Impurity control-region

The so-called impurity-control region, where the opposite situation takes place and the electroneutrahty condition can be simplified to [Me jj] = 2[Vm]. 

Gold has been used for many years as a minority carrier lifeline controller in Si. As such, it is introduced in a controlled manner, usually by diffusion into transistor structures to decrease the carrier lifetime in the base region in order to increase the switching speed (Ravi, 1981). Conversely, the uncontrolled presence of Au is clearly deleterious to the performance of devices, both because of the increased recombination within the structure and the increase of pipe defects, which can cause shorting of the device. These pipe defects consist of clusters of metallic impurities at dislocations bounding epitaxial stacking faults. 

The unique electronic properties of semiconductor devices arise at the regions where p-typc and ra-typc materials ate in close proximity, as in p-n junctions. Typical impurity levels ate about 0.0001 at %, and their inclusion and distribution need to be very strictly controlled during preparation. Without these deliberately introduced point defects, semiconductor devices of the type now commonly available would not be possible. 

The net result of such regulation is that there are well-developed standards for the way in which quantitative and, increasingly, qualitative measurements are carried out. There are also well-defined limits for various categories of impurity, all of which tends to emphasise the importance of measurements on the final API rather than on the process that produces it, for which the requirement is simply that the process remains under control . The importance of this summary is that at present, whatever the means of production, there is a requirement that the API will reach certain standards of purity as demonstrated by end-point measurement. Those standards effectively demand quantification of impurities down to about 0.05% for related organic impurities, in the region of ppb to low ppm for known toxins and low ppm levels for inorganics such as catal3Tic metals. 

---