3.10 Summary of Chapter 3 3.11 References for Chapter 3 4 Syntax Analysis 4.1 Introduction 4.2 Context-Free Grammars 4.3 Writing a Grammar 4.4 Top-Down Parsing 4.5 Bottom-Up Parsing 4.6 Introduction to LR Parsing: Simple LR 4.7 More Powerful LR Parsers 4.8 Using Ambiguous Grammars 4.9 Parser Generators 4.10 Summary of Chapter 4 4.11 References for Chapter 4 5 Syntax-Directed Translation 5.1 Syntax-Directed Definitions 5.2 Evaluation Orders for SDD's 5.3 Applications of Syntax-Directed Translation 5.4 Syntax-Directed Translation Schemes 5.5 Implementing L-Attributed SDD's 5.6 Summary of Chapter 5 5.7 References for Chapter 5 6 Intermediate-Code Generation 6.1 Variants of Syntax Trees 6.2 Three-Address Code 6.3 Types and Declarations 6.4 Translation of Expressions 6.5 Type Checking 6.6 Control Flow 6.7 Backpatching 6.8 Switch-Statements 6.9 Intermediate Code for Procedures 6.10 Summary of Chapter 6 6.11 References for Chapter 6 7 Run-Time Environments 7.1 Storage Organization 7.2 Stack Allocation of Space 7.3 Access to Nonlocal Data on the Stack 7.4 Heap Management 7.5 Introduction to Garbage Collection 7.6 Introduction to Trace-Based Collection 7.7 Short-Pause Garbage Collection 7.8 Advanced Topics in Garbage Collection 7.9 Summary of Chapter 7 7.10 References for Chapter 7 8 Code Generation 8.1 Issues in the Design of a Code Generator 8.2 The Target Language 8.3 Addresses in the Target Code 8.4 Basic Blocks and Flow Graphs 8.5 Optimization of Basic Blocks 8.6 A Simple Code Generator 8.7 Peephole Optimization 8.8 Register Allocation and Assignment 8.9 Instruction Selection by Tree Rewriting 8.10 Optimal Code Generation for Expressions 8.11 Dynamic Programming Code-Generation 8.12 Summary of Chapter 8 8.13 References for Chapter 8 9 Machine-Independent Optimizations 9.1 The Principal Sources of Optimization 9.2 Introduction to Data-Flow Analysis 9.3 Foundations of Data-Flow Analysis 9.4 Constant Propagation 9.5 Partial-Redundancy Elimination 9.6 Loops in Flow Graphs 9.7 Region-Based Analysis 9.8 Symbolic Analysis 9.9 Summary of Chapter 9 9.10 References for Chapter 9 10 Instruction-Level Parallelism 10.1 Processor Architectures 10.2 Code-Scheduling Constraints 10.3 Basic-Block Scheduling 10.4 Global Code Scheduling 10.5 Software Pipelining 10.6 Summary of Chapter 10 10.7 References for Chapter 10 11 Optimizing for Parallelism and Locality 11.1 Basic Concepts 11.2 Matrix Multiply: An In-Depth Example 11.3 Iteration Spaces 11.4 Affine Array Indexes 11.5 Data Reuse 11.6 Array Data-Dependence Analysis 11.7 Finding Synchronization-Free Parallelism 11.8 Synchronization Between Parallel Loops 11.9 Pipelining 11.10 Locality Optimizations 11.11 Other Uses of Affine Transforms 11.12 Summary of Chapter 11 11.13 References for Chapter 11 12 Interprocedural Analysis 12.1 Basic Concepts 12.2 Why Interprocedural Analysis? 12.3 A Logical Representation of Data Flow 12.4 A Simple Pointer-Analysis Algorithm 12.5 Context-Insensitive Interprocedural Analysis 12.6 Context-Sensitive Pointer Analysis 12.7 Datalog Implementation by BDD's 12.8 Summary of Chapter 12 12.9 References for Chapter 12 A A Complete Front End A.1 The Source Language A.2 Main A.3 Lexical Analyzer A.4 Symbol Tables and Types A.5 Intermediate Code for Expressions A.6 Jumping Code for Boolean Expressions A.7 Intermediate Code for Statements A.8 Parser A.9 Creating the Front End B Finding Linearly Independent Solutions Index
8,437 citations
TL;DR: Optical trapping of dielectric particles by a single-beam gradient force trap was demonstrated for the first reported time, confirming the concept of negative light pressure due to the gradient force.
