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Under forward bias, the current is carried by majority holes flowing
from the p-region to the n-region, and majority electrons flowing from the nregion to the p-region. There are plenty of these carriers available, but they
are faced with a ‘potential barrier’ in the transition region that restricts the
numbers of carriers which can cross it. The height of this barrier decreases as
the forward bias voltage increases, more carriers can pass, and hence the
diode current increases rapidly with the voltage.
With reverse bias, the current is carried by minority electrons flowing
across the junction from the p-region and minority holes from the n-region.
There is no potential barrier to hinder this movement, but the constraint to the
current flow is simply a very small supply of minority carriers ready to cross
the junction. Hence the reverse current is much smaller than the forward
current. The supply of minority carriers is not affected by the reverse bias
voltage, and so the reverse saturation current is essentially constant over a
wide range of the voltage.
The forward characteristic of junction diodes can be described by the
equation, often called the
diode equation:
KV
IIe
DS
D
1,
where IS is the reverse saturation current and K is a temperature dependent
parameter which varies somewhat between devices, but is often
–1
approximately equal to 35 V
for silicon diodes at room temperature.
In practice, the voltage across a diode is in the range from 0.6 to 0.7 V
when the typical range of currents found in low power electronic circuits is
about from 0.1 to 10 mA. This is a good rule of thumb for circuit design.
Another rule of thumb is that for a forward bias of less than about 0.4 V,
or with a reverse bias, the current flowing is likely to be quite negligible
compared to other currents in the circuit.
An ideal diode would be an insulator under reverse bias and have no
resistance (and hence drop no voltage) under forward bias. Real diodes only
approximate to this ideal, but are close enough for many practical purposes.
Diodes of all shapes and sizes are used in a variety of applications: for
example, converting a.c. to d.c. in a car alternator or a mains power supply;
extracting the wanted signal from the radio frequency carrier in a radio or
TV; protecting a PDA when the batteries are inserted wrongly; improving the
switching speed of digital circuits. Light emitting diodes are a particular form
of p-n junction device made not from silicon but from compound
semiconductors such as gallium arsenide phosphide (GaAsP). These emit
light when a forward current is passed, and are commonly used in bicycle
lamps, and various forms of indicator and display.
51
When considering which diode to use, there are three main electronic
parameters which determine its suitability for the application.
The maximum forward current, I
1.
. For all forms of a diode there is a
F
high current level at which the device will be irreversibly damaged. Much of
this is due to heating, and diodes for the high current use have to be in a
package with the high thermal conductivity so that the temperature rise can
I
be minimized. The safe maximum value of
depends on such factors as the
F
external temperature and the way the diode is mounted on a circuit board or
heat sink. Values can range from less than 100 mA for a small signal diode to
thousands of amps for a power rectifier.
Forward voltage drop, V
2.
. As it is shown above, the exponential
F
form of the diode equation means that for a particular range of forward
currents, the voltage across the diode junction does not vary a lot. For a
silicon diode in the milliamp range, a value of 0.65 V is a good rule of
thumb. The forward voltage drop is greater than this value with higher
currents, and may be increased even further in a practical device by the
voltage drop across the resistance of the semiconductor material. If a lower
VF is needed, there are special diodes such as Schottky barrier diodes. These
contain a junction between a metal and a semiconductor which has a much
higher value of
Is than a silicon p-n junction and hence a much lower value of
VF than ordinary silicon diodes.
Peak reverse voltage, V
3.
. When a small reverse bias voltage is
R
applied to the diode, only a small leakage current flows. As the reverse bias
increases, however, it reaches a value at which the semiconductor junction
will break down, allowing a large and potentially damaging current to flow.
Hence it is important to ensure that the peak voltage in a circuit is less than
V
this critical
. There is, however, a particular type of diode called a Zener
R
diode in which the reverse bias breakdown is sharply defined, reproducible
and reversible. These devices are one form of voltage reference, used to set
particular voltage values in a circuit.
The next section will describe rectifiers, illustrating one important
application of diodes in the conversion of alternating current to direct current.
4.3. P-N Diode Applications
Half-Wave Rectification
In practice most d.c. power units use full-wave rectification, which is
achieved either by the use of the full-wave rectifier circuit with a centretapped transformer winding, or by a
bridge rectifier with a single secondary
transformer winding. Both these circuits will be explained after the
description of a simpler
half-wave rectification circuit.
52
v
in
V
m
VD
0
t
T
-V
a.c.
R
V
outv
Vm-V
m
v
out
F
0
t
(a) (b)
Fig. 4.2. Half-wave rectification: (a) the circuit of half-wave rectifier; (b) the input
and output waveforms
Figure 4.2(a) shows the circuit of a d.c. power unit with the half-wave
rectification but without smoothing. The rectifier is a silicon p-n junction
diode, which readily conducts the current when it is forward biased by more
than about 0.65 V, and will behave like an open circuit when reverse biased
or when the forward bias is less than about 0.4 V (see section 4.2).
