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 n- type 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!!