Download LED, LCD characteristics, Tunnel diode, Schottky diode

LED, LCD characteristics, Tunnel diode, Schottky diode
1.2.1 LED CHARACTERISTICS
A light-emitting diode (LED) is a semiconductor device that emits visible light
when an electric current passes through it. The light is not particularly bright, but
in most LEDs it is monochromatic, occurring at a single wavelength. The output
from an LED can range from red (at a wavelength of approximately 700
nanometers) to blue-violet (about 400 nanometers). Some LEDs emit infrared (IR)
energy (830 nanometers or longer); such a device is known as an infraredemitting diode (IRED).
An LED or IRED consists of two elements of processed material called P-type
semiconductors and N-type semiconductors. These two elements are placed in
direct contact, forming a region called the P-N junction. In this respect, the LED or
IRED resembles most other diode types, but there are important differences. The
LED or IRED has a transparent package, allowing visible or IR energy to pass
through. Also, the LED or IRED has a large PN-junction area whose shape is
tailored to the application.
Benefits of LEDs, compared with incandescent and fluorescent illuminating
devices, include:
 Low power requirement: Most types can be operated with battery power
supplies.
 High efficiency: Most of the power supplied to an LED or IRED is converted
into radiation in the desired form, with minimal heat production.
 Long life: When properly installed, an LED or IRED can function for decades.
Operation:When the device is forward-biased, electrons cross the pn junction
from the n-type material and recombine with holes in the p-type material. The
free electrons are in the conduction band and at a higher energy than the holes in
the valence band. The difference in energy between the electrons and the holes
corresponds to the energy of visible light. When recombination takes place, the
recombining electrons release energy in the form of photons. The emitted light
tends to be monochromatic (one color) that depends on the band gap (and other
factors). A large exposed surface area on one layer of the semiconductive
material permits the photons to be emitted as visible light. This process, called
electroluminescence, is illustrated in below. Various impurities are added during
the doping process to establish the wavelength of the emitted light. The
wavelength determines the color of visible light. Some LEDs emit photons that are
not part of the visible spectrum but have longer wavelengths and are in the
infrared(IR) portion of the spectrum.
The forward voltage across an LED is considerably greater than for a silicon diode.
Typically, the maximum VF for LEDs is between 1.2 V and 3.2 V, depending on the
material. Reverse breakdown for an LED is much less than for a silicon rectifier
diode (3 V to 10 V is typical). The LED emits light in response to a sufficient
forward current, as shown in Figure. The amount of power output translated into
light is directly proportional to the forward current, as indicated in graph. An
increase in IF corresponds proportionally to an increase in light output. The light
output (both intensity and color) is also dependent on temperature. Light
intensity goes down with higher temperature as indicated in the graph.
LED Semiconductor Materials:
The semiconductor gallium arsenide (GaAs) was used in early LEDs and emits IR
radiation, which is invisible. The first visible red LEDs were produced using gallium
arsenide phosphide (GaAsP) on a GaAs substrate. The efficiency was increased
using a gallium phosphide (GaP) substrate, resulting in brighter red LEDs and also
allowing orange LEDs. Later, GaP was used as the light-emitter to achieve pale
green light. By using a red and a green chip, LEDs were able to produce yellow
light. The first super-bright red, yellow, and green LEDs were produced using
gallium aluminum arsenide phosphide (GaAlAsP). By the early 1990s ultrabright
LEDs using indium gallium aluminum phosphide (InGaAlP) were available in red,
orange, yellow, and green.
Blue LEDs using silicon carbide (SiC) and ultrabright blue LEDs made of gallium
nitride (GaN) became available. High intensity LEDs that produce green and blue
are also made using indium gallium nitride (InGaN). High-intensity white LEDs are
formed using ultrabright blue GaN coated with fluorescent phosphors that absorb
the blue light and reemit it as white light.
Characteristics:
An LED emits light over a specified range of wavelengths as indicated by the
spectral output curves in Figure below. The curves in part (a) represent the light
output versus wavelength for typical visible LEDs, and the curve in part (b) is for a
typical infrared LED. The wavelength is expressed in nanometers (nm). The
normalized output of the visible red LED peaks at 660 nm, the yellow at 590 nm,
green at 540 nm, and blue at 460 nm. The output for the infrared LED peaks at
940 nm.
The graphs in Figure below show typical radiation patterns for small LEDs. LEDs
are directional light sources (unlike filament or fluorescent bulbs). The radiation
pattern is generally perpendicular to the emitting surface; however, it can be
altered by the shape of the emitter surface and by lenses and diffusion films to
favor a specific direction. Directional patterns can be an advantage for certain
applications, such as traffic lights, where the light is intended to be seen only by
certain drivers. Figure(a) shows the pattern for a forward-directed LED such as
used in small panel indicators. Figure(b) shows the pattern for a wider viewing
angle such as found in many super-bright LEDs. A wide variety of patterns are
available from manufacturers; one variation is to design the LED to emit nearly all
the light to the side in two lobes.
