PROJECT REPORT SHEET
PROJECT DESCRIPTION: Special applications diodes
NAME: YONG GU PARK
STUDENT ID: C61103
Clipper faults
Clippers
A clipper is a circuit that is used to eliminate a portion of an input signal. There are two basic
types of clippers: series clippers and shunt clippers. The series clipper contains a diode that is in
series with the load. The shunt clipper contains a diode that is in parallel with the load.
Basic clippers
The series clipper is a familiar circuit. The half-wave rectifier is nothing more than a series
clipper. When the diode in the series clipper is conducting, the load waveform resembles the
input waveform. When the diode is not conducting, the output is approximately 0 V (Figure 4.2).
The direction of the diode determines the polarity of the output waveform. If the diode symbol
(in the schematic diagram) points toward the source, the circuit is a positive series clipper,
meaning that it clips the positive alternation of the input. If the diode symbol points toward the
load, the circuit is a negative series clipper, meaning that it clips the negative alternation of the
input (Figure 4.11).
Ideally, a series clipper has an output of
when the diode is conducting (ignoring the
voltage across the diode). When the diode is not conducting, the input voltage is dropped across
the diode, and
.
Unlike a series clipper, a shunt clipper provides an output when the diode is not conducting.
When the diode is off (not conducting), the component acts as an open. When this is the case,
and form a voltage divider, and the output from the circuit is found using
When the diode in the circuit is on (conducting), it shorts out the load. In this case, the circuit
ideally has an output of
. Again, this relationship ignores the voltage across the diode. In
practice, the output from the circuit is generally assumed to equal 0.7 V, depending upon
whether the circuit is a positive shunt clipper or a negative shunt clipper. The direction of the
diode determines whether the circuit is a positive or negative shunt clipper. The series currentlimiting resistor ( ) is included to prevent the conducting diode from shorting out the source.
A biased clipper is a shunt clipper that uses a dc voltage source to bias the diode. The biasing
voltage ( ) determines the voltage at which the diode begins conducting. The diode in the
biased clipper turns on when the load voltage reaches a value of
. In practice, the dc
biasing voltage is usually set using a potentiometer and a dc supply voltage
A biased clipper
Clippers are used in a variety of systems, most commonly to perform one of two functions:
Altering the shape of a waveform
Protecting circuits from transients
The first application is apparent in the operation of half-wave rectifiers. As you know, these
circuits are series clippers that change an alternating voltage into a pulsating dc waveform. A
transient is an abrupt current or voltage spike of extremely short duration. Left unprotected,
many circuits can be damaged by transients. Clippers can be used to protect sensitive circuits
from the effects of transients, as illustrated in Figure 4.12.
Clamper faults
A clamper is a circuit that is designed to shift a waveform above or below a dc reference voltage
without altering the shape of the waveform. This results in a change in the dc average of the
waveform. Both of these statements are illustrated in Figure 4-3. (The clamper has changed the
dc average of the input waveform from 0 V to +5 V without altering its shape.)
A clamper with its input and (ideal) output waveforms
There are two basic types of clampers:
A positive clamper shifts its input waveform in a positive direction, so that it lies above a dc
reference voltage. For example, the positive clamper in Figure 4-3 shifts the input waveform so
that it lies above 0 V (the dc reference voltage).
A negative clamper shifts its input waveform in a negative direction, so that it lies below a dc
reference voltage.
Both types of clampers, along with their input and output waveforms. The direction of the diode
determines whether the circuit is a positive or negative clamper.
Clamper operation is based on the concept of switching time constants. The capacitor charges
through the diode and discharges through the load. As a result, the circuit has two time constants:
For the charge cycle,
and
(where
is the resistance of the diode)
For the discharge cycle,
and
(where
is the resistance of the load)
Since is normally much greater than
, the capacitor charges much more quickly than it
discharges. As a result, the input waveform is shifted.
A biased clamper allows a waveform to be shifted above (or below) a dc reference other than 0
V.
Several biased clampers
The circuit (a) uses a dc supply voltage (V) and a potentiometer to set the potential at the cathode
of
. By varying the setting of
, the dc reference voltage for the circuit can be varied
between approximately 0 V and the value of the dc supply voltage.