Abstract: Optical trapping of dielectric particles by a single-beam gradient force trap was demonstrated for the first reported time. This confirms the concept of negative light pressure due to the gradient force. Trapping was observed over the entire range of particle size from 10 μm to ~25 nm in water. Use of the new trap extends the size range of macroscopic particles accessible to optical trapping and manipulation well into the Rayleigh size regime. Application of this trapping principle to atom trapping is considered.
6,434 citations
TL;DR: The purpose of this tutorial paper is to give an introduction to the theory of Markov models, and to illustrate how they have been applied to problems in speech recognition.
Abstract: The basic theory of Markov chains has been known to mathematicians and engineers for close to 80 years, but it is only in the past decade that it has been applied explicitly to problems in speech processing. One of the major reasons why speech models, based on Markov chains, have not been developed until recently was the lack of a method for optimizing the parameters of the Markov model to match observed signal patterns. Such a method was proposed in the late 1960's and was immediately applied to speech processing in several research institutions. Continued refinements in the theory and implementation of Markov modelling techniques have greatly enhanced the method, leading to a wide range of applications of these models. It is the purpose of this tutorial paper to give an introduction to the theory of Markov models, and to illustrate how they have been applied to problems in speech recognition.
4,546 citations
TL;DR: A new conceptual framework and a minimization principle together provide an understanding of computation in model neural circuits that represent an approximation to biological neurons in which a simplified set of important computational properties is retained.
Abstract: A new conceptual framework and a minimization principle together provide an understanding of computation in model neural circuits. The circuits consist of nonlinear graded-response model neurons organized into networks with effectively symmetric synaptic connections. The neurons represent an approximation to biological neurons in which a simplified set of important computational properties is retained. Complex circuits solving problems similar to those essential in biology can be analyzed and understood without the need to follow the circuit dynamics in detail. Implementation of the model with electronic devices will provide a class of electronic circuits of novel form and function.
2,019 citations
TL;DR: It is shown how the matrix analytic methodology can incorporate practical system considerations such as finite buffers and a class of overload control mechanisms discussed in the literature.
Abstract: We study the performance of a statistical multiplexer whose inputs consist of a superposition of packetized voice sources and data. The performance analysis predicts voice packet delay distributions, which usually have a stringent requirement, as well as data packet delay distributions. The superposition is approximated by a correlated Markov modulated Poisson process (MMPP), which is chosen such that several of its statistical characteristics identically match those of the superposition. Matrix analytic methods are then used to evaluate system performance measures. In particular, we obtain moments of voice and data delay distributions and queue length distributions. We also obtain Laplace-Stieitjes transforms of the voice and data packet delay distributions, which are numerically inverted to evaluate tails of delay distributions. It is shown how the matrix analytic methodology can incorporate practical system considerations such as finite buffers and a class of overload control mechanisms discussed in the literature. Comparisons with simulation show the methods to be accurate. The numerical results for the tails of the voice packet delay distribution show the dramatic effect of traffic variability and correlations on performance.
1,305 citations
TL;DR: This survey gives an overview of some general decomposition results and the methodology used to obtain these results for two vacation models and attempts to provide a methodological overview to illustrate how the seemingly diverse mix of problems is closely related in structure and can be understood in a common framework.
Abstract: Queueing systems in which the server works on primary and secondary (vacation) customers arise in many computer, communication, production and other stochastic systems. These systems can frequently be modeled as queueing systems with vacations. In this survey, we give an overview of some general decomposition results and the methodology used to obtain these results for two vacation models. We also show how other related models can be solved in terms of the results for these basic models. We attempt to provide a methodological overview with the objective of illustrating how the seemingly diverse mix of problems is closely related in structure and can be understood in a common framework.
1,136 citations
Book•
01 Jan 1986TL;DR: In this paper, the current status and future applications of lightwave transmission of longwavelength semiconductor lasers emitting near 1.3 and 1.55-mu m are described, and bit-error-rate curves for a transmission experiment at 8 Gb/s over 76 km of fiber are shown.
