In this chapter, the types of GaAs devices will be briefly defined and introduced for the purpose of referring to the types of applications of GaAs devices. This introduction will not include details of the physical principles or the operation of the devices. GaAs devices are categorised broadly by whether they are electronic devices or light-based (photonic) devices. Electronic devices are of three basic types: bipolar transistors, field effect transistors and diodes. All three types have important commercial applications. The purpose of this introduction is to provide a level of understanding useful for a discussion of applications of GaAs devices. Four broad categories of photonic devices will be introduced: light emitting diodes (LEDs), laser diodes (LDs), photodetectors and waveguides. As for electronic devices, a rudimentary introduction of device types is useful for a discussion of applications of GaAs devices.

The surface quality of GaAs is very important in the reproducible fabrication of devices. There are two rather distinct aspects of surface quality: cleanliness and electronic passivation. Both aspects are the topics of this chapter. We begin with a practical discussion of surface treatments aimed at the removal of organic contaminants and native oxides from the semiconductor surfaces. We will then proceed to a review of the electronic properties of the gallium arsenide surface, especially those of its native oxide, and discuss approaches to improve those properties, i.e. to passivate the surface to improve device performance.

Dry etch processes can provide excellent profile control. The energetic ion bombardment increases the etch rate on the exposed surface relative to those regions protected by the mask, so vertical sidewalls with negligible undercutting are readily achieved. Depending on the balance between chemical and physical contributions to the dry etch, it is possible to vary the profile from isotropic/crystallographic to vertical to angled. With vertical profiles, smaller critical dimensions are achievable. The ion enhancement also removes the dependence on the pattern alignment relative to the wafer crystal planes. The proper balance of chemical etching and physical sputtering allows dry etch processes such as RIE, HDPE, RIBE and CAIBE to produce devices with small feature sizes and vertical profiles. Although some electronic damage results from the ion bombardment that permits fine features and profile control, these processes are and will remain the basis of most III-V device fabrication.

Schottky contacts are one of the most widely studied aspects of GaAs technology. In part, this is due to the fact that no single Schottky contact satisfies all of the requirements for an ideal Schottky contact to GaAs. Another factor driving more research has been the desire for higher Schottky barrier heights. Yet another factor has been the poorly understood nature and reproducibility of the metal/GaAs interface. In spite of the existence of contacts with greater interface stability, TiPtAu exhibits more than sufficient reliability against gate sinking in GaAs-based FETs. However, TiPtAu should be used with hydrogen getters for hermetic packages or be replaced with non-Ti and non-Pt materials. Many choices of refractory contacts can be used for high-temperature self-aligned FETs. Some of the best are WSi_0.45 and WSiN because these remain amorphous to sufficiently high temperatures. A wide variety of processing choices are available to tailor the gate structure to the desired application using available equipment.

GaAs HBTs are greatly affected by the unintentional introduction of defects in certain key parts of their structure. These process sensitive parts of the HBT structure need to be understood and controlled by process engineers. HBT operation, testing and performance characterisation supply the basic information that materials, processing and device engineers need to solve and understand the often complex material and process interactions. Reliability characterisation is an equally important guide for HBT engineers to the extent that such information is available. Many HBT process choices are available and some of the main ones were reviewed in this chapter. As in most facets of engineering, knowledge and information are a starting point for a job that needs to be done better than is now possible.