Environmental Engineering Reference
In-Depth Information
This process causes a current to flow through the diode that is
exponentially related to voltage
as follows
where is the junction area and the scale current density which is
inversely proportional to the doping concentrations. The product
is
often expressed in terms of a scale current and denoted as
As far as the charge stored in the device is concerned, we have two
contributions under the forward bias condition. The first is given by the
charge stored in the depletion region, that can be evaluated by
substituting for in (1.12), assuming there is an abrupt junction. In the
same manner, this charge yields a small signal junction capacitance that can
be expressed by (1.13) and (1.14). In any case, since this contribution is
negligible, the junction capacitance is often modeled with a capacitive value
of where is expressed by (1.14) or (1.17), depending on whether the
junction is assumed to be abrupt or not.
The second contribution takes into account the charge due to minority
carrier concentrations close to the junction that are responsible for the
diffusion current. This component yields a diffusion capacitance,
which
is proportional to the current
as follows [
1
]-[2]
