Page 963 - Basic Electrical Engineering

For determining the reverse characteristic, the supply connection has to be
reversed. Under the reverse-biased condition, the junction resistance is very
high and ideally no current should flow. But due to minority charge carriers,

a negligibly small current of the order of microamperes will flow. This
current is also called leakage current of the diode. It gets saturated to its

initial value of a few microamperes or even less than that. Increase of
negative biasing, i.e., increase of negative voltage across the diode does not

increase this reverse current. However, if the reverse voltage is increased to a
large value, at one stage, the p–n junction will break down with a sudden rise

in reverse current. The reverse voltage at which the diode breaks down and a
large reverse current starts flowing is called the breakdown voltage. At this
reverse breakdown voltage, current continues to increase.

14.5.2 An Ideal Diode

An ideal diode will conduct in one direction and oppose any current flow in

the other direction. An ideal diode will have zero forward resistance and
infinite reverse resistance. An ideal diode is difficult to realize. If certain
assumptions are made, we may realize a near ideal diode. For example, we

may ignore the reverse current I and assume that forward voltage drop, V F
R
as constant at 0.3 V for germanium and 0.7 V for silicon (for an ideal diode,
I = 0 and V = 0). The V–I characteristic for a diode which is near real is
R
F
shown in Fig. 14.10 (a). The equivalent circuit is shown in Fig. 14.10 (b).
The biased diode is assumed to have a constant forward voltage drop, V and
F
no series resistance. In the equivalent circuit of a practical diode a voltage
source V (equal to 0.3 V for the germanium diode and 0.7 V for the silicon
F
diode) has been shown in series with an ideal diode so as to represent voltage
drop across an ideal divode equal to zero. An example will clarify this

concept.