The zener clamper (b) uses a zener diode to set the dc reference voltage for the circuit. The dc
reference voltage for this circuit is approximately equal to
. Note that zener clampers
are limited to two varieties:
Negative clampers with positive dc reference voltages
Positive clampers with negative dc reference voltages
Voltage multiplier faults
Voltage Multipliers
Voltage doublers
A voltage multiplier provides a dc output voltage that is a multiple of the circuit’s peak input
voltage. For example, a voltage doubler with a peak input of 10 V provides a dc output that is
approximately 20 V.
Each of the circuits provides a dc load voltage that is approximately twice the value of the peak
source voltage. The half-wave doubler gets its name from the fact that the output capacitor ( )
is charged during the positive half-cycle of the input signal. In contrast, the output capacitor in
the full-wave doubler ( ) is charged during both alternations of the input cycle. Note that the
output from a full-wave doubler has less ripple than the output from a comparable half-wave
doubler.
The voltage tripler is very similar to the half-wave voltage doubler. If you compare the tripler to
the circuit (a), you will see that the circuit made up of , , , and is actually a half-wave
voltage doubler. This circuit charges
the input cycle,
of
and
is charged to approximately
is approximately
combination of
to a value of
and
,
. Since
and
. During the negative alternation of
. The voltage across the series combination
and the load are in parallel with the series
are also approximately equal to
.
A voltage tripler
The voltage quadrupler contains two half-wave voltage doublers. The circuit made up of
, and
charges
to a value of
the load.
to a value of
. The circuit made up of
. The combined charge of
is applied to
,
,
, and
,
,
charges
(the filter capacitor) and
A voltage quadrupler
Voltage multipliers reduce source current by roughly the same factor that they increase source
voltage. For example, a voltage tripler produces a dc output voltage that is approximately three
times the peak source voltage. At the same time, its maximum output current is roughly one-third
the value of the source current. As such, voltage multipliers are commonly used in high-voltage,
low-current applications. They can also be used to produce dual-polarity output voltages in
power supply applications.
Varactor diodes
The varactor diode requires the reverse bias to be applied across the diode in a way that does not
affect the operation of the tuned circuit of which it is part. Care must be taken to isolate the bias
voltage from the tuning circuit so that the RF performance is not impaired.
Typical circuit using a varactor diode for tuning
Typically the cathode is earthed or run at the DC common potential. The other end can then have
the bias potential applied. The bias circuitry needs to be isolated for RF signals from the tuned
circuit to prevent any degradation of the performance. Either a resistor or an inductor can be used
for this as the diodes operate under reverse bias and present a high DC resistance.
Applying varactor tuning voltage via resistor and inductor
Inductors can operate well under some situations as they provide a low resistance path for the
bias. However they can introduce spurious inductance and under some circumstances they may
cause spurious oscillations to occur when used in an oscillator. Resistors may also be used. The
resistance must be high enough to isolate the bias circuitry from the tuned circuit without
lowering the Q. They must also be low enough to control the bias on the diode against the effects
of the RF passing through the diode. A value of 10 kohms is often a good starting point.
The varactor diodes may be driven in either a single or back to back configuration. The single
varactor configuration has the advantage of simplicity. The back-to-back configuration
overcomes the problem of the RF modulating the tuning voltage as the effect is cancelled out - as
the RF voltage rises, the capacitance on one diode will increase and the other decrease. The
back-to-back configuration also halves the capacitance of the single diode as the capacitances
from the two diodes are placed in series with each other. It should also be remembered that the
series resistance will be doubled and this will affect the Q.
Varactor back-to-back drive
When designing a circuit using varactor diodes, care must be taken to ensure that the diodes do
not become forward biased. Sometimes, especially when using low levels of reverse bias, the
signal in the RF section of the circuit may be sufficient over some sections of the cycle to
overcome the bias and drive the diode into forward conduction. This leads to the generation of
spurious signals and other nasty unwanted effects.
Tunnel diodes
Tunnel diode schematic symbol
The tunnel diode was found many microwave applications because semiconductor devices of
the day could not reach these frequencies. Although not widely used today, it is still sometimes
mentioned and it is a fascinating device.
The tunnel diode was discovered by a Ph.D. research student named Esaki in 1958 while he was
investigating the properties of heavily doped germanium junctions for use in high speed bipolar
transistors. In the course of his research he produced some heavily doped junctions and as a
result found that they produced an oscillation at microwave frequencies as a result of the
tunneling effect. It was subsequently found that other materials including gallium arsenide also
produced the same effect.
