Impurities electronic devices

Several factors detennine how efficient impurity atoms will be in altering the electronic properties of a semiconductor. For example, the size of the band gap, the shape of the energy bands near the gap and the ability of the valence electrons to screen the impurity atom are all important. The process of adding controlled impurity atoms to semiconductors is called doping. The ability to produce well defined doping levels in semiconductors is one reason for the revolutionary developments in the construction of solid-state electronic devices. 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

Semiconductors have a considerably smaller band gap (e.g. silicon 1.17 eV). Their conductivity, which is zero at low temperatures but increases to appreciable values at higher temperatures, depends greatly on the presence of impurities or, if added advertently, dopants. This makes it possible to manipulate the band gap and tune the properties of semiconductors for applications in electronic devices [C. Kit-tel. Introduction to Solid State Physics (1976), Wiley Sons, New York N. Ashcroft and N.D Mermin, Solid State Physics (1976), Saunder College]. 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

Apart from hydrocarbons and gasoline, other possible fuels include hydrazine, ammonia, and methanol, to mention just a few. Fuel cells powered by direct conversion of liquid methanol have promise as a possible alternative to batteries for portable electronic devices (cf. below). These considerations already indicate that fuel cells are not stand-alone devices, but need many supporting accessories, which consume current produced by the cell and thus lower the overall electrical efficiencies. The schematic of the major components of a so-called fuel cell system is shown in Figure 22. Fuel cell systems require sophisticated control systems to provide accurate metering of the fuel and air and to exhaust the reaction products. Important operational factors include stoichiometry of the reactants, pressure balance across the separator membrane, and freedom from impurities that shorten life (i.e., poison the catalysts). Depending on the application, a power-conditioning unit may be added to convert the direct current from the fuel cell into alternating current. 

Silicon s tetravalent pyramid crystalline structure, similar to tetravalent carbon, results in a great variety of compounds with many practical uses. Crystals of sihcon that have been contaminated with impurities (arsenic or boron) are used as semiconductors in the computer and electronics industries. Silicon semiconductors made possible the invention of transistors at the Bell Labs in 1947. Transistors use layers of crystals that regulate the flow of electric current. Over the past half-century, transistors have replaced the vacuum tubes in radios, TVs, and other electronic equipment that reduces both the devices size and the heat produced by the electronic devices. 

FIGURE 19.4 (a) Purifica- tion of silicon by zone refining. The heater coil sweeps the molten zone and the impurities to the lower end of the rod. After the rod has cooled, the impurities are removed by cutting off the rod s lower end. (b) A rod of ultrapure silicon and silicon wafers cut from the rod. Silicon wafers are used to produce the integrated-circuit chips found in solid-state electronic devices. 

Second, the stoichiometry of the melt and of impurities introduced during processing must be controlled to the level demanded by application. Although these constraints vary with application, more control is clearly better in that the demands on purity and spatial uniformity of the material are becoming more stringent with the increasing miniaturization of electronic devices. 

Semiconductors are materials with electrical conducting properties somewhere between those of insulators and conductors. Semiconductors are prepared from semimetals, most commonly silicon. Semiconductors are used in many electronic devices including computers. What makes these materials so popular is the ability to control the conductivity by the addition of small amounts of impurities called doping agents. 

Most of the III-V nitride materials utilised in optoelectronic or electronic devices contain a high density of structural defects. At present, the relationship between these defects and electrically active deep levels is only speculative. To shed light on the role of structural defects and impurities in III-V nitrides, it is important to detect and characterise deep levels in these novel semiconductors. 

The spectacular success of the semiconductor industry is based on the production of materials selectively designed for specialized applications in electronic and optical devices. By carefully controlled doping of semiconductors with selected impurities—electron donors or electron acceptors—the conductivity and other properties can be modulated with great precision. Fig. 12.8 shows schematically how doped semiconductors work. In an intrinsic semiconductor (a), conducting electron-hole pairs can only by produced by thermal or photoexcitation across the band gap. In (b), addition of a small concentration of an electron donor creates an impurity band just below the conduction band. Electrons can then Jump across a much-reduced gap to the conduction band and act as negatively-charged current carriers. This produces a n-type semiconductor. In (c), an electron acceptor creates an empty impurity band just above the valence band. In this case electrons can jump from the valence band to leave positive holes. These can also conduct electricity, since electrons falling into positive holes create new holes, a sequence... 

In practice, nothing is absolutely pure, so the word substance is an idealization. Among the purest materials ever prepared are silicon (Fig. 1.5) and germa-ninm. These elements are used in electronic devices and solar cells, and their electronic properties require either high purity or else precisely controlled concentrations of deliberately added impurities. Meticulous chemical and physical methods have enabled scientists to prepare germanium and silicon with concentrations less than one part per billion of impurities. Anything more would alter their electrical properties. 

Crystals also have electrical and magnetic properties now being used in computers and other electronic devices. Crystals are almost always imperfect and contain impurities (atoms of other elements). These are utilized in semiconductors. For methods of growing crystals, see nucleation. 

Typical transfer molding compositions for encapsulation of electronic devices are mixtures of an epoxy novolac resin, a phenolic resin hardener, a catalyst, large amounts of inorganic filler (e.g., Si02) flame-retardant ingredients, internal lubricants, carbon black, and sometimes other additives such as getters to trap ionic impurities (34,35), corrosion-protection materials, and stress-relief ingredients. 

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