SiGe Field Effect Transistors

The first research effort aimed at the growth of epitaxial SiGe thin films on Si substrates can be dated back to 1975, when Kasper et al[1] published their results of Si/Ge superlattice grown on Si substrates using molecular beam epitaxy (MBE), they termed it UHV Epitaxy. Publication in this field was scarce for the next 8 years as the electronic and optical properties measurable in the material system were dominated by materials defects, especially dislocations derived from the large (4%) lattice mismatch between Ge and Si. In 1983, Bean and coworkers [2,3] conducted systematic studies on the material issues, and outlined the parameter space in which high quality, pseudomorphic SiGe thin films could be grown on Si substrates. Such improvement in material quality brought about a rapid expansion of the field. Electronic transport study [4] was followed by an array of demonstrative devices ranging from field effect transistors (FET's) [5,6] to photodetectors [7,8]. Heterojunction bipolar transistors (HBT's), has been carried one step beyond the demonstrative devices stage. Significant effort has been put into integrating HBT's with the integrated circuit technology [9,10,11]. In the meantime, a systematic understanding of the energy band structure [12], fundamental electronic properties [13], and material issues [14,15] has been developed. The introduction of compositional graded, relaxed SiGe buffer layers [16] opened up another dimension in this arena. One can now adjust the strain in a SiGe film from compression all the way to tension, while maintaining the threading dislocation density at the surface of the buffer layer to below 10 sup 6 cm sup (-2). Such relazed buffer layers allowed the fabrication of high mobility two dimensional electron gas (2DEG) and hole gas (2DHG) with the highest mobility ever reported on Si substrates [17,18,19,20].