Introduction
A diode is an electrical device
allowing current to move through it in one direction with far greater
ease than in the other. The most common type of diode in modern circuit
design is the semiconductor diode, although other diode
technologies exist. Semiconductor diodes are symbolized in schematic
diagrams as such:
When placed in a simple battery-lamp
circuit, the diode will either allow or prevent current through the
lamp, depending on the polarity of the applied voltage:
When the polarity of the battery is such
that electrons are allowed to flow through the diode, the diode is said
to be forward-biased. Conversely, when the battery is "backward"
and the diode blocks current, the diode is said to be reverse-biased.
A diode may be thought of as a kind of switch: "closed" when
forward-biased and "open" when reverse-biased.
Oddly enough, the direction of the diode
symbol's "arrowhead" points against the direction of electron
flow. This is because the diode symbol was invented by engineers, who
predominantly use conventional flow notation in their schematics,
showing current as a flow of charge from the positive (+) side of the
voltage source to the negative (-). This convention holds true for all
semiconductor symbols possessing "arrowheads:" the arrow points in the
permitted direction of conventional flow, and against the permitted
direction of electron flow.
Diode behavior is analogous to the
behavior of a hydraulic device called a check valve. A check
valve allows fluid flow through it in one direction only:
Check valves are essentially
pressure-operated devices: they open and allow flow if the pressure
across them is of the correct "polarity" to open the gate (in the
analogy shown, greater fluid pressure on the right than on the left). If
the pressure is of the opposite "polarity," the pressure difference
across the check valve will close and hold the gate so that no flow
occurs.
Like check valves, diodes are essentially
"pressure-" operated (voltage-operated) devices. The essential
difference between forward-bias and reverse-bias is the polarity of the
voltage dropped across the diode. Let's take a closer look at the simple
battery-diode-lamp circuit shown earlier, this time investigating
voltage drops across the various components:
When the diode is forward-biased and
conducting current, there is a small voltage dropped across it, leaving
most of the battery voltage dropped across the lamp. When the battery's
polarity is reversed and the diode becomes reverse-biased, it drops
all of the battery's voltage and leaves none for the lamp. If we
consider the diode to be a sort of self-actuating switch (closed in the
forward-bias mode and open in the reverse-bias mode), this behavior
makes sense. The most substantial difference here is that the diode
drops a lot more voltage when conducting than the average mechanical
switch (0.7 volts versus tens of millivolts).
This forward-bias voltage drop exhibited
by the diode is due to the action of the depletion region formed by the
P-N junction under the influence of an applied voltage. When there is no
voltage applied across a semiconductor diode, a thin depletion region
exists around the region of the P-N junction, preventing current through
it. The depletion region is for the most part devoid of available charge
carriers and so acts as an insulator:
If a reverse-biasing voltage is applied
across the P-N junction, this depletion region expands, further
resisting any current through it:
Conversely, if a forward-biasing voltage
is applied across the P-N junction, the depletion region will collapse
and become thinner, so that the diode becomes less resistive to current
through it. In order for a sustained current to go through the diode,
though, the depletion region must be fully collapsed by the applied
voltage. This takes a certain minimum voltage to accomplish, called the
forward voltage:
For silicon diodes, the typical forward
voltage is 0.7 volts, nominal. For germanium diodes, the forward voltage
is only 0.3 volts. The chemical constituency of the P-N junction
comprising the diode accounts for its nominal forward voltage figure,
which is why silicon and germanium diodes have such different forward
voltages. Forward voltage drop remains approximately equal for a wide
range of diode currents, meaning that diode voltage drop not like that
of a resistor or even a normal (closed) switch. For most purposes of
circuit analysis, it may be assumed that the voltage drop across a
conducting diode remains constant at the nominal figure and is not
related to the amount of current going through it.
In actuality, things are more complex
than this. There is an equation describing the exact current through a
diode, given the voltage dropped across the junction, the temperature of
the junction, and several physical constants. It is commonly known as
the diode equation:
The equation kT/q describes the voltage
produced within the P-N junction due to the action of temperature, and
is called the thermal voltage, or Vt of the junction.
At room temperature, this is about 26 millivolts. Knowing this, and
assuming a "nonideality" coefficient of 1, we may simplify the diode
equation and re-write it as such:
You need not be familiar with the "diode
equation" in order to analyze simple diode circuits. Just understand
that the voltage dropped across a current-conducting diode does
change with the amount of current going through it, but that this change
is fairly small over a wide range of currents. This is why many
textbooks simply say the voltage drop across a conducting, semiconductor
diode remains constant at 0.7 volts for silicon and 0.3 volts for
germanium. However, some circuits intentionally make use of the P-N
junction's inherent exponential current/voltage relationship and thus
can only be understood in the context of this equation. Also, since
temperature is a factor in the diode equation, a forward-biased P-N
junction may also be used as a temperature-sensing device, and thus can
only be understood if one has a conceptual grasp on this mathematical
relationship.
A reverse-biased diode prevents current
from going through it, due to the expanded depletion region. In
actuality, a very small amount of current can and does go through a
reverse-biased diode, called the leakage current, but it can be
ignored for most purposes. The ability of a diode to withstand
reverse-bias voltages is limited, like it is for any insulating
substance or device. If the applied reverse-bias voltage becomes too
great, the diode will experience a condition known as breakdown,
which is usually destructive. A diode's maximum reverse-bias voltage
rating is known as the Peak Inverse Voltage, or PIV, and
may be obtained from the manufacturer. Like forward voltage, the PIV
rating of a diode varies with temperature, except that PIV increases
with increased temperature and decreases as the diode becomes
cooler -- exactly opposite that of forward voltage.
Typically, the PIV rating of a generic
"rectifier" diode is at least 50 volts at room temperature. Diodes with
PIV ratings in the many thousands of volts are available for modest
prices.
- REVIEW:
- A diode is an electrical
component acting as a one-way valve for current.
- When voltage is applied across a diode
in such a way that the diode allows current, the diode is said to be
forward-biased.
- When voltage is applied across a diode
in such a way that the diode prohibits current, the diode is said to
be reverse-biased.
- The voltage dropped across a
conducting, forward-biased diode is called the forward voltage.
Forward voltage for a diode varies only slightly for changes in
forward current and temperature, and is fixed principally by the
chemical composition of the P-N junction.
- Silicon diodes have a forward voltage
of approximately 0.7 volts.
- Germanium diodes have a forward
voltage of approximately 0.3 volts.
- The maximum reverse-bias voltage that
a diode can withstand without "breaking down" is called the Peak
Inverse Voltage, or PIV rating.
|
