专业英语01.docVIP

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