Applications:
Typical applications include:
 Indicator lights: These can be two-state (i.e., on/off), bar-graph, or
alphabetic-numeric readouts. Standard LEDs are used for indicator lamps
and readout displays on a wide variety of instruments, ranging from
consumer appliances to scientific apparatus. A common type of display
device using LEDs is the seven-segment display. Combinations of the
segments form the ten decimal digits as illustrated in Figure below. Each
segment in the display is an LED. By forward-biasing selected combinations
of segments, any decimal digit and a decimal point can be formed. Two
types of LED circuit arrangements are the common anode and common
cathode as shown.
 Traffic Signals: LEDs are quickly replacing the traditional incandescent bulbs
in traffic signal applications. Arrays of tiny LEDs form the red, yellow, and
green lights in a traffic light unit. An LED array has three major advantages
over the incandescent bulb: brighter light, longer lifetime (years vs.
months), and less energy consumption (about 90% less). LED traffic lights
are constructed in arrays with lenses that optimize and direct the light
output. Figure below illustrates the concept of a traffic light array using red
LEDs. A relatively low density of LEDs is shown for illustration. The actual
number and spacing of the LEDs in a traffic light unit depends on the
diameter of the unit, the type of lens, the color, and the required light
intensity. With an appropriate LED density and a lens, an 8- or 12inch traffic
light will appear essentially as a solid-color circle. LEDs in an array are
usually connected either in a series-parallel or a parallel arrangement. A
series connection is not practical because if one LED fails open, then all the
LEDs are disabled. For a parallel connection, each LED requires a limiting
resistor. To reduce the number of limiting resistors, a series-parallel
connection can be used, as shown in Figure .
 LCD panel backlighting: Specialized white LEDs are used in flat-panel
computer displays.
 Fiber optic data transmission: Ease of modulation allows wide
communications bandwidth with minimal noise, resulting in high speed and
accuracy.
 Remote control: Most home-entertainment "remotes" use IREDs to
transmit data to the main unit.
1.2.2 LCD Characteristics
LCD (liquid crystal display) is the technology used for displays in notebook and
other smaller computers. Like light-emitting diode (LED) and gas-plasma
technologies, LCDs allow displays to be much thinner than cathode ray tube (CRT)
technology. LCDs consume much less power than LED and gas-display displays
because they work on the principle of blocking light rather than emitting it.
An LCD is made with either a passive matrix or an active matrix display display
grid. The active matrix LCD is also known as a thin film transistor (TFT) display. The
passive matrix LCD has a grid of conductors with pixels located at each
intersection in the grid. A current is sent across two conductors on the grid to
control the light for any pixel. An active matrix has a transistor located at each
pixel intersection, requiring less current to control the luminance of a pixel. For
this reason, the current in an active matrix display can be switched on and off
more frequently, improving the screen refresh time (your mouse will appear to
move more smoothly across the screen, for example).
Some passive matrix LCD's have dual scanning, meaning that they scan the grid
twice with current in the same time that it took for one scan in the original
technology. However, active matrix is still a superior technology.
Construction:
Simple facts that should be considered while making an LCD:
 The basic structure of LCD should be controlled by changing the applied
current.
 We must use a polarized light.
 Liquid crystal should able be to control both of the operation to transmit or
can also able to change the polarized light.
As mentioned above that we need to take two polarized glass pieces filter in the
making of the liquid crystal. The glass which does not have a polarized film on the
surface of it must be rubbed with a special polymer which will create microscopic
grooves on the surface of the polarized glass filter. The grooves must be in the
same direction of the polarized film. Now we have to add a coating of pneumatic
liquid phase crystal on one of the polarized filter of the polarized glass. The
microscopic channel cause the first layer molecule to align with filter orientation.
When the right angle appears at the first layer piece, we should add a second
piece of glass with the polarized film. The first filter will be naturally polarized as
the light strikes it at the starting stage.
Thus the light travels through each layer and guided on the next with the help of
molecule. The molecule tends to change its plane of vibration of the light in order
to match their angle. When the light reaches to the far end of the liquid crystal
substance, it vibrates at the same angle as that of the final layer of the molecule
vibrates. The light is allowed to enter into the device only if the second layer of
the polarized glass matches with the final layer of the molecule.
Operation:
Liquid crystal displays (LCDs) are a passive display technology. This means they do
not emit light; instead, they use the ambient light in the environment. By
manipulating this light, they display images using very little power. This has made
LCDs the preferred technology whenever low power consumption and compact
size are critical. Liquid crystal (LC) is an organic substance that has both a liquid
form and a crystal molecular structure. In this liquid, the rod-shaped molecules
are normally in a parallel array, and an electric field can be used to control the
molecules. Most LCDs today use a type of liquid crystal called Twisted Nematic
(TN).
A Liquid Crystal Display (LCD) consists of two substrates that form a "flat bottle"
that contains the liquid crystal mixture. The inside surfaces of the bottle or cell
are coated with a polymer that is buffed to align the molecules of liquid crystal.
The liquid crystal molecules align on the surfaces in the direction of the buffing.
For Twisted Nematic devices, the two surfaces are buffed orthogonal to one
another, forming a 90 degree twist from one surface to the other, see figure
below.
