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Unformatted text preview: Introduction to Microelectronics Edwin C. Kan Page 2-1 1/21/2009 Chapter 2 I NTRODUCTION TO S EMICONDUCTORS 2.1 Semiconductor Materials Semiconductor used in electronics is the class of material where the conductivity of the material can be controlled to vary over many orders of magnitude, for example, in silicon we can tune the conductivity using either doping or electric field for modulation larger than 10 10 . In contrast, metal has high conductivity and insulator has very low conductivity, both of whose conductivity can hardly be modulated even over 3 times. There are three common kinds of semiconductors: • Elemental semiconductor: Si, Ge (Group IV elements) • Compound semiconductor (fixed composition): SiC, GaAs, GaN, and ZnO (Group IV with equal composition, Group III-V and Group II-VI) • Alloy: Si 1-x Ge x , Al 1-x Ga x As, Hg 1-x Cd x Te, etc., where x can be any number between 0 and 1 in composition. If the semiconductors cannot be made pure, then the electrical conductivity will be dominated by the uncontrollable impurity (controllable impurity is useful, as shown later), and we cannot achieve a high order of conductivity modulation. Usually for industrial grade semiconductors, the uncontrollable impurity is lower than 10-9 in composition (or below 1013 cm-3), and there are often chemical “getters” which make the uncontrollable impurity to be electrically inactive and immobile in the material. The two solid-state materials that can be made purest are Germanium and Silicon (lucky us), and there are many known getters such as carbon and fluoride. The pure semiconductors also have charge carriers with higher electrical mobility, which will be introduced later. Semiconductors used in electronics also have very regular atomic structure called the crystal lattice, as any defect in crystal lattice behaves similarly to uncontrollable impurities that will diminish electrical conduction and its modulation. To obtain the good crystal quality, industrial-grade semiconductors are made in very high-temperature (for example, silicon at 1,450 o C) and crystallized in a controllable manner by a seed material. The dominant semiconductor for electronics today is silicon (larger than 98% market share), with SiGe, Ge and GaAs, SiC and GaN trailing far behind. This is mainly because of the cost of manufacturing and the high degree of material perfection in both Si and SiO 2 . For optoelectronics and lighting, GaAs and GaN play a more important role because silicon has low conversion efficiency from electric power to light. However, due to cost issues, solar cells today are still mostly made by silicon even though GaAs can give better efficiency. The uncontrollable impurity and defect concentrations in industry-grade silicon are less than 10 13 cm-3 , which is about 10-9 to 10-10 in composition as mentioned before....
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This note was uploaded on 08/30/2009 for the course ECE 3150 taught by Professor Spencer during the Spring '07 term at Cornell University (Engineering School).
- Spring '07