8.2.1 A Two-Path View

In developing a small-signal model for the dc drain current, we should recall that, in the general case, this current does not consist only of the channel (drain-to-source current) I _DS. As we have seen in Sec. 6.6, assuming V _DS > 0, high electric fields near the drain can result in impact ionization, which can cause a parasitic current I _DB to flow from the drain to the substrate. We assume that all of this current exits through the substrate terminal B. This current must be added to I _DS, in order to obtain the total drain current:

One can thus view the MOS transistor as containing two paths, as shown in Fig. 8.1. Since the channel path leads to the source, it is convenient, as we will see, to express I _DS as a function of voltages defined with respect to the source. Similarly, since the drain-to-substrate path leads to the substrate, it is convenient to express I _DB as a function of voltages defined with respect to the substrate. These conventions are indicated in the figure. ^{[ ]}

Although the above approach could be generalized to include a gate current, as mentioned in Sec. 6.6 this current is negligible during normal operation. We thus assume for the dc gate current:

Assume that the terminal voltages are changed by a small amount. The resulting changes ? I _D,