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The Early effect, named after its discoverer James M. Early, is the variation in the width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage. A greater reverse bias across the collector–base junction, for example, increases the collector–base depletion width, decreasing the width of the charge neutral portion of the base. In Figure 1 the neutral (i.e. active) base is green, and the depleted base regions are hashed light green. The neutral emitter and collector regions are dark blue and the depleted regions hashed light blue. Under increased collector–base reverse bias, the lower panel of Figure 1 shows a widening of the depletion region in the base and the associated narrowing of the neutral base region. The collector depletion region also increases under reverse bias, more than does that of the base, because the collector is less heavily doped. The principle governing these two widths is charge neutrality. The narrowing of the collector does not have a significant effect as the collector is much longer than the base. The emitter–base junction is unchanged because the emitter–base voltage is the same. Base-narrowing has two consequences that affect the current: *There is a lesser chance for recombination within the "smaller" base region. *The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the emitter junction increases. Both these factors increase the collector or "output" current of the transistor with an increase in the collector voltage. This increased current is shown in Figure 2. Tangents to the characteristics at large voltages extrapolate backward to intercept the voltage axis at a voltage called the Early voltage, often denoted by the symbol VA. ==Large-signal model== In the forward active region the Early effect modifies the collector current () and the forward common-emitter current gain (), as typically described by the following equations: : Where * is the collector–emitter voltage * is the thermal voltage ; see thermal voltage: role in semiconductor physics * is the Early voltage (typically 15 V to 150 V; smaller for smaller devices) * is forward common-emitter current gain at zero bias. Some models base the collector current correction factor on the collector–base voltage ''V''CB (as described in base-width modulation) instead of the collector–emitter voltage ''V''CE. Using ''V''CB may be more physically plausible, in agreement with the physical origin of the effect, which is a widening of the collector–base depletion layer that depends on ''V''CB. Computer models such as those used in SPICE use the collector–base voltage ''V''CB.〔( Orcad PSpice Reference Manual named PSpcRef.pdf ), p. 209. This manual is included with the free version of Orcad PSpice, but they do not maintain a copy on line. If the link given here expires, try Googling PSpcRef.pdf.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Early effect」の詳細全文を読む スポンサード リンク
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