Abstract: The current status and future applications to lightwave transmission of long-wavelength semiconductor lasers emitting near 1.3 and 1.55- mu m are described. Bit-error-rate curves for a transmission experiment at 8 Gb/s over 76 km of fiber are shown, and schematics of a high-speed distributed feedback laser and a multielectrode distributed Bragg reflector laser are presented. >
1,119 citations
TL;DR: The experimental discovery of a continuous shift in the optical frequency of a soliton pulse as it travels down the fiber is described, caused by a Raman self-pumping of the soliton.
Abstract: We describe the experimental discovery of a continuous shift in the optical frequency of a soliton pulse as it travels down the fiber. The effect is caused by a Raman self-pumping of the soliton, by which energy is transferred from the higher to the lower-frequency parts of its spectrum. For 120-fsec pulses, we have observed net frequency shifts as great as 10% of the optical frequency.
1,095 citations
TL;DR: The theory of Raman effects, which causes a continuous downshift of the mean frequency of pulses propagating in optical fibers, agrees well with recent measurements.
Abstract: Raman effects cause a continuous downshift of the mean frequency of pulses propagating in optical fibers. For solitons in silica fibers, the effect varies roughly with the inverse fourth power of the pulse width. At 1.5-μm wavelength in a fiber with 15 psec/nm/km time-of-flight dispersion, a soliton of 250-fsec duration is predicted to shift by its own spectral width after about 100m of propagation. The theory agrees well with recent measurements.
1,051 citations
TL;DR: It is pointed out that a coherent state propagating through an amplitude dispersive medium will, under suitable conditions, evolve into a quantum superposition of two coherent states 180\ifmmode^\circ\else\textdegree\fi{} out of phase with each other.
Abstract: It is pointed out here that a coherent state propagating through an amplitude dispersive medium will, under suitable conditions, evolve into a quantum superposition of two coherent states 180\ifmmode^\circ\else\textdegree\fi{} out of phase with each other. The response of a homodyne detector to this superposition of macroscopically distinguishable states is calculated. Signatures which an experimentalist might look for in the homodyne detector's output in order to verify the generation of such states are described.
1,037 citations
TL;DR: In this paper, a Lie-group-theoretical approach to the analysis of interferometers is presented, which can achieve phase sensitivity Δo approaching 1/N, where N is the total number of quanta entering the interferometer, provided that the light entering the input ports is prepared in a suitable quantum state.
Abstract: A Lie-group-theoretical approach to the analysis of interferometers is presented. Conventional interferometers such as the Mach-Zehnder and Fabry-Perot can be characterized by SU(2). We introduce a class of interferometers characterized by SU(1,1). These interferometers employ active elements such as four-wave mixers or degenerate-parametric amplifiers in their construction. Both the SU(2) and SU(1,1) interferometers can in principle achieve a phase sensitivity Δo approaching 1/N, where N is the total number of quanta entering the interferometer, provided that the light entering the input ports is prepared in a suitable quantum state. SU(1,1) interferometers can achieve this sensitivity with fewer optical elements.
Book•
01 Jan 1986TL;DR: The authors found that Japanese tonal patterns are sparsely specified, which suggests that they are much more similar to English intonational structures than earlier descriptions would have allowed, and they also showed that Japanese tone structures can be found to have a more concrete sort than hitherto suspected.
Abstract: Comparisons between Japanese and English prosodics have usually either focused on the strikingly apparent phonetic differences between the stress patterns of English and the tonal accent patterns of Japanese or concentrated upon formal similarities between the abstract arrangements of the stresses and tones. A recent investigation of tone structure in Japanese (Pierrehumbert & Beckman forthcoming), however, has convinced us that if the proper prosodic phenomena are compared, far more pervasive similarities can be discovered and of a much more concrete sort than hitherto suspected. In particular, there is now extensive evidence that Japanese tonal patterns are very sparsely specified, which suggests that they are much more similar to English intonational structures than earlier descriptions would have allowed.
TL;DR: This is the fourteenth edition of a quarterly column that provides continuing coverage of new developments in the theory of NP-completeness, and readers who have results they would like mentioned (NP-hardness, PSPACE- hardness, polynomialtime-solvability, etc.), or open problems they wouldlike publicized, should send them to David S. Johnson.
