Capacitance, Diffusion Capacitance
Capacitance
The charge in the diode carrying current IQ is known to be
Q = IQτF + QJ
where τF is the forward transit time of charge carriers:[2] The first term in the charge is the charge in transit across the diode when the current IQ flows. The second term is the charge stored in the junction itself when it is viewed as a simple capacitor; that is, as a pair of electrodes with opposite charges on them. It is the charge stored on the diode by virtue of simply having a voltage across it, regardless of any current it conducts.
In a similar fashion as before, the diode capacitance is the change in diode charge with diode voltage:
Diffusion Capacitance
is the junction capacitance and the first term is called the diffusion capacitance, because it is related to the current diffusing through the junction.
Diffusion capacitance is the capacitance due to transport of charge carriers between two terminals of a device, for example, the diffusion of carriers from anode to cathode in forward bias mode of a diode or from emitter to base (forward biased junction in active region) for a transistor.
In a semiconductor device with a current flowing through it at a particular moment there is necessarily some charge in the process of transit through the device. If the applied voltage changes to a different value and the current changes to a different value, a different amount of charge will be in transit in the new circumstances.
The change in the amount of transiting charge divided by the change in the voltage causing it is the diffusion capacitance.
The adjective "diffusion" is used because the original use of this term was for junction diodes, where the charge transport was via the diffusion mechanism. See Fick's law.
the amount of charge in transit through the device at this particular moment, denoted Q, is given by
Q = I(V)τF.
Consequently, the corresponding diffusion apacitance:Cdiff.is