The primary optoelectronic devices of importance in lightwave communications systems are emitters, amplifiers, modulators, and detectors. Emitters are the front-end components of a lightwave communications system. The signal is input into the fiber using emitters. The most important emitters are light emitting diodes (LEDs) and lasers. As we will see, most communications systems use lasers due to their much higher power and relatively large modulation bandwidth as compared with LEDs. In addition to front-end emitters, modern lightwave communications systems utilize optical amplifiers for long distance communications. The natural attenuation of lightwave signals propagating through a fiber optic cable can be compensated by the use of amplifiers placed at periodic spatial intervals. In this way, very long distance fiber transmission lines can be made for transcontinental and transoceanic communications. The most attractive amplifiers in long distance lightwave networks are all optical devices since these structures are less costly and typically less noisy than their optoelectronic alternatives. In this chapter, we discuss the two most important semiconductor emitters, LEDs and lasers. The operating principles of these devices as well as various device types will be presented. Since the basic physics utilized in lasers, i.e., stimulated emission, is common to optical amplifiers, the chapter includes a discussion of the two most important optical amplifiers used in lightwave communications systems. These are EDFAs and SOAs. Finally, the chapter concludes with a discussion of photodetectors.

LEDs

The LED has become one of the most ubiquitous compound semiconductor devices. It is commonly employed in numerous applications including clocks, appliances, calculators, lighting, and signs. LEDs make up the largest share of commercial optical semiconductor products.

A bipolar junction transistor (BJT) consists of two p–n junctions with three terminals. The device comprises three different doped layers. These are alternating p, n, and p layers or n, p, and n layers. The first type of device is called a pnp BJT, while the second device is an npn BJT. The first layer is called the emitter. The second layer is called the base and the third layer is called the collector. Thus for a pnp BJT, the emitter and collector regions are p-type while the base is n-type. The two p–n junctions are then formed between the emitter and base and collector and base. The vast majority of BJT applications are in analog electronics, but BJTs can also be employed in digital circuitry.

BJT operation

A BJT can be used in several circuit configurations. In most applications the input signal is across two of the BJT leads while the output signal is extracted from a second pair of leads. Since there are only three leads for a BJT, one of the leads must be common to both the input and the output circuitry. Hence we call the different circuit configurations common emitter, common base, and common collector to identify the lead common to both the input and the output. These configurations are shown in Fig. 4.1. The most commonly employed configuration is the common emitter configuration.

A BJT can be biased into one of four possible modes. These are saturation, active, inverted, and cutoff.

Solid state devices and their associated circuits used in wireless systems operate from the ultra-high frequency (UHF) to millimeter-wave frequencies. Each system requires various RF functionalities such as switching, amplification, mixing, filtering, sampling, dividing, and combining. Some of these functions are produced using a mixture of active and passive components. The active components are typically transistors, primarily FETs and BJTs.

The power amplifier is the critical portion of the RF front end in a wireless telecommunications system. The two most prevalent applications of power amplifiers in wireless systems are in cellular telephones and base stations. Power amplifier performance depends to some extent on some or all of the following: power, gain, efficiency, linearity, reliability, and thermal management issues. Any one of these specifications can be traded off against any other one. Additionally, none of these issues is solely dependent upon the device itself. For example, reliability and thermal management depend upon packaging and bonding in addition to their dependence on the circuit elements. The performance attributes are also coupled. The higher the device output power the more important thermal management becomes. Thus optimization for one parameter often impacts another device parameter that in turn may require different optimization. In the first part of this chapter we will discuss some of the performance metrics and examine how they vary with device type and design when used in power amplifiers. In addition to providing high power, the frequency of operation of a power transistor used in wireless systems must be high. Therefore, transistors must be optimized to deliver high output power at high frequency.

Reflection high-energy electron diffraction (RHEED or R-HEED) is a technique for surface structural analysis that is remarkably simple to implement, requiring at the minimum only an electron gun, a phosphor screen, and a clean surface. Its interpretation, however, is complicated by an unusually asymmetric scattering geometry and by the necessity of accounting for multiple scattering processes. First performed by Nishikawa and Kikuchi (1928a, b) at nearly the same time as the discovery of electron diffraction by Davison and Germer (1927a, b), RHEED has assumed modern importance because of its compatibility with the methods of vapor deposition used for the epitaxial growth of thin films. We take RHEED to encompass the energy range from about 8 to 20 keV, though it can be employed at electron energies as high as 50 to 100 keV.

Because of its small penetration depth, owing to the interaction between incident electrons and atoms, RHEED is primarily sensitive to the atomic structure of the first few planes of a crystal lattice. Diffraction from a structure periodic in only two dimensions therefore underlies the observed pattern, and the positions of the elastically scattered beams can be computed from single-scattering expressions. Nonetheless, because the elastic scattering is comparable to the inelastic scattering, multiple scattering processes are also crucial, and these must be included to obtain the correct intensity. The RHEED geometry – an incident beam directed at a low angle to the surface – has a very strong effect on both the diffraction and its interpretation.

Introduction

In order to determine the atomic positions of atoms in the first few surface layers, the RHEED intensity must be measured as a function of the scattering angles and then compared with dynamical calculations. There are two methods, which are the intensity rocking curve method (Maksym, 1985; Ichimiya et al., 1993b for example) and the azimuthal plot method (Mitura and Maksym, 1993). In the rocking curve method, diffraction intensities along several reciprocal rods are measured as functions of the incident angle of the electron beam. For azimuthal plots, one measures the specular intensity as a function of azimuth from a given direction of the incident and beam and at a given incident angle. For both methods, integrated intensities are measured in order to reduce the morphological effects of the surfaces (Ichimiya, 1987a; Appendix C). These two methods are equivalent and just involve different methods of collecting the data. In either case a model of the surface structure must be assumed for comparison with the measurement. Convergent-beam RHEED uses a combination of both methods (Ichimiya et al., 1980; Smith, 1992; Smith et al., 1992). In this case the incident beam is not parallel but cone-like, as shown in Fig. 7.11. Dynamical analyses of the relative intensities of different RHEED spots also give knowledge of surface structures (Hashizume et al., 1994). In this chapter we will examine a particularly useful method of comparing dynamical calculations with measured rocking curves.

Introduction

Kinematic theories describe the motion of physical processes without consideration of the forces involved. In electron diffraction the kinematic approach has come to mean singlescattering analysis since in this view symmetry and energy conservation, and not the details of the potential, largely determine the diffraction pattern (see Chapter 6). But in fact singlescattering analysis is more than this. The strength of the interaction is included by means of a scattering factor, the mean potential is included by the refraction of the incident angle when a beam enters the crystal and some multiple-scattering processes are included in the diffraction of disordered systems by considering diffraction from blocks of atoms. In addition inelastic processes, related to the imaginary part of the potential, are included by a factor that describes absorption. As a result kinematic theory is an exceedingly useful approximate analysis that serves as a starting point for much of the dynamical theory. In contrast, the exact dynamical theory, which will be described in Chapters 12–14, is an analysis in which the potential is included from the beginning and in which multiple scatterings are the main diffraction process. But the results and trends of dynamical theory are difficult to visualize in simple ways. In this chapter we present the basic kinematic theory for electron diffraction from surfaces.