Semiconductors: elements having conductivity between a good conductor and insulator
• 2 Classes of Semiconductor
1.Single crystal: have repetitive crystal structure (ex: Ge and Si)
2.Compound: constructed of two or more semiconductor materials of different atomic structures. (ex:
GaAs, CdS, GaN, GaAsP)
• Ge, Si and GaAs – most frequently used in the construction of electronic devices
• 1939 – discovery of diodes
• 1947 – discovery of transistor
Germanium was used almost exclusively because:
•
o Relatively easy to find
o Available in large quantities
o Relatively easy to refine to obtain very high levels of purity which is an important
aspect in the fabrication process
But diodes and transistors constructed using Ge suffered from low levels of reliability due to its
sensitivity to change in temperature
• 1954 – first silicon transistor was introduced
Silicon:
•
o have improved temperature sensitivity
o One of the most abundant materials on earth
o Has the benefit of years of development
o Is the leading semiconductor materials for electronic components and ICs
• 1970’s – first GaAs transistor was developed
• GaAs Transistor: had a speed of operation up to 5 times that of Silicon
GaAs:
•
o was more difficult to manufacture at high levels of purity
o was more expensive
o Had little design support
o Often used as the base material for new high-speed, very large scale integrated
(VLSI) circuit designs
To fully appreciate why Si, Ge, and GaAs are the semiconductors of choice for the electronics
industry requires some understanding of the atomic structure of each and how the atoms are bound
together to form a crystalline structure.
The fundamental components of an atom are:
• electron
• proton
• neutron
In the lattice structure, neutrons and protons form the nucleus and electrons appear in fixed orbits
around the nucleus. The Bohr model for the three materials is provided in the following figure.
