An ideal
diode doesn't conduct any current at all if its cathode is more positive
than its anode (or, by convention, the voltage across it is negative).
On the other hand, an ideal diode conducts with no resistance at all if
the anode is more positive than its cathode (or the voltage across the
diode is positive).
A diode whose anode is more positive than its cathode is also said to be
'forward-biased', while one whose cathode is more positive than its
anode is 'reverse-biased'. The voltage-current curve of an ideal
diode would therefore look like what's shown in Figure 1.
Figure 1.
The Voltage-Current Curve of an Ideal Diode
Of course, in
the real world, there is no such thing as an ideal diode. A real diode
would only start to conduct when the voltage across it exceeds a value
referred to as its
cut-in
voltage
(also known as 'breakpoint', 'offset', 'threshold', or 'forward'
voltage). When the diode voltage is below the cut-in voltage,
there is almost no current flowing through a diode. Once the diode
voltage exceeds the cut-in voltage, the current flowing through the
diode increases sharply with the voltage. The typical cut-in
voltage for a silicon diode is about 0.6 V while a germanium diode has a
typical cut-in voltage of about 0.2 V.
A real diode
also exhibits some resistance when it is conducting. In fact, the 'on'
resistance of a diode is not a fixed value - it roughly varies with the
current inversely. A real diode also exhibits a certain amount of
current through it when it is reverse-biased. This current is known as
its reverse-bias current, or simply reverse current. It is also
referred to as 'leakage' current.
If the
reverse-bias voltage applied to a diode becomes large enough, the diode
junction begins to break down, allowing the diode to conduct even in
reverse-bias. This phenomenon occurs at what is known as the reverse
breakdown voltage, which is very much larger than the forward cut-in
voltage of the diode. For a silicon diode, for instance, the
reverse breakdown voltage would be typically around 50 V. Thus,
the voltage-current curve of a real diode is shown 2.
Figure 2.
The Voltage-Current Curve of a Real Diode (not drawn to scale)
See Also:
Junction
Diode;
p-n
Junction; More Articles
