Diffusion Capacitance is the capacitance that happens due to transport of charge carriers between two terminals of a device, for example, the diffusion of carriers from anode to cathode in a forward biased diode or from emitter to base in a forward-biased junction of a transistor.
[note 1][citation needed] In a semiconductor device with a current flowing through it (for example, an ongoing transport of charge by diffusion) at a particular moment there is necessarily some charge in the process of transit through the device.
To implement this notion quantitatively, at a particular moment in time let the voltage across the device be
In this case the amount of charge in transit through the device at this particular moment, denoted
is In the event the quasi-static approximation does not hold, that is, for very fast voltage changes occurring in times shorter than the transit time
That problem is a subject of continuing research under the topic of non-quasistatic effects.