电子科学与技术专业英语2.1.3 Internal Current Patterns.ppt

Specialized English School of Physics Optoelectronic Engineering W. Zhang A detailed examination of current components in the biased BJT reveals a relatively complicated picture．Fortunately，there exist approximations that are simple and yet quite accurate through wide ranges of conditions. This is another way of saying that some of the most complicated features of the device are often small enough to be neglected safely. Figure 2-4 is a physical representation of a BJT in the ”common-emitter orientation shown first in Figure 2 - 1(a) . The large primary arrow directed from collector to emitter is identically , representing one-dimensional electron transport. The labels IC and IE show that it constitutes both the collector-terminal current and the emitter-terminal current, which typically differ by only about one percent． Shown also are other current “strands” that depart from the primary current or join it. The relative magnitudes of the secondary currents are represented qualitatively by their widths, but for clarity all of these widths are grossly exaggerated with respect to that of the primary arrow. The circled values give typical currents one might encounter in a small BJT (a “milliwatt device) under bias, and it is evident that they differ by many orders of magnitude. The most important of the secondary currents is the one labeled IB, the base-terminal current．It differs in numerous and major ways from the primary electron current that has occupied us up to this point：First ．it is a majority-carrier current，or a hole current in the NPN device. Second，it flows laterally from the base contact into the active region of the BJT． Refer back to Figure 2－2, which shows the commonplace (diffused) BJT in a relatively realistic cross section. The portion of the base region contiguous with (or “under) the emitter region is the active base region, sometimes also termed the intrinsic base region．（This use of the term does not have doping co