Tunnel diode structure
The tunnel diode is similar to a standard p-n junction in many respects except that the doping
levels are very high. Also the depletion region, the area between the p-type and n-type areas,
where there are no carriers is very narrow. Typically it is in the region of between five to ten
nano-metres - only a few atom widths.
As the depletion region is so narrow this means that if it is to be used for high frequency
operation the diode itself must be made very small to reduce the high level of capacitance
resulting from the very narrow depletion region.
Mode of operation
The characteristic curve for a tunnel diode shows an area of negative resistance. When forward
biased the current in the diode rises at first, but later it can be seen to fall with increasing voltage,
before finally rising again. The reason for this is that there are a number of different components
to forming the overall curve. The main two are the normal diode current across the junction, and
the current arising from the tunneling effect. It is this last component that is of interest in a tunnel
diode.
Tunneling is an effect that is caused by quantum mechanical effects when electrons pass through
a potential barrier. It can be visualized in very basic terms by them "tunneling" through the
barrier.
The tunneling only occurs under certain conditions. This means that it peaks when a certain
voltage is placed across the junction. This results in the current increasing to a point beyond that
which would be expected for a standard pn junction. As the voltage across the diode is increased
the effect reduces and the current through the device falls. This results in a negative resistance
region on the curve of te diode that can be used to provide gain.
Advantages and disadvantages
One of the main reasons for the early success of the tunnel diode was its high speed of operation
and the high frequencies it could handle. This resulted from the fact that while many other
devices are slowed down by the presence of minority carriers, the tunnel diode only uses
majority carriers, i.e. holes in an n-type material and electrons in a p-type material. The minority
carriers slow down the operation of a device and as a result their speed is slower. Also the
tunneling effect is inherently very fast.
The tunnel diode is rarely used these days and this results from its disadvantages. Firstly they
only have a low tunneling current and this means that they are low power devices. While this
may be acceptable for low noise amplifiers, it is a significant drawback when they are sued in
oscillators as further amplification is needed and this can only be undertaken by devices that
have a higher power capability, i.e. not tunnel diodes. The third disadvantage is that they are
problems with the reproducibility of the devices resulting in low yields and therefore higher
production costs.
Applications
Although the tunnel diode appeared promising some years ago, it was soon replaced by other
semiconductor devices like IMPATT diodes for oscillator applications and FETs when used as
an amplifier. Nevertheless the tunnel diode is a useful device for certain applications.
Schottky diodes
The Schottky diode or Schottky Barrier diode is an electronics component that is widely used
for radio frequency (RF) applications as a mixer or detector diode. The Schottky diode is also
used in power applications as a rectifier, again because of its low forward voltage drop leading to
lower levels of power loss compared to ordinary PN junction diodes. Although normally called
the Schottky diode these days, named after Schottky, it is also sometimes referred to as the
surface barrier diode, hot carrier diode or even hot electron diode.
Despite the fact that Schottky barrier diodes have many applications in today's high tech
electronics scene, it is actually one of the oldest semiconductor devices in existence. As a metalsemiconductor devices, its applications can be traced back to before 1900 where crystal
detectors, cat's whisker detectors and the like were all effectively Schottky barrier diodes.
Structure
The Schottky barrier diode can be manufactured in a variety of forms. The most simple is the
point contact diode where a metal wire is pressed against a clean semiconductor surface. This
was how the early Cat's Whisker detectors were made, and they were found to be very unreliable,
requiring frequent repositioning of the wire to ensure satisfactory operation. In fact the diode that
is formed may either be a Schottky barrier diode or a standard PN junction dependent upon the
way in which the wire and semiconductor meet and the resulting forming process.
Point contact Schottky diode
Although point contact diodes were manufactured many years later, these diodes were also
unreliable and they were subsequently replaced by a technique in which metal was vacuum
deposited.
Deposited metal Schottky barrier diode
One of the problems with the simple deposited metal diode is that breakdown effects are noticed
around the edge of the metalised area. This arises from the high electric fields that are present
around the edge of the plate. Leakage effects are also noticed. To overcome these problems a
guard ring of P+ semiconductor fabricated using a diffusion process is used along with an oxide
layer around the edge. In some instances metallic silicides may be used in place of the metal.