Figure 4.2(b) shows the input and output waveforms of the circuit. The
transformer output waveform is the continuous line, whilst the voltage across
the load resistor
R is shown as a dashed line. The difference between them is
caused by the presence of the silicon diode. When the transformer output
voltage is sufficiently positive for the diode to conduct, the voltage applied to
V
the load resistance is the a.c. voltage minus the voltage
dropped across the
F
diode. When the transformer output voltage is negative, no current flows, so
there is zero voltage drop across the load, as shown. The voltage dropped
across the diode depends a little on the current that is supplied. It is usually
equal to 0.65 V or 0.7 V at a small current, but it is likely to be about 1 V at a
current of 1 ampere or so. The diode voltage drop is one reason why the
amplitude of the transformer output must be more than the final d.c. output.
Full-Wave Rectification
The full-wave rectifier circuit, shown in fig. 4.3(a), can be analysed
mostly in the same way as the half-wave rectifier. Note that the secondary
winding of the transformer is centre tapped in this circuit (that is, a
connection is made to the centre of the secondary winding, dividing it into
two halves in series). Relative to the centre tap, the sinusoids from each half
of the secondary winding are out of phase with each other.
53
Each diode is connected across one half of the winding, so one diode is
reverse biased during one half-cycle of the waveform and the other is reverse
biased during the other half-cycle. At any particular time, therefore, only one
half of the transformer is supplying current. But the diodes are arranged so
that the current through the load resistance is in the same direction during
each half-cycle of the a.c. waveform. The circuit is essentially two half-wave
rectifiers in parallel, each using a different half of the a.c. waveform to
produce the same current through the load.
i
VD1
(Vm-VF)/R
v
a.c.
0
T
VD1
VD2
R
v
out
Vm-V
i
VD2
0
v
out
F
0
(Vm-VF)/R
t
t
t
(a) (b)
Fig. 4.3. Full-wave rectification: (a) the full-wave rectifier circuit; (b) the currents of
the diodes and the output waveform
The current waveforms in these two diodes are shown in fig. 4.3(b).
These currents add together to produce the voltage across the load. The
output voltage waveform is also shown in fig. 4.3(b).
The Bridge Rectifier
An alternative way of producing full-wave rectification is shown
in fig. 4.4(a).
VD
In this circuit there are always two diodes conducting, either
VD
, as shown in fig. 4.4(b), or VD2 and VD3, as shown in fig. 4.4(c). The
4
and
1
circuit ensures that though the sign of the output voltage from the transformer
is reversed with each half-cycle of the a.c. supply, the voltage across the load
has the same sign: positive in this case. The output voltage of this
arrangement is therefore almost the same as that of the full-wave rectifier of
fig. 4.3. The amplitude of output voltage in a bridge rectifier circuit equals
54
2
.
VVV
out m m F
i
v
VD1
TV
a.c.
VD2
VD3
VD4
R
V
out
(a)
VD3
i
R
V
out
i
VD1
i
R
V
out
i
VD4
i
i
VD2
i
Fig. 4.4. The bridge rectifier: (a) the circuit; (b) the equivalent circuit during the
positive half-cycle; (c) the equivalent circuit during the negative half-cycle
i
(c)(b)
4.4. Special Diodes
Diodes are known for their unidirectional current property. Basically, diodes
are used for rectifying waveforms, and can be used within power supplies or
within radio detectors. They can also be used in circuits where ‘one way’ effect of
diode is required. Diodes transmit electric currents in one direction, however, the
manner in which they do so can vary. Several types of diodes are available for the
use in the electronics design. Some of diode types are:
Zener Diode: This type of the diode provides a stable reference
voltage. It is a very useful type and has a wide application. The diode runs in
reverse bias, and breaks down at a certain voltage. A stable voltage is
produced, if the current flowing through the resistor is limited.
Schottky Diodes: These diodes feature a lower forward voltage drop
if compared to the ordinary silicon p-n junction diodes. The voltage drop may
55
be in the range from 0.15 to 0.4 volts at a low current, if compared to the 0.6
volts for a silicon diode.
Photodiode: Photodiodes are usually used to detect light. Generally,
these diodes operate in reverse bias, wherein even a small current flow,
resulting from the light, can be detected. Photodiodes can be used to generate
electricity, used as solar cells and even in photometry.
Light Emitting Diode (LED): It is one of the most popular types of
diodes. When this diode permits the transfer of electric current between the
electrodes, light is produced. The color of light depends on the energy gap of
the semiconductor.
