Preface
This book was written to serve both the research and user communities—to provide scientists embarking on avalanche photodiode (APD) research with a rigorous treatment of the fundamentals while also offering engineers practical information on APD characteristics and application. The overarching objective was to help readers build physical intuition about APDs and APD photoreceivers by giving clear explanations of “why” rather than simply stating “what.”
Unfortunately, there is inherent tension between the level of theoretical detail useful to those with an academic or research objective and the brevity most useful to those seeking a quick engineering reference. Rare is the individual who will read this book from cover to cover. Many sections of the book are structured as derivations in order to provide the clearest exposition of why a particular result is so, and what assumptions the result embodies. Those seeking quick answers will be better served by skipping to the “textbook” result at the end of such a section.
Those readers without a background in semiconductor physics who also wish to understand details of APD function are encouraged to read the Appendix first. A background in quantum mechanics is assumed—or at least a willingness to accept the validity of standard quantum-mechanical results—but otherwise, the Appendix is a self-contained primer that covers the background needed to understand contemporary APD research.
Chapter 1 explains how APDs work and how their device characteristics relate to their structure, concluding with a brief survey of different kinds of APD. Chapter 2 is a detailed mathematical treatment of APD and photoreceiver sensitivity, teaching the statistical tools used to quantify noise and the methods by which various figures of merit are calculated from device parameters. Chapter 3 relates the effective range and range error of lidar systems built using APDs to photoreceiver circuit characteristics like bandwidth and the sensitivity metrics developed in Chapter 2. Finally, Chapter 4 provides details of models useful for InGaAs APD design and discusses InGaAs APD design principles and aspects of their manufacture.
Types of avalanche photodiode 1
Avalanche photodiodes (APDs) are semiconductor detectors that convert photons to electrons and then multiply the electrons, resulting in an amplified photocurrent signal. APDs are useful for sensing weak optical signals, boosting the amplified output above circuit noise in analog photoreceivers or generating output strong enough to directly trigger transition of digital logic in photon-counting receivers. APDs are categorized by their intended mode of operation and the optical waveband to which they are responsive—a function of the semiconductor alloys from which they are fabricated. Within a given category, APDs may be further differentiated by their method of manufacture, and structural optimization for a given application.
This chapter sketches the landscape of APD technology, beginning with an overview of APD function and structure. Figures of merit used to quantify APD performance are introduced and several types of APD of contemporary interest are described. Subsequent chapters focus specifically on linear-mode InGaAs APDs, examining their design, performance, and application to range-finding and lidar in greater detail.
1.1 APD function
Selected topics in semiconductor physics are presented in the Appendix for readers seeking background on the physical concepts and processes mentioned in this section. APDs are a type of photodiode. When operated under reverse bias, a region forms inside a photodiode in which the macroscopic electric field is strong and the material is depleted of mobile charge carriers. Because carrier velocity saturates in strong electric fields, reverse current through this depletion region is primarily governed by the number of charge carriers available to transport current rather than by the applied voltage that drives their motion. Under optical illumination, photovoltaic absorption increases the supply of mobile carriers by promoting electrons out of valence band states in which they cannot transport current into conduction band states in which they can (Fig. 1.1). The vacated valence band states behave as positive charge carriers called holes which also transport current. The charge carriers generated by optical absorption are collectively called photocarriers, and the portion of the current they transport is termed photocurrent. Reverse-biased photodiodes can be used to sense light because the generation rate of photocarriers—and therefore the photocurrent—is proportional to incident optical power.
Heat can also promote electrons across the energy gap between valence and conduction bands, and in strong electric fields, quantum tunneling of electrons out of the valence band and into the conduction band generates additional carriers. These processes operate continuously, resulting in current that flows through a reversebiased diode even when there is no light to generate photocarriers. This dark current is always present in a photodiode, including when an optical signal is generating photocurrent, and is a source of noise that introduces error in optical power measurements. APDs are photodiodes that internally multiply charge carriers via impact ionization, increasing the photocurrent that flows in response to a given incident optical power level. Carriers generated by impact ionization can themselves initiate further impact ionization, leading to branching chains of ionization events likened to an avalanche.
The process is diagrammed in Fig. 1.2, where electrons and holes are depicted drifting in opposite directions in an applied electric field. The avalanche gain process also amplifies unwanted dark current and the shot noise already present on the diode’s current prior to multiplication. In many APDs, statistical fluctuation of the avalanche gain adds excess noise to the multiplied current. In general, the avalanche gain process amplifies an APD’s noise current by a factor at least as large as the gain applied to its signal photocurrent. Consequently, when considered as an isolated component, APDs are always less sensitive than an equivalent photodiode without any avalanche gain. However, photodiodes are never used as isolated components—they are always part of a photoreceiver circuit that has other noise sources. APDs are useful when downstream sources of noise in a photoreceiver—such as amplifier circuit noise—are stronger than the noise from the APD. Measures of APD sensitivity are largely meaningless outside the context of specific photoreceiver circuits because an APD’s contribution to the sensitivity of a photoreceiver depends fundamentally on the magnitude of a receiver circuit’s amplifier noise. Together with the APD’s excess noise characteristics, the amplifier noise determines the maximum avalanche gain at which it is fruitful to operate a linear-mode APD. The sensitivity advantage to be gained by using a particular APD depends on the amplifier with which it is paired, and cannot be determined from the APD’s characteristics alone. Relevant sensitivity calculations are presented in Chapter 2.