SiGe, GaAs, and InP Heterojunction Bipolar Transistors

CHAPTER TWO

Material Properties and Technologies

In the first part of this chapter we present the material properties of SiGe, GaAs, and InP semiconductors, such as bandgaps, lattice constants, velocity-field characteristics, conduction- and valence-band discontinuities, heavy doping effect, and carrier mobilities. In the second part of the chapter we illustrate the semiconductor technologies and fabrication of SiGe/Si-, AlGaAs/GaAs-, and InP-based heterojunction bipolar transistors.

2.1 SiGe AND GROUP III/V COMPOUND SEMICONDUCTORS

2.1.1 Bandgaps and Lattice Constants

When Si epitaxial layers are grown on Si substrates, there is a natural matching of the crystal lattice, and high-quality single-crystal layer result. On the other hand, it is often desirable to obtain epitaxial layers that differ somewhat from the substrate. This can be accomplished easily if the lattice structure and lattice constant match for the two materials. For example, GaAs and AlAs both have the zincblende structure, with a lattice constant of about 5.65 Å. As a result, epitaxial layers of the ternary alloy AlGaAs can be grown on GaAs substrates with little lattice mismatch [1]. Similarly, GaAs can be grown on Ge substrate.

Figure 2.1 shows the energy bandgap as a function of lattice constant for several elemental and compound semiconductors. Since AlAs and GaAs have similar lattice constants, the ternary alloy AlGaAs has essentially the same lattice constant over the entire range of compositions from AlAs ...