TL;DR: It is shown that amplifier noise causes a soliton's group velocity to undergo a random-walk process, which limits the system's product of length times bit rate, in one example, to about 24 000 GHz-km.
Abstract: In an optical-communications system using soliton pulse transmission, periodic amplification is needed to maintain the energy of the solitons. We show that amplifier noise causes a soliton's group velocity to undergo a random-walk process. The resultant timing errors at the receiver limit the system's product of length times bit rate, in one example, to about 24 000 GHz-km.
Book•
01 Jan 1986TL;DR: This document discusses the representation of processes in the distributed UNIX system, and some of the mechanisms used to achieve this representation are described.
Abstract: 1. General Review of the System. 2. Introduction to the Kernel. 3. The Buffer Cache. 4. Internal Representation of Files. 5. System Calls for the File System. 6. The System Representation of Processes. 7. Process Control. 8. Process Scheduling and Time. 9. Memory Management Policies. 10. Interprocess Communication. 11. Multiprocessor Systems. 12. Distributed UNIX System.
TL;DR: The first observation of optically trapped atoms is reported, with estimates that about 500 atoms are confined in a volume of about ${10}^{3}$ \ensuremath{\mu}$ m3 at a density of about £10^{11}$-${10]^{12}$ and in good quantitative agreement with theoretical expectations.
Abstract: We report the first observation of optically trapped atoms Sodium atoms cooled below ${10}^{\ensuremath{-}3}$ K in "optical molasses" are captured by a dipole-force optical trap created by a single, strongly focused, Gaussian laser beam tuned several hundred gigahertz below the ${D}_{1}$ resonance transition We estimate that about 500 atoms are confined in a volume of about ${10}^{3}$ \ensuremath{\mu}${\mathrm{m}}^{3}$ at a density of ${10}^{11}$-${10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ Trap lifetimes are limited by background pressure to several seconds The observed trapping behavior is in good quantitative agreement with theoretical expectations
01 Jan 1986
TL;DR: The C Programming Language as discussed by the authors provides a complete, authoritative coverage of the C language, its standard library, and key design techniques, as well as its locales and exception safety.
Abstract: From the Publisher:
Written by Bjarne Stroustrup, the creator of C, this is the world's most trusted and widely read book on C.
For this special hardcover edition, two new appendixes on locales and standard library exception safety have been added. The result is complete, authoritative coverage of the C language, its standard library, and key design techniques. Based on the ANSI/ISO C standard, The C Programming Language provides current and comprehensive coverage of all C language features and standard library components.
For example:
abstract classes as interfaces class hierarchies for object-oriented programming templates as the basis for type-safe generic software exceptions for regular error handling namespaces for modularity in large-scale software run-time type identification for loosely coupled systems the C subset of C for C compatibility and system-level work standard containers and algorithms standard strings, I/O streams, and numerics C compatibility, internationalization, and exception safety
Bjarne Stroustrup makes C even more accessible to those new to the language, while adding advanced information and techniques that even expert C programmers will find invaluable.
TL;DR: This paper analyzes a model of a multiplexer for packetized voice and data using the index of dispersion for intervals (IDI), which describes the cumulative covariance among successive interarrival times.
Abstract: This paper analyzes a model of a multiplexer for packetized voice and data. A major part of the analysis is devoted to characterizing the aggregate packet arrival process resulting from the superposition of separate voice streams. This is done via the index of dispersion for intervals (IDI), which describes the cumulative covariance among successive interarrival times. The IDI seems very promising as a measurement tool to characterize complex arrival processes. This paper also describes the delays experienced by voice and data packets in the multiplexer using relatively simple two-parameter approximations.
TL;DR: In this paper, the authors present x-ray-diffraction profiles from a variety of different crystals which are characteristically diffuse in the direction perpendicular to the surface through which the incident and diffracted beams pass, but sharp in both parallel directions.
Abstract: We present x-ray-diffraction profiles from a variety of different crystals which are characteristically diffuse in the direction perpendicular to the surface through which the incident and diffracted beams pass, but sharp in both parallel directions. We show that these effects arise from truncation of the crystal lattice at the surface. To explain the precise form of the momentum-transfer dependence of the intensity across the reciprocal-space zone, it is necessary to include the effects of surface roughness on an atomic scale. Such measurements therefore allow highly sensitive roughness determinations to be made. Understanding the origin of these streaks of intensity will have significant impact on the practice of x-ray crystallographic determinations of surface structure.
