Section B3: The Practical Diode
OK, the
ideal diode
is an extraordinarily wellbehaved creature that allows
us to deal with its nonlinearities in unbelievably reasonable terms. But...and
there’s always a but... we have to look at the deviations from ideal that
materials, fabrication and miscellaneous stuff introduces. We’re going to
approach this by removing some of the ideal approximations to define a
semiideal case. Then, as promised by the title of this section, practical
considerations will be introduced and we’ll end up with a pretty realistic
device.
To begin, recall from our discussion of
diffusion
that the free electrons in
the ntype material and the free holes in the ptype material will move
across the junction until
equilibrium
is reached. The width of the resulting
depletion region
is related to the
barrier potential
and is a function of
applied bias
.
I realize that all these terms are somewhat overwhelming, but if
they are not comfortable, please review The
pn Junction
discussion.
These concepts are fundamental to the design, operation and
manipulation of all semiconductor devices and the sooner they are
internalized, the less “painful” future material will be!
Now
, with that said, let’s look at how a diode actually works...
The
diode current equation
presented in Equation 3.26 defines an
exponential relationship between the diode current (the dependent variable)
and the diode voltage (the independent variable) and is derived from the
physics of Section A. This equation holds over at least seven orders of
magnitude of current and is truly valuable in defining the behaviors of
semiconductor diodes (and later bipolar junction transistors). NOTE: Please
refer to notation conventions for a description as to significance of upper and
lower case characters.
)
1
(
−
=
T
D
nV
v
O
D
e
I
i
(Equation 3.26)
where:
i
D
is the current through the diode
v
D
is the voltage (potential difference) measured across the diode
terminals
I
O
is the reverse saturation current
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n
is a device constant between 0.5 and 2 that is dependent on
material, diode
construction, and operational considerations.
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 Spring '09
 Mechatronics, pn junction, OC, Von

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