Introduction

In the twenty-first century, p-n junction diode devices are revolutionising electronics, much as transistors did in the twentieth century. Diodes had been developed well before the transistor, and the properties of diodes were initially exploited in power supplies, radios, early logic circuits, and other more specialised applications. Diodes took a distant second place to transistors in the hierarchy of electronic devices after the transistor was developed. This paradigm has now changed decisively: Two semiconductor devices based directly on the p-n junction diode are currently enjoying unparalleled industrial growth. These two devices are the photovoltaic () solar cell and the light-emitting diode ().

The consequences of this development constitute a revolution in two major industrial sectors:

1. Energy production has relied on hydrocarbons and nuclear power, and although these will continue to be important, the direct conversion of solar radiation into useful power is the key to a long-term, sustainable energy supply. Ninety-seven per cent of all renewable energy on earth is in the form of solar radiation. The twenty-first century has already seen the rapid growth of a global solar PV industry in conjunction with the involvement of governments worldwide. A scale of production and deployment of PVs that is unprecedented is now underway. The worldwide consumption of silicon semiconductor material for the entire microelectronics industry has been overtaken by its use for solar cells alone.
2. The twenty-first century has already witnessed the ongoing displacement of incandescent lamps, fluorescent lamps, and discharge lamps by LEDs. The world's major lighting companies are now dedicating their efforts to LED lighting products. Governments are recognising the benefits of LED lighting in their quest for sustainability.
3. More recently, both inorganic LEDs and organic light-emitting diodes (s) are enabling self-emissive displays in key display markets including handheld devices, televisions, and digital billboards. LEDs have also completely replaced fluorescent lamp backlighting in the well-established liquid crystal display () industry.

The purpose of this book is to present the physical concepts required for a thorough understanding of p-n junctions starting with introductory quantum mechanics, solid state physics, and semiconductor fundamentals. This leads to both inorganic and organic semiconductors and the associated p-n junction devices with a major emphasis on PV and LEDs. An introduction to transistors is also included since it builds readily on the p-n junction.

The book is aimed at senior undergraduate levels (years 3 and 4). The theory of the p-n junction can be quite dry in the absence of context. Students are inspired and motivated as they readily appreciate the relevance of both solar cells and LEDs. Chapter 1 motivates and presents introductory quantum mechanics for students who have not seen this elsewhere. As such, this book is designed to be accessible to all students with an interest in semiconductor devices. This is intentional since solar cells and LEDs involve a wide range of science and engineering concepts.

In Chapter 2, the physics of solid-state electronic materials is covered in detail starting from the basic behaviour of electrons in crystals. The quantitative treatment of electrons and holes in energy bands is presented along with the important concepts of excess carriers that become significant once semiconductor devices are connected to sources of power or illuminated by sunlight. A series of semiconductor materials and their important properties are reviewed. The behaviour of semiconductor surfaces and trapping concepts are introduced since they play an important role in solar cell and LED device performance.

In Chapter 3, the basic physics and important models of p-n junction devices are presented. The diode is presented as a semiconductor device that can be understood from band theory covered in Chapter 2. Diode device concepts are extended to include tunnelling, thermionic emission, metal-semiconductor contact phenomena, and the heterojunction.

Chapter 4 introduces the theory of radiation, a topic frequently overlooked in books on semiconductor devices. The deeper understanding of photon emission and absorption processes gained from this chapter is highly relevant to subsequent chapters on solar cells and LEDs. In this chapter, the physics of photon creation is explained with a minimum of mathematical complexity. Radiation theory of the oscillating electronic dipole is treated classically and then using simple quantum mechanics. The key role of the exciton in organic molecules is presented as preparation for OLEDs and organic solar cells in Chapter 7. In addition, line-shapes predicted for direct-gap semiconductors are derived. Finally, the subject of photometric units introduces the concepts of luminance and colour coordinates that are essential to a discussion of organic and inorganic LED devices.

Chapter 5 covers inorganic solar cells. The p-n junction fundamentals introduced in Chapter 3 are further developed to include illumination of the p-n junction. Readily understood modelling is used to explain the behaviour of a solar cell. Realistic solar cell structures and models are presented along with the attendant surface recombination and bulk absorption issues that must be understood in practical solar cells. A series of solar cell technologies are reviewed starting with bulk single and multicrystalline silicon solar cell technology. Amorphous silicon materials and device concepts are presented. Solar cells made using semiconductors such as CdTe are introduced followed by multijunction solar cells using layered, lattice-matched III-V semiconductor stacks.

Chapter 6 considers the basic LED structure and its operating principles. The measured lineshape of III-V LEDs is compared with the predictions of Chapter 4. LEDs are engineered to maximise radiative recombination, and key energy loss mechanisms are discussed. The series of developments that marked the evolution of today's high-efficiency LED devices is presented starting from the semiconductors and growth techniques of the 1960s. This is followed by an in-depth presentation of wider band-gap semiconductors culminating in nitride materials and their synthesis methods for the LED industry. The double heterojunction is introduced and the resulting energy well is analyzed. Strategies to optimise optical outcoupling are discussed. Finally, the concept of spectral down-conversion using phosphor materials and the white LED are introduced along with topics of current importance including the 'green gap'.

Chapter 7 introduces new concepts required for an understanding of organic semiconductors, in which conjugated molecular bonding gives rise to p bands and HOMO and LUMO levels. The organic LED is introduced by starting with the simplest single active layer polymer-based LED followed by successively more complex small-molecule LED structures. The roles of the various layers, including electrodes and carrier injection and transport layers, are discussed and the relevant candidate molecular materials are described. Concepts from Chapter 4, including the molecular exciton and singlet and triplet states, are used to explain efficiency limitations in the light generation layer of small-molecule OLEDs. In addition, the opportunity to use phosphorescent and delayed-fluorescence host-guest light-emitting layers to improve device efficiency is explained. The organic solar cell is introduced and the concepts of exciton generation and exciton dissociation are described in the context of the heterojunction and the bulk heterojunction. The interest in the use of fullerenes and other related nanostructured materials is explained for the bulk heterojunction. The most recent breakthrough in perovskites as a revolutionary hybrid organic/inorganic semiconductor material is presented.

Finally, Chapter 8 introduces, carefully explains, and models the two transistor types for which the p-n junction is most clearly relevant. Both the bipolar junction transistor () and the junction field effect transistor () permit the use of this book for introductory semiconductor device courses that are designed to include three-terminal devices and the concept of amplification. This lays the groundwork for subsequent courses on metal oxide field-effect transistors (s) and other devices.

This Second Edition has been brought up to date throughout and colour has been added liberally throughout the book. A much improved and expanded set of homework problems has been developed. In addition to two new chapters, a more thorough treatment of solid-state physics to better develop band theory is included. Recent developments in telluride/selenide/sulfide solar cells, cadmium-free thin film solar cells, perovskite solar cells, triplet-harvesting strategies for OLEDs, phosphorescent, and thermally activated delayed fluorescence dopants, and LED optical outcoupling are included. A discussion of the LED colour-rendering index has been added, and a more in-depth analysis of carrier diffusion and recombination in solar...