TL;DR: This paper presents a model for analyzing the performance of transmission strategies in a multihop packet radio network where each station has adjustable transmission radius and shows that the network can achieve better performance by suitably controlling the transmission range.
Abstract: This paper presents a model for analyzing the performance of transmission strategies in a multihop packet radio network where each station has adjustable transmission radius. A larger transmission radius will increase the probability of finding a receiver in the desired direction and contribute bigger progress if the transmission is successful, but it also has a higher probability of collision with other transmissions. The converse is true for shorter transmission range. We illustrate our model by comparing three transmission strategies. Our results show that the network can achieve better performance by suitably controlling the transmission range. One of the transmission strategies, namely transmitting to the nearest forward neighbor by using adjustable transmission power, has desirable features in a high terminal density environment.
TL;DR: The first observation of the modulational instability of light waves in dielectric material using a neodymium-doped yttrium aluminum garnet laser operated at 1.319 \ensuremath{\mu}m and single-mode optical fibers with anomalous group-velocity dispersion is reported.
Abstract: We report the first observation of the modulational instability of light waves in dielectric material using a neodymium-doped yttrium aluminum garnet laser operated at 1.319 \ensuremath{\mu}m and single-mode optical fibers with anomalous group-velocity dispersion. The observed results are in good agreement with the theoretical predictions. The relationship between the modulation instability and parametric four-wave mixing and the interplay with stimulated Raman and Brillouin scatterings are also presented.
TL;DR: The experiment confirmed at the classical level that the angle of rotation of linearly polarized light in this fiber gives a direct measure of Berry's phase, and the topological nature of this effect was verified, i.e., the rotation was found to be independent of deformations of fiber path if the solid angle of the path in momentum space stayed constant.
Abstract: We report the first experimental verification of Berry's topological phase. The key element in the experiment was a single-mode, helically wound optical fiber, inside which a photon of a given helicity could be adiabatically transported around a closed path in momentum space. The experiment confirmed at the classical level that the angle of rotation of linearly polarized light in this fiber gives a direct measure of Berry's phase. The topological nature of this effect was also verified, i.e., the rotation was found to be independent of deformations of fiber path if the solid angle of the path in momentum space stayed constant.
01 Aug 1986
TL;DR: A transformation is used to obtain simple algorithms for computing the Voronoi diagram of point sites, of line segment sites, and of weighted point sites with sweepline technique.
Abstract: We present a transformation that can be used to compute Voronoi diagrams with a sweepline technique. The transformation is used to obtain simple algorithms for computing the Voronoi diagram of point sites, of line segment sites, and of weighted point sites. All algorithms have O(n log n) worst case running time and use O(n) space.
TL;DR: In this paper, the authors review recent advances in our current level of understanding of the physics underlying transport and optical properties of Ge x Si 1-x /Si strained-layer heterostructures.
Abstract: This paper reviews recent advances in our current level of understanding of the physics underlying transport and optical properties of Ge x Si 1-x /Si strained-layer heterostructures. Included are discussions of critical (maximum) layer thicknesses, effects of coherency strain on the bandgaps of both Si and Ge x Si 1-x and the influence of layer strains on the band alignments of Ge x Si 1-x /Si strained-layer heterostructures. Transport studies will center on the modulation doping results of both n and p type heterostructures. Indeed, these earlier transport studies provided essential information which led to an understanding of the band-alignment in these strained layer heterostructures. Recent measurements of the indirect bandgap of Ge x Si 1-x strained layers on
TL;DR: On etudie les relations entre les codes [n,k] lineaires a deux poids, les ensembles projectifs et certains graphes fortement reguliers as mentioned in this paper.
Abstract: On etudie les relations entre les codes [n,k] lineaires a deux poids, les ensembles (n,k,h 1 h 2 ) projectifs et certains graphes fortement reguliers
TL;DR: In this paper, a model for polarisation dispersion in single-mode fibres of arbitrary length and configuration is proposed based on the existence of principal states of polarisation, i.e., two orthogonal input states of polarity whose corresponding output states exhibit zero dispersion.