Avalanche Diode: This type of a diode operates in the reverse bias,
and uses avalanche effect. The avalanche breakdown takes place across the
entire p-n junction, when the voltage drop is constant and is independent of
the current. Generally, the avalanche diode is used for photo-detection,
wherein high levels of sensitivity can be obtained by the avalanche process.
Laser Diode: This type of a diode is different from the LED type, as
it produces coherent light. These diodes are used in DVD and CD drives,
laser pointers, etc. Laser diodes are more expensive than LEDs. However,
they are cheaper than other laser generators.
Varicap Diode or Varactor Diode: This type of a diode uses a
reverse bias placed upon it, which varies the width of the depletion layer
depending on the voltage applied to the diode. This diode acts as a capacitor.
By altering the bias on the diode, the width of the depletion region changes,
thereby varying the capacitance.
Rectifier Diode: These diodes are used to rectify alternating power
inputs in power supplies. They can rectify current levels that range from an
amp upwards. If low voltage drops are required, Schottky diodes can be used,
however, generally they are p-n junction diodes.
(a) (b) (c) (d) (e) (f) (g) (h)
Fig. 4.5. Diode symbols: (a) diode; (b) Zener diode; (c) bidirectional Zener diode;
(d) tunnel diode; (e) Schottky diode; (f) varicap diode; (g) photodiode;
(h) light emitting diode
Diodes are used widely in electronics, from design to production.
Besides the above mentioned types, other diodes are PIN diodes, point
contact diodes, signal diodes, step recovery diodes, tunnel diodes and gold
doped diodes. The diode type to transfer the electric current depends on the
type and amount of the transmission, as well as on specific applications.
56
4.4.1. Zener Diode
A Zener diode is a special kind of a diode which allows the current to
flow in the forward direction in the same manner as an ideal diode, but it also
permits it to flow in the reverse direction when the voltage is above a certain
value known as the breakdown voltage, ‘Zener knee voltage’ or ‘Zener
voltage’. The device was named after Clarence Zener, who discovered this
electrical property.
A conventional solid-state diode does not allow the significant current if
it is reverse-biased below its reverse breakdown voltage. When the reverse
bias breakdown voltage is exceeded, a conventional diode is subject to the high
current due to the avalanche breakdown. Unless this current is limited by the
circuitry, the diode will be permanently damaged due to the overheating. In the
case of a large forward bias, the diode exhibits a voltage drop due to its
junction built-in voltage and internal resistance. The amount of the voltage
drop depends on the semiconductor material and doping concentrations.
A Zener diode exhibits almost the same properties, except the device is
specially designed to have a greatly reduced breakdown voltage, the so-called
Zener voltage. The volt-ampere characteristic of a Zener diode showing the
breakdown region is shown in fig. 4.6. By contrast with the conventional
device, a reverse-biased Zener diode exhibits a controlled breakdown and
allows the current to keep the voltage across the Zener diode close to the
Zener breakdown voltage. For example, a diode with a Zener breakdown
voltage of 4,7 V exhibits a voltage drop of very nearly 4.7 V across a wide
range of reverse currents. The Zener diode is therefore ideal for various
applications such as the generation of a reference voltage (e. g. for an
amplifier stage), or as a voltage stabilizer for low-current applications.
The Zener diode’s operation depends on the heavy doping of its p-n
junction allowing electrons to tunnel from the valence band of the p-type
material to the conduction band of the n-type material. In the atomic scale,
this tunneling corresponds to the transport of valence band electrons into the
empty conduction band states. This occurs as a result of the reduced barrier
between these bands and high electric fields that are induced due to the
relatively high levels of dopings on both sides. The breakdown voltage can
be controlled quite accurately in the doping process. While tolerances within
0.05 % are available, the most widely used tolerances are 5 % and 10 %.
Breakdown voltage for commonly available zener diodes can vary widely
from 1.2 volts to 200 volts.
57
AnodeCathode Forward
AK
Current
+I
F
Forward
Region
Bias
Zener
Z
Voltage
“Zener”
Breakdown
Region
-I
R
V
Reverse
Current
Forward Bias
F
+V
F
Reverse Bias
-V
R
I
Z(min)
I
Z(max)
-V
Fig. 4.6. Zener diode symbol and I-V characteristics
Another mechanism that produces a similar effect is the avalanche effect
as in the
avalanche diode. These two types of the diode are in fact
constructed in the same way and both effects are present in diodes of this
type. In silicon diodes up to 5.6 volts, the Zener effect is predominant and
shows a marked negative temperature coefficient. Above 5.6 volts, the
avalanche effect becomes predominant and exhibits a positive temperature
coefficient. In a 5.6 V diode, the two effects occur together and their
temperature coefficients nearly cancel each other out, thus a 5.6 V diode is
the component of choice in temperature-critical applications. Modern
manufacturing techniques have produced devices with the voltage lower than
5.6 V with negligible temperature coefficients, but as higher voltage devices
are encountered, the temperature coefficient rises dramatically. A 75 V diode
has 10 times the coefficient of a 12 V diode.
