Semiconductors

Semiconductors are a group of materials with electrical characteristics in between metals and insulators. The electrical characteristics of semiconductors can be altered to a dramatic extent by several ways including dopants, temperature, optical excitation, stress and other ways. This as well as other reasons makes semiconductors the material of choice for many electronic devices. Current electronic devices such as IC including the microprocessors used in personal computers, lasers, communication devices and a vast array of other electronic devices are made using semiconductors.

Semiconductors are a group of materials typically referring to group IV elements of the periodic table or combinations of groups III and V elements or groups II and VI elements. Semiconductors from group IV are elemental semiconductors because they are composed of only one element. These semiconductors include Si and Ge. Si is by far the most studied and utilized semiconductor today since it is the material used to make microprocessors. Carbon is not usually referred to as a semiconductor. The story of the transistor and integrated circuits is one of the most interesting and dramatic stories of technological innovation and development. From the invention of the first transistor on November 17, 1947 by William Shockley, Walter Brattain and John Bardeen to modern day IC’s with close to a billion transistors operating at extremely high speeds, the continual march of technological progress has occurred despite dire predictions and perceived physical impossibilities. A good historical account of the development of transistors can be found at http://www.pbs.org/transistor/index.html .

Compound semiconductors are semiconductors with two different elements. III-V compound semiconductors include GaAs, GaN, InP, InSb and are composed of one group III element and one group V element in a one-to-one ratio. These semiconductors are used mostly for optical devices such as lasers and photodetectors. II-VI semiconductors include CdSe, ZnS and are composed of one group II element and one group VI element in a one-to-one ratio. These semiconductors are also used for optical applications. Two other types of semiconductors are encountered and are composed of several species of group III elements combined with several species of group V elements. Ternary semiconductors are composed of the three elements with the total group III elements and group V elements still having a one-to-one ratio. Quartenary semiconductors are composed of the four elements with the total group III elements and group V elements still having a one-to-one ratio. They are written as follows:

Ternary: Al_xGa_1-xAs where x is the mole fraction of AlAs

Quaternary: In_xGa_1-xAs_yP_1-y where x is the mole fraction of InAsP

and y is the mole fraction of InGaAs

A list of many of the most commonly used semiconductors are listed in Table 1.1 of Advanced Semiconductor Fundamentals (referred to from now on as ASM). The various applications that the different semiconductors are used for are determined by their electronic and optical characteristics that can drastically vary from one semiconductor to another. These characteristics and applications will be studied in this course starting with the most important aspect of these materials, namely their crystalline structure.