Semiconductor Materials: Ge, Si, And GaAs

The construction of every discrete (individual) solid-state (hard crystal structure) electronic device or integrated circuit begins with a semiconductor material of the highest quality. 

Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator. 

In general, semiconductor materials fall into one of two classes: single-crystal and compound. 

Single-crystal semiconductors such as germanium (Ge) and silicon (So) have a repetitive crystal structure, whereas compound semiconductors such as gallium arsenide (GaAs), 

Cadmium sulfide (CdS), gallium nitride (GaN), and gallium arsenide phosphide (GaAsP) are constructed of two or more semiconductor materials of different atomic structures. 

The three semiconductors used most frequently in the construction of electronic devices are Ge, Si, and GaAs. 

In the first few decades following the discovery of the diode in 1939 and the transistor in 1947 germanium was used almost exclusively because it was relatively easy to find and was available in fairly large quantities. It was also relatively easy to refine to obtain very high levels of purity, an important aspect in the fabrication process. However, it was discovered in the early years that diodes and transistors constructed using germanium as the base material suffered from low levels of reliability due primarily to it's sensitivity to changes in temperature. At the time, scientists were aware that another material, silicon, had improved temperature sensitivities, but the refining process for manufacturing silicon of very high levels of purity was still in the development stages. Finally, however, in 1954 the first silicon transitory was introduced, and silicon quickly became the semiconductor material of choice. Not only is silicon less temperature sensitive, but it is one of the most abundant materials on earth, removing any concerns about availability, the flood gates now opened to this new material, and the manufacturing and design technology improved steadily through the following years to the current high level of sophistication. 

As time moved on, however, the field of electronics became increasingly sensitive to issues of speed. Computers were operating at higher and higher speeds, and communication systems were operating at higher levels of performance. A semiconductor material capable of meeting these new needs had to be found. 

The result was the development of the first GaAs transistor in the early 1970s. This new transistor had speeds of operation up to five times that of Si. The problem, however, was that because of the years of intense design efforts and manufacturing improvements using Si, Si transistor networks for most applications were cheaper to manufacture at had the advantage of highly efficient design strategies. GaAs was more difficult to manufacture at high levels of purity, was more expensive, and had little design support in the early years of development, however, in time the demand for increased speed resulted in more funding for GaAs research, to the point that today it is often used as the base material for nee high-speed, very large scale integrated (VLSI) circuit designs.