8.4 Diffusion or Storage Capacitance

In forward bias，the excess charge stored in the neutral，or diffusion region，of the diode leads to a delay whenever an attempt is made to change the voltage across the junction. 10. In forward bias，the excess charge stored in the neutral，or diffusion，region of the diode leads to a delay whenever an attempt is made to change the voltage across the junction. 提示：or表示“即”，不要翻译为“或者”；注意根据上下文灵活翻译whenever。 Because this region is electrically neutral，there is not only an excess of minority carriers（already shown in Fig.8.4）but also an equal excess of majority carriers，so that their charges balance and the region is neutral. All of these carriers must be re-adjusted in number when，for example，the external circuit causes an alternating voltage to appear across the diode. This results in a flow of charge in and out of the diode which is modeled by the diffusion capacitancesometimes called the storage capacitance. This small-signal capacitance can be calculated from the change ΔQ of the excess minority carrier charge Q，stored in the neutral diffusion regions of the diode，which accompanies a small change ΔV in the applied voltage. 11. This small-signal capacitance can be calculated from the change ΔQ of the excess minority carrier charge Q，stored in the neutral diffusion regions of the diode，which accompanies a small change ΔV in the applied voltage. 提示：过去分词stored修饰Q；which引导的非限制性定语从句修饰ΔQ。 The ΔQ charge is alternately being charged and discharged through the junction as the voltage across the junction changes. 12. The ΔQ charge is alternately being charged and discharged through the junction as the voltage across the junction changes. 提示：is alternately being charged and discharged是现在进行时被动语态。

Fig.8.4 Minority carrier concentration changes with changing forward-bias voltage.

Define Cd as the dQ/dV，and the following result can be derived

where τt is called effective transit time .

The small-signal diffusion capacitance Cd is directly proportional to the current through the junction，becoming larger than the depletion layer capacitance for all reasonable forward biases. 13. The small-signal diffusion capacitance Cd is directly proportional to the current through the junction，becoming larger than the depletion layer capacitance for all reasonable forward biases. 提示：由becoming组成的现在分词短语作状语，becoming的逻辑主语是主句中的主语；is directly proportional to表示与……成正比；注意根据上下文灵活翻译reasonable一词。 Cd is always associated with the diode differential resistance rd，so the forward-biased diode is inevitably lossy，and can not be used as a way of making a good capacitor.

Fig.8.5 Voltage dependence of the small-signal junction capacitance Cj and the small signal diffusion capacitance Cd of a junction diode.

From the foregoing we see that the relative significance of charge storage in the space-charge region（as represented by Cj）and charge storage in the quasi-neutral regions depends strongly on the junction voltage. 14. From the foregoing we see that the relative significance of charge storage in the space-charge region（as represented by Cj）and charge storage in the quasi-neutral regions depends strongly on the junction voltage. 提示：From the foregoing表示“从前面的分析中”；主语用we并不是一定要说明行为者，翻译时不一定必须表示为“我们……”；significance表示大小，不要机械翻译为“意义”、“重要性”。 Under reverse bias，storage in the quasi-neutral regions is negligible and the storage represented by the junction capacitancedominates. Under forward bias，although Cj increases（because Wd decreases），the exponential factor in the formula for Cd generally makes diffusion capacitance and its associated charge storage dominant.

Fig.8.5 illustrates the voltage dependence of the Cd and Cj，showing that with a forward bias of more than a few hundred millivolts the diffusion capacitance normally dominates，and Cj is often negligible. 15. Fig.8.5 illustrates the voltage dependence，showing that with a forward bias of more than a few hundred millivolts the diffusion capacitance normally dominates，and Cj is often negligible. The converse applies for reverse bias voltages. 提示：现在分词showing及其后面的宾语从句作状语；showing后面是由and连接的两个宾语从句，此处and可以翻译为“而”；后面一句中The converse applies表示与前面一句表示的情况相反。 The converse applies for reverse bias voltages.

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