Non-ideal diode behaviors

The preceding discussion is for an ideal diode. Many silicon devices operate very nearly as perfect ideal diodes. Our task when engineering materials for diodes and related devices is to understand not only how they work when right, but also to understand how defects in the materials relate to non-ideal behaviors. In this section, some of the major observable malfunctions in diodes are described without going into the causes of the problems extensively. Later, when we come to discussions of specific defects in materials we will consider how they lead to the sorts of misbehaviors described here. 

Shunt resistances can be taken into account easily in the diode equation by addition of a term Jshimt=Vappiied/Rshimt Together with the series resistance of Equation 3.31, this yields  

Note that the dJ/dV derivative is the inverse of the derivative shown in Equation 3.31. 

Recombination in the space charge region dramatically increases the voltage needed to achieve a given level of current injection. It can be shown that the exponent in the diode equation is modified to yield  

As the reverse bias rises, at some point the cascade begins. Worse yet is the fact that the additional current and non-ionizing collisions heat the system and increase the rate at which minority carriers are created, increasing the rate of the process and hence the reverse current. Therefore, the resistance continues to drop rapidly as the current increases. Unless some other series resistance limits the current, this leads to the diode acting as a fuse and burning out. 

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In this equation the added term a is called the diode ideality factor. The value of the ideality factor ranges from 1 for an ideal diode to at most 2. The latter occurs when the minority carrier diffusion lengths (before recombination) are small relative to the depletion width, W. The consequence of non-ideal diode behavior is shown in Figure 3.11. Effectively, the forward resistance of the diode can be doubled at a given voltage. 

Non-ideal diode behaviors (reverse leakage, series and shunt resistances, nonideal behavior, reverse breakdown) and the ideality factor. 

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