Deposited metal and oxide film Schottky diode
There are a number of points of interest from the fabrication process. The most critical element
in the manufacturing process is to ensure a clean surface for an intimate contact of the metal with
the semiconductor surface, and this is achieved chemically. The metal is normally deposited in a
vacuum either by the use of evaporation or sputtering techniques. However in some instances
chemical deposition is gaining some favour, and actual plating has been used although it is not
generally controllable to the degree required.
When silicides are to be used instead of a pure metal contact, this is normally achieved by
depositing the metal and then heat treating to give the silicide. This process has the advantage
that the reaction uses the surface silicon, and the actual junction propagates below the surface,
where the silicon will not have been exposed to any contaminants. A further advantage of the
whole Schottky structure is that it can be fabricated using relatively low temperature techniques,
and does not generally need the high temperature steps needed in impurity diffusion.
Characteristics
The Schottky diode is what is called a majority carrier device. This gives it tremendous
advantages in terms of speed because it does not rely on holes or electrons recombining when
they enter the opposite type of region as in the case of a conventional diode. By making the
devices small the normal RC type time constants can be reduced, making these diodes an order
of magnitude faster than the conventional PN diodes. This factor is the prime reason why they
are so popular in radio frequency applications.
The diode also has a much higher current density than an ordinary PN junction. This means that
forward voltage drops are lower making the diode ideal for use in power rectification
applications.
Its main drawback is found in the level of its reverse current which is relatively high. For many
uses this may not be a problem, but it is a factor which is worth watching when using it in more
exacting applications.
The overall I-V characteristic is shown below. It can be seen that the Schottky diode has the
typical forward semiconductor diode characteristic, but with a much lower turn on voltage. At
high current levels it levels off and is limited by the series resistance or the maximum level of
current injection. In the reverse direction breakdown occurs above a certain level. The
mechanism is similar to the impact ionisation breakdown in a PN junction.
Applications
The Schottky barrier diodes are widely used in the electronics industry finding many uses as
diode rectifier. Its unique properties enable it to be used in a number of applications where other
diodes would not be able to provide the same level of performance. In particular it is used in
areas including:
PIN diodes
PIN diodes (p-i-n diodes) are used in many electronics applications, and in particular PIN diodes
find uses in electronic switching applications. However it found its first applications in 1952 as a
low frequency high power rectifier. The PIN diode is also used in a number of microwave
applications, although it took until around 1960 before its use became more popular in this
application. A further use is as a photo-detector (photodetector or photo-diode) where its
structure is particularly suited to absorbing light.
PIN diode structure
The PIN diode consists of a semiconductor diode with three layers. The usual P and N regions
are present, but between them is a layer of intrinsic material a very low level of doping. This may
be either N-type or P-type, but with a concentration of the order of 13^13 cm^-3 which gives it a
resistivity of the order of one k-ohm cm.
The thickness of the intrinsic layer is normally very narrow, typically ranging from 10 to 200
microns. The outer P and N-type regions are then heavily doped.
There are two ways in which the PIN diode can be realised. One is to fabricate the p-i-n diode in
a planar structure, and the other is to use a mesa structure. When the planar structure is fabricated
an epitaxial film is grown onto the substrate material and the P+ region is introduced either by
diffusion or ion implantation. The mesa structure has layers grown onto the substrate. These
layers have the dopants incorporated. In this way it is possible to control the thickness of the
layers and the level of dopants more accurately and a very thin intrinsic layer can be fabricated if
required. This is ideal for high frequency operation. A further advantage of the mesa structure is
that it provides a reduced level of fringing capacitance and inductance as well as an improved
level of surface breakdown.
PIN diode with a planar construction
PIN diodes are widely made of silicon, and this was the semiconductor material that was used
exclusively until the 1980s when gallium arsenide was introduced.
PIN diode characteristics
The main feature of the PIN diode is the intrinsic layer between the P-type and N-type regions.
This enables it to provide properties such as a high reverse breakdown voltage, and a low level of
capacitance. For microwave applications it offers carrier storage when it is forward biased.
It is found that at low levels of reverse bias the depletion layer become fully depleted. Once fully
depleted the p-i-n diode capacitance is independent of the level of bias because there is little net
charge in the intrinsic layer.