All such diodes are usually marketed under the term of ‘Zener diode’.
The Zener Diode Regulator
Zener diodes can be used to produce a stabilized voltage output with
low ripple under varying load current conditions (fig. 4.7). A small current
passes through the diode from a voltage source via a suitable current limiting
R
resistor (
a voltage drop of
). Then the Zener diode will conduct sufficient current to maintain
S
V
. We remember from the previous chapters that the d.c.
out
58
output voltage from the half or full-wave rectifiers contains ripple superimposed
onto the d.c. voltage. By connecting a simple zener stabilizer circuit as shown
below across the output of the rectifier, a more stable output voltage can be
produced.
R
The resistor,
current flow through the diode with the voltage source,
across the combination. The stabilised output voltage
is connected in series with the Zener diode to limit the
S
V
being connected
S
V
is taken from
out
across the Zener diode. The Zener diode is connected with its cathode
terminal connected to the positive rail of the d.c. supply so it is reverse biased
R
and will be operating in its breakdown condition. Resistor
is selected to
S
limit the maximum current flowing in the circuit.
I
+V
DC input voltage
from rectifier or
smoothing circuit
Vin (V
0V
S
R
S
I
I
Z
VD
L
V
out
R
L
)
S
Fig. 4.7. Zener diode regulator
With no load connected to the circuit, the load current is zero (IL = 0), and
all the circuit current passes through the Zener diode which in turn dissipates its
R
maximum power. Also a small value of the series resistor
diode current when the load resistance
R
is connected. In this case, the power
L
results in a greater
S
dissipation of the diode increases. So care must be taken when selecting the
appropriate value of series resistance so that the zener’s maximum power rating
is not exceeded under this no-load or high-impedance condition.
The load is connected in parallel with the Zener diode, so the voltage
R
across
is always the same as the zener voltage, (VR = VZ). There is a
L
minimum zener current for which the voltage stabilization is effective and the
zener current must stay above this value operating under the load within its
breakdown region all the time. The upper limit of the current is, of course,
V
dependent upon the power rating of the device. The supply voltage
V
be greater than
.
Z
must
S
One small problem with zener diode stabiliser circuits is that the diode
can sometimes generate electrical noise on top of the d.c. supply as it tries to
stabilise the voltage. Normally this is not a problem for most applications but
the addition of a large value decoupling capacitor across the Zener’s output
may be required to give additional smoothing.
59
To summarize, a Zener diode is always operated in its reverse biased
condition. A voltage regulator circuit can be designed using a Zener diode to
maintain a constant d.c. output voltage across the load in spite of variations in
the input voltage or changes in the load current. The zener voltage regulator
consists of a current limiting resistor RS connected in series with the input
voltage VS with the Zener diode connected in parallel with the load RL in this
reverse biased condition. The stabilized output voltage is always selected to
be the same as the breakdown voltage VZ of the diode.
4.4.2. Schottky Diode
The Schottky diode (named after German physicist Walter H. Schottky;
also known as hot carrier diode) is a semiconductor diode with a low forward
voltage drop and a very fast switching action. The cat’s-whisker detectors
used in the early days of the wireless can be considered primitive Schottky
diodes.
When the current flows through a diode there is a small voltage drop
across the diode terminals. A normal silicon diode has a voltage drop
between 0.6–1.7 volts, while a Schottky diode voltage drop is between
approximately 0.15–0.45 volts. This lower voltage drop can provide higher
switching speed and better system efficiency.
A metal-semiconductor junction is formed between a metal and a
semiconductor, creating a Schottky barrier (instead of a semiconductor–
semiconductor junction as in conventional diodes). Typical metals used are
molybdenum, platinum, chromium or tungsten; and the semiconductor would
typically be n-type silicon. The metal side acts as the anode and n-type
semiconductor acts as the cathode of the diode. This Schottky barrier results
in both very fast switching and low forward voltage drop.
Reverse Recovery Time
The most important difference between p-n and Schottky diodes is
reverse recovery time, when the diode switches from non-conducting to
conducting state and vice versa. Where in a p-n diode the reverse recovery
time can be hundreds of nanoseconds and less than 100 ns for fast diodes,
Schottky diodes do not have recovery time, as there is nothing to recover
from (i. e. no charge carrier depletion region at the junction). The switching
time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for
special high-capacity power diodes. With p-n junction switching, there is also
a reverse recovery current, which brings increased EMI noise in high-power
semiconductors. This is not so important with Schottky diodes switching
instantly with only slight capacitive loading.
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