7.4 Ideal Current-Voltage Relationship

1. Assumptions

The ideal current-voltage relationship of a pn junction is derived on the basis of the following four assumptions.

（1）The abrupt depletion layer approximation applies. The space charge regions have abrupt boundaries and the semiconductor is neutral outside of the depletion region.

（2）The Maxwell-Boltzmann approximation applies to carrier statistics.

（3）The concept of low injectionapplies.

（4）The individual electron and hole currents are constant throughout the depletion region.

Fig.7.3（a）zero bias，（b）reverse bias，and（c）forward bias.

2. Ideal-diode equation

By solving the continuity equationfor minority carriers in the quasi-neutral regions near a pn junction based on the above assumptions，it is possible to obtain current-voltage characteristicsin the steady state for several simple cases with important practical applications. 13. By solving the continuity equationfor minority carriers in the quasi-neutral regions near a pn junction based on the above assumptions，it is possible to obtain current-voltage characteristicsin the steady state for several simple cases with important practical applications. 提示：动名词短语solving the continuity equation 是By的介词宾语，其后面又带有多个修饰短语；主句中it是先行主语，代表从to obtain…到句尾的动词不定式短语；with important practical applications修饰several simple cases，可翻译为“几种具有重要实用价值的简单情况”。 These solutions lead to the following celebrated Shockley equation，or ideal diode law .

ID=IS［exp（qVa/kT）］-1 （7.1）

where ID is the diode current and ISthe diode reverse saturation current .

3. The current-voltage characteristic

The ideal I-Vcharacteristic predicted by the above equation is illustrated in Figs.7.4（a）and（b）in the linear and semilog plots respectively. In these universal curves I and V are plotted as multiples of IS and kT/qrespectively. 14. In these universal curves I and V are plotted as multiples of IS and kT/qrespectively. 提示：these universal curves表示“通用曲线”；注意，如果对照英语直译主句，会存在汉语表达的内容不清晰、可读性差的问题。应该在正确理解主句表示的内容后，采用意译的方式，表达的含义将清晰、明确。 In the forward direction for Va＞ 3kT/q，the rate of current rise is constant ［Fig.7.4（b）］；at 300 K for every decade change of current，the voltage changes by 59.5 mV（=2.3 kT/q）. 15. In the forward direction for Va> 3kT/q，the rate of current rise is constant ［Fig.7.4（b）］；at 300 K for every decade change of current，the voltage changes by 59.5mV（=2.3 kT/q）. 提示：every decade change of current表示电流变化一个数量级。 Note that the reverse current saturates for reverse voltages of more than about-3（kT/q）. 16. Note that the reverse current saturates for reverse voltages of more than about-3（kT/e）. 提示：由于负数的绝对值越大，表示负数代数值越小，因此该句不能翻译为大于-3（kT/q），而要翻译为比-3（kT/q）更负。

Fig.7.4 The current-voltage characteristic on two different scales.

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