材料科学与工程专业英语

2.6 semiconductor

Following the discussion of intrinsic ,elemental semiconductors we note that the fermi function indicates that the number of charge carriers increases exponentially with temperature. This effect so dominates the conductivity of semiconductors that conductivity also follows an exponential increase with temperature(an example of an arrhenius equation ).This increase is in sharp contrast to the behavior of metals.

We consider the effect of impurities in extrinsic,elemental semiconductors.Doping a group IV a material(such as Si) with a group V a impurity (such as P)produces an n-type semiconductor in which negative charge carriers(conduction electrons)dominate.The “extea”electron from the group V A addition produces a donor level in the energy band structure of the semiconductor.As with instrinsic semiconductors,extrinsic semiconduction exhibits arrhenius behavior.in n-type material, the temperature span between the regions of extrinsic and insrinsic behavior is called the exhaustion range .A p-type semiconductor is produced by doping a group IV a material with a group III a impurity(such as Al).The group III A element has a “missing”electron

producing an acceptor level in the band stucture and leading to formation of positive charge carriers (electron holes). The region between extrinsic and instrinsic behavior for p-type semiconductors is called the saturation range . Hall effect measurements can distinguish between n-type and p-type conduction.

Compound semiconductors usually have an MX composition with an average of four valence electrons per atom .The III-V and II-VI compounds are the common examples .amorphous semiconductors are the non-crystalline materials with semiconducting behavior.Elemental and compound material are both found in this category .To appreciate the applications of semiconductors,we review a few decades.the solid state rectifier (or diode) contains a single p-n junction .Current flows readily when this junction is forward biased but is almost completely choked off when reverse biased.the transistor is a device consisting of a pair of nearby pn junctions.The net result is a solid state amplifier. Replacing vacuum tubes with solid state elements such as these produced substantial miniaturization of electrical circuits.Further miniaturization has resulted by the production of microcircuis consisting of precise parrerns of n-type and

p-type regions on a single crystal chip.

The major electrical properties needed to specify an intrinsic semiconductor are band gap,electron mobility,hole mobility,and conduction electron density (=electron hole density ) at room temperature.For extrinsic semiconductors,one needs to specify either the donor level (for n-type material) or the acceptor level (for p-type material).

2.7 composites

One category of structural engineering material is that of composites .These materials involve some combination of two or more components from the “fundamental”materal types .A key philosophy in selecting composite materials is that they provide the “best of both worlds”that is ,attrative properties from each component. A classic example is fiberglass.The strength of small diameter glass fibers is combined with the ductility of the polymetric matrix.The combination of these two components provides a product superior to either component alone .Many composites,such as fiberglass,involve combinations that cross over the boundaries of different kinds of materials. Others,such as concrete,involve different component from within a single material type.In general,we shall use a fairly narrow definition of