Compound Semiconductor Crystals

Abstract

AbstractAs we learned from previous chapters, silicon has a diamond crystal structure with pure covalent bonding. Due to its nature as a single element, the Si crystal structure is highly symmetric. Since there are two different elements in binary III–V compounds, the basic diamond structure turns into zinc-blende crystal structure, which destroys some of the symmetry observed in Si. This lower symmetry in crystal structure leads to band structures with different features which, in turn, give the unique properties not available to Si. Using GaAs as an example, the main advantages over Si are (a) Larger bandgap: The larger bandgap leads to a low intrinsic carrier concentration (ni) suitable for high-temperature operation. (b) Smaller effective mass: The effective mass is inversely proportional to the carrier mobility. The higher mobility contributes to high-speed operation. (c) Direct bandgap: The location of the lowest conduction band minimum also varies with a decreasing degree of crystal symmetry in III–V compounds. Some of them have both the conduction band minimum and the valence band maximum located at the zone center. These are called direct bandgap semiconductors. This allows for their use in photonic device and transfer electron device applications. (d) Heterostructures: Additionally, binary III–V compounds can form ternary and quaternary compounds, which leads to an expanded selection of lattice constants and bandgap energies. New heterostructure and quantum effect devices then become possible.In this chapter, the basic structural, transport, doping, and surface properties of III–V binaries are introduced first. The variation of lattice constant and energy gap in ternary and quaternary alloys are discussed next.