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pn <strong>Junction</strong> Capacitance: Forward Bias<br />
Excess charge stored in neutral region near edges of space charge<br />
region is<br />
Q<br />
=<br />
D<br />
i D<br />
τ<br />
T<br />
Coulombs<br />
t T is called diode transit time and depends on size and type of diode.<br />
The Q depends on i D and in turn on v D . This capacitance behavior is<br />
referred to as diffusion capacitance and given by:<br />
C<br />
j<br />
=<br />
dQ<br />
dv<br />
D<br />
D<br />
=<br />
dQ<br />
di<br />
D<br />
D<br />
di<br />
dv<br />
D<br />
D<br />
≅<br />
iDτ<br />
T<br />
V<br />
T<br />
[F]<br />
i<br />
D<br />
=<br />
I<br />
S<br />
⎡ ⎛ v<br />
⎢exp<br />
⎜<br />
⎣ ⎝ V<br />
D<br />
T<br />
⎞ ⎤<br />
⎟ − 1⎥<br />
⎠ ⎦<br />
≈<br />
I<br />
S<br />
⎛ v<br />
exp<br />
⎜<br />
⎝ V<br />
D<br />
T<br />
⎞<br />
⎟<br />
⎠<br />
Diffusion capacitance is proportional to current and becomes quite<br />
large at high currents.<br />
ELE2110A © 2008<br />
Lecture 02 - 27
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