When the PIN diode is forward biased both types of current carrier are injected into the intrinsic
layer where they combine. It is this process that enables the current to flow across the layer.
The particularly useful aspect of the PIN diode occurs when it is used with high frequency
signals, the diode appears as a resistor rather than a non linear device, and it produces no
rectification or distortion. Its resistance is governed by the DC bias applied. In this way it is
possible to use the device as an effective RF switch or variable resistor producing far less
distortion than ordinary PN junction diodes.
Applications
The PIN diode is used in a variety of different applications from low frequencies up to high radio
frequencies. The properties introduced by the intrinsic layer make it suitable for a number of
applications where ordinary PN junction diodes are less suitable.
In the first instance the diode can be used as a power rectifier. Here the intrinsic layer gives it a
high reverse breakdown voltage, and this can be used to good effect in many applications.
Although the p-i-n diode finds many applications in the high voltage arena, it is probably for
radio frequency applications where it is best known. The fact that when it is forward biased, the
diode is linear, behaving like a resistor, can be put to good use in a variety of applications. It can
be used as a variable resistor in a variable attenuator, a function that few other components can
achieve as effectively. The PIN diode can also be used as an RF switch. In the forward direction
it can be biased sufficiently to ensure it has a low resistance to the RF that needs to be passed,
and when a reverse bias is applied it acts as an open circuit, with only a relatively small level of
capacitance.
Another useful application of the PIN diode is for use in RF protection circuits. When used with
RF, the diode normally behaves like a resistor when a small bias is applied. Hover this is only
true for RF levels below a certain level. Above this the resistance drops considerably. Thus it can
be used to protect a sensitive receiver from the effects of a large transmitter if it is placed across
the receiver input.
PIN diode attenuator and switch circuit
Finally the PIN diode finds many applications as a photodiode, although this will be explained
separately.
Summary
The PIN diode is an ideal component to provide electronics switching in many areas of
electronics. It is particularly useful for RF design applications and for providing the switching, or
attenuating element in RF switches and RF attenuators. The PIN diode is able to provide much
higher levels of reliability than RF relays that are often the only other alternative.
Step-recovery diodes
The step recovery diode or SRD is a form of semiconductor diode that can be used as a charge
controlled switch and it has the ability to generate very sharp pulses. In view of its method of
operation, it is also called the "Snap-off" diode, "charge storage" diode or "memory varactor".
The step recovery diode finds a number of applications in microwave radio frequency electronics
as pulse generator or parametric amplifier. It finds uses in a number of different roles including
very short pulse generation, ultra fast waveform generation, comb generation, and high order
frequency multiplication. The step recovery diode is also capable of working at moderate power
levels, and this gives it a distinct advantage over some other radio frequency technologies that
are available.
The step recovery diode, SRD is not as common as many other forms of semiconductor diode,
but it can be very useful in many microwave radio frequency applications.
Step recovery diode structure
The step recovery diode is fabricated with the doping level gradually decreasing as the junction
is approached or as a direct PIN structure. This reduces the switching time because there are
fewer charge carriers in the region of the junction and hence less charge is stored in this region.
This allows the charge stored in this region of the step recovery diode to be released more
rapidly when changing from forward to reverse bias. A further advantage is that the forward
current can also be established more rapidly than in the ordinary junction diode.
Step recovery diode operation
The step recovery diode is used as what is termed a charge controlled switch. When the step
recovery diode is forward biased and charge enters it, the diode appears as a normal diode and it
behaves in much the same way. When diodes switch from forward conduction to reverse cut-off,
a reverse current flows briefly as stored charge is removed. When all the charge is removed it
suddenly turns off or snaps off. It is the abruptness with which the reverse current ceases that
enables the step recovery diode to be used for the generation of microwave pulses and also for
waveform shaping.
To explain this in more detail, under normal forward bias conditions the diode will conduct
normally. Then if it is quickly reverse biased it will initially appear as a low impedance, typically
less than an ohm. Once the charge that is stored in the device is depleted, the impedance will
very abruptly increase to its normal reverse impedance which will be very high. This transition
occurs very quickly, typically well under a nanosecond.
This property allows the step recovery diode to be used in pulse shaping (sharpening) and in
pulse generator circuits. The high harmonic content of the signal produced by any repetitive
waveforms from step recovery diode circuits enables them to be used as comb generators where
a comb of harmonically related frequencies are generated.