Diffusion Capacitance is the

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...

that happens due to transport of

charge carrier In physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. The term is used ...

s between two terminals of a device, for example, the diffusion of carriers from anode to cathode in a forward biased

diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...

or from emitter to baseforward-biased

junction Junction may refer to: Arts and entertainment * ''Junction'' (film), a 2012 American film * Jjunction, a 2002 Indian film * Junction (album), a 1976 album by Andrew Cyrille * Junction (EP), by Basement Jaxx, 2002 * Junction (manga), or ''Hot ...

of a

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...

.The "forward biased" in this context means that the diode/transistor allows the current to flow. In a

semiconductor device A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity li ...

with a current flowing through it (for example, an ongoing transport of charge by

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

) 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 laws of diffusion Fick's laws of diffusion describe diffusion and were derived by Adolf Fick in 1855. They can be used to solve for the diffusion coefficient, . Fick's first law can be used to derive his second law which in turn is identical to the diffusion equ ...

. To implement this notion quantitatively, at a particular moment in time let the voltage across the device be

V

. Now assume that the voltage changes with time slowly enough that at each moment the current is the same as the DC current that would flow at that voltage, say

I=I(V)

(the ''

quasistatic approximation Quasistatic approximation(s) refers to different domains and different meanings. In the most common acceptance, quasistatic approximation refers to equations that keep a static form (do not involve time derivatives) even if some quantities are all ...

''). Suppose further that the time to cross the device is the forward transit time

_F

. In this case the amount of charge in transit through the device at this particular moment, denoted

Q

, is given by ::

Q=I(V)_F

. Consequently, the corresponding diffusion capacitance:

C_

. is ::

C_ =\begin\frac\end=\begin\frac\end _F

. 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

_F

, the equations governing time-dependent transport in the device must be solved to find the charge in transit, for example the

Boltzmann equation The Boltzmann equation or Boltzmann transport equation (BTE) describes the statistical behaviour of a thermodynamic system not in a state of equilibrium, devised by Ludwig Boltzmann in 1872.Encyclopaedia of Physics (2nd Edition), R. G. Lerne ...

. That problem is a subject of continuing research under the topic of non-quasistatic effects. See Liu , and Gildenblat ''et al.''Hailing Wang, Ten-Lon Chen, and Gennady Gildenblat, ''Quasi-static and Nonquasi-static Compact MOSFET Models'' http://pspmodel.asu.edu/downloads/ted03.pdf {{Webarchive, url=https://web.archive.org/web/20070103055142/http://pspmodel.asu.edu/downloads/ted03.pdf , date=2007-01-03

Notes

References notes

External links

Junction capacitance
Capacitance Electrical parameters Semiconductors