Following section describes the diffusion capacitance in brief.
- The junction capacitance dominates the reactance of a p-n junction under reverse bias; for forward bias, however, the charge storage, or diffusion capacitance, Cs becomes dominant.
- It has been recently shown that the various time-dependent current components, as well as the boundary conditions, affect the diffusion capacitance in forward bias.
- We need to specify at which terminals the stored charges are extracted or "reclaimed" and where the relevant voltage drops occur.
- For long diodes whose dimensions are larger than the diffusion lengths, there is no diffusion capacitance.
- For a short diode, the reclaimable charge is two-thirds of the total stored charge.
- The diffusion capacitance due to stored holes on the n side of length c is:
- There is a similar contribution from the stored electrons on the p side (Fig. given below).
- In practice, most Si p-n junctions behave like short diodes, while laser diodes made in direct band gap (short lifetime) semiconductors often correspond to the long-diode case.
Similarly, we can determine the a-c conductance by allowing small changes in the current. For example, for a long diode, we get