Abstract: We describe a model for polarisation dispersion in single-mode fibres of arbitrary length and configuration that is based on the existence of principal states of polarisation. These are two orthogonal input states of polarisation whose corresponding output states exhibit zero dispersion in their state of polarisation to first order.
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TL;DR: Linda consists of a few simple operators designed to support and simplify the construction of explicitly-parallel programs, which is to make it largely unnecessary to think about the coupling between parallel processes.
Abstract: Linda consists of a few simple operators designed to support and simplify the construction of explicitly-parallel programs. Linda has been implemented on ATandT Bell Labs' S/Net multicomputer and, in a preliminary way, on an Ethernet-based MicroVAX network and an Intel iPSC hypercube. Parallel programming is often described as being fundamentally harder than conventional, sequential programming, but in our experience (limited so far, but growing) it isn't. Parallel programming in Linda is conceptually the same order of task as conventional programming in a sequential language. Parallelism does, though, encompass a potentially difficult problem. A conventional program consists of one executing process, of a single point in computational time-space, but a parallel program consists of many, and to the extent that we have to worry about the relationship among these points in time and space, the mood turns nasty. Linda's mission, however, is to make it largely unnecessary to think about the coupling between parallel processes. Linda's uncoupled processes, in fact, never deal with each other directly. A parallel program in Linda is a spatially and temporally unordered bag of processes, not a process graph.
TL;DR: This paper presents a unifying procedure, called Facet, for the automated synthesis of data paths at the register-transfer level that minimizes the number of storage elements, data operators, and interconnection units.
Abstract: This paper presents a unifying procedure, called Facet, for the automated synthesis of data paths at the register-transfer level. The procedure minimizes the number of storage elements, data operators, and interconnection units. A design generator named Emerald, based on Facet, was developed and implemented to facilitate extensive experiments with the methodology. The input to the design generator is a behavioral description which is viewed as a code sequence. Emerald provides mechanisms for interactively manipulating the code sequence. Different forms of the code sequence are mapped into data paths of different cost and speed. Data paths for the behavioral descriptions of the AM2910, the AM2901, and the IBM System/370 were produced and analyzed. Designs for the AM2910 and the AM2901 are compared with commercial designs. Overall, the total number of gates required for Emerald's designs is about 15 percent more than the commercial designs. The design space spanned by the behavioral specification of the AM2901 is extensively explored.
TL;DR: A data compression scheme that exploits locality of reference, such as occurs when words are used frequently over short intervals and then fall into long periods of disuse, is described and proves that it never performs much worse than Huffman coding and can perform substantially better.
Abstract: A data compression scheme that exploits locality of reference, such as occurs when words are used frequently over short intervals and then fall into long periods of disuse, is described. The scheme is based on a simple heuristic for self-organizing sequential search and on variable-length encodings of integers. We prove that it never performs much worse than Huffman coding and can perform substantially better; experiments on real files show that its performance is usually quite close to that of Huffman coding. Our scheme has many implementation advantages: it is simple, allows fast encoding and decoding, and requires only one pass over the data to be compressed (static Huffman coding takes two passes).
TL;DR: In this article, a simple expression for the low field mobility in the miniband conduction regime is derived; localization effects, hopping conduction, and effective mass filtering are discussed.
Abstract: New results on the physics of tunneling in quantum well heterostructures and its device applications are discussed. Following a general review of the field in the Introduction, in the second section resonant tunneling through double barriers is investigated. Recent conflicting interpretations of this effect in terms of a Fabry-Perot mechanism or sequential tunneling are reconciled via an analysis of scattering. It is shown that the ratio of the intrinsic resonance width to the total scattering width (collision broadening) determines which of the two mechanisms controls resonant tunneling. The role of symmetry is quantitatively analyzed and two recently proposed resonant tunneling transistor structures are discussed. The third section deals with perpendicular transport in superlattices. A simple expression for the low field mobility in the miniband conduction regime is derived; localization effects, hopping conduction, and effective mass filtering are discussed. In the following section, experimental results on tunneling superlattice photoconductors based on effective mass filtering are presented. In the fifth section, negative differential resistance resulting from localization in a high electric field is discussed. In the last section, the observation of sequential resonant tunneling in superlattices is reported. We point out a remarkable analogy between this phenomenon and paramagnetic spin resonance. New tunable infrared semiconductor lasers and wavelength selective detectors based on this effect are discussed.