Download Section B2: The Ideal Diode

Section B2: The Ideal Diode
As mentioned in the introduction to this section, the
diode is the simplest of the nonlinear devices. The
circuit symbol for a diode is shown to the right and
illustrates its two-terminal nature. What this means is
that there are two places to connect the diode to the
external circuit, the anode and cathode. We’ll get to
this more in a couple minutes but, for normal
operating conditions, the anode is more positive than the cathode and
current will flow in the direction indicated.
An ideal diode is simply a pn junction where the change from p-type to ntype material is assumed to occur instantaneously, also referred to as an
abrupt junction. The easiest way to conceptualize this is that we have two
separate “chunks” of doped semiconductor that somehow we are able to
“stick” together with no change in either segment (the same approach we
took when talking about the pn junction).
Under the ideal diode assumption, we also ignore the effect of the depletion
region and the built-in voltage that we talked about in Section A6. What this
allows us to do is look at diode characteristics without all that pesky physical
stuff getting in the way. Anyway, with this treatment, we can assume
instantaneous changes in current, and we get the general shape of the
current-voltage (IV) curve for an ideal diode as shown below.
Note: This figure is slightly different from the one in your text, but is more
common in the literature and, to my mind; more clearly illustrates what’s
going on. The figure above also illustrates a simple case of a piecewise
linear plot. The piecewise linear approximation allows us to treat different
regions separately and, within these regions, consider the nonlinear device
as a linear element. Specifically, for the ideal diode, at vd=0 the diode
instantaneously changes between the forward bias region and reverse bias
region. Comparing these IV characteristics to our first figure, the diode is
conducting, or “on,” in the forward bias region and the current is flowing in
the direction of the arrow in the diode symbol. Conversely, the diode is not
conducting, or “off,” in the reverse bias region – when the current attempts
to flow against the direction of the arrow in the diode symbol (it hits the
bar).
Although we’ll see that, with certain modifications from ideal, this holds true
for all diodes, in the ideal diode case this is particularly nice. As illustrated in
the figures below, if the ideal diode is
¾ forward biased: the ideal curve has infinite slope, the resistance
associated with the diode in the forward bias region is zero (Rf=0) and
the diode may be replace with a short.
¾ reverse biased: the ideal curve has zero slope, the resistance associated
with the diode in the reverse bias region is infinity (Rr64)and the diode
may be replaced with an open.
Hmmm... this is interesting! If we can define one of these cases, then life
(and the math) becomes much easier!!