This helical structure has the ability to control light. A polarizer is applied to the
front and an analyzer/reflector is applied to the back of the cell. When randomly
polarized light passes through the front polarizer it becomes linearly polarized. It
then passes through the front glass and is rotated by the liquid crystal molecules
and passes through the rear glass. If the analyzer is rotated 90 degrees to the
polarizer, the light will pass through the analyzer and be reflected back through
the cell. The observer will see the background of the display, which in this case is
the silver gray of the reflector.
The LCD glass has transparent electrical conductors plated onto each side of the
glass in contact with the liquid crystal fluid and they are used as electrodes. These
electrodes are made of Indium-Tin Oxide (ITO). When an appropriate drive signal
is applied to the cell electrodes, an electric field is set up across the cell. The liquid
crystal molecules will rotate in the direction of the electric field. The incoming
linearly polarized light passes through the cell unaffected and is absorbed by the
rear analyzer. The observer sees a black character on a sliver gray background,
see figure below. When the electric field is turned off, the molecules relax back to
their 90 degree twist structure. This is referred to as a positive image, reflective
viewing mode. Carrying this basic technology further, an LCD having multiple
selectable electrodes and selectively applying voltage to the electrodes, a variety
of patterns can be achieved.
Many advances in TN LCDs have been produced. Super Twisted Nematic (STN)
Liquid Crystal material offers a higher twist angle (>=200° vs. 90°) that provides
higher contrast and a better viewing angle. However, one negative feature is the
birefringence effect, which shifts the background color to yellow-green and the
character color to blue. This background color can be changed to a gray by using a
special filter. The most recent advance has been the introduction of Film
compensated Super Twisted Nematic (FSTN) displays. This adds a retardation film
to the STN display that compensates for the color added by the birefringence
effect. This allows a black and white display to be produced.
Advantages:
 LCD’s consumes less amount of power compared to CRT and LED
 LCD’s are consist of some microwatts for display in comparison to some mill
watts for LED’s
 LCDs are of low cost
 Provides excellent contrast
 LCD’s are thinner and lighter when compared to cathode ray tube and LED
Disadvantages:





Require additional light sources
Range of temperature is limited for operation
Low reliability
Speed is very low
LCD’s need an AC drive
Applications:
Liquid crystal technology has major applications in the field of science and
engineering as well on electronic devices.
 Liquid crystal thermometer
 Optical imaging
 The liquid crystal display technique is also applicable in visualization of the
radio frequency waves in the waveguide
 Used in the medical applications
1.2.3 TUNNEL DIODE
Tunnel diode is a highly doped semiconductor device and is used mainly for low
voltage high frequency switching applications. It works on the principle of
Tunneling effect. 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. Then in 1973, Esaki
received the Nobel prize for Physics for his work on the tunnel diode.
After the work by Esaki, other researchers demonstrated that other materials also
showed the tunnelling effect. Holonyak and Lesk demonstrated a Gallium
Arsenide device in 1960, and others demonstrated Indium tin, and then in 1962
the effect was demonstrated in materials including Indium Arsenide, Indium
Phosphide and also Silicon.
The circuit symbol of Tunnel diode is shown below. The tunnel diode is a two
terminal device with p type semiconductor acting as anode and n type
semiconductor as cathode.
Construction:
The tunnel diode is similar to a standard p-n junction in many respects except that
the doping levels are very high. Densities of the order of 5x10^19 cm^-3 are
common.
The main difference is that the depletion region is the area between the p-type
and n-type areas. 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.
In terms of the material used for these diodes, the favoured semiconductor is
germanium. Tunnel diodes are usually fabricated from germanium, gallium or
gallium arsenide. These all have small forbidden energy gaps and high ion
motilities. Silicon is not used in the fabrication of tunnel diodes due to low (Ip,I
v)value.
A small tin dot is soldered or alloyed to a heavily doped pellet of n-type Ge, GaSb
or GaAs. The pellet is then soldered to anode which is also used for heat
dissipation. The cathode contact is connected to the tin dot via a mesh screen
used to reduce inductance. The diode has a ceramic body and a hermetically
sealing lid on top.
Tunnel diode structures generally fall into one of three basic structures:
 Ball alloy: This type of tunnel diode format is fabricated as a mesa
structure. To achieve this form of structure, the fabrication technique
involves bringing an alloy containing the required dopants into contact with
a heavily doped substrate. The temperature used is around 500°C at which
point the dopants quickly melt and diffuse into the substrate. The overall
structure geometry is then defined by etching the diode to the required
proportions.
 Pulsed bond: This is a relatively straightforward structure to create,
although careful process control is required during the fabrication process.
The diode is created by using a wire coated with an alloy containing the
required dopants. This is pressed hard onto the heavily doped substrate,
and then a voltage pulse is applied. The effect of this is that the junction
forms by a process of local alloying.
Despite this, there are drawbacks to this process because it can only produce a
small junction, and the exact properties, including the area of the junction cannot
be controlled tightly.
 Planar structure: Planar technology can be used to create the diode. Using
this approach for the fabrication process, the heavily doped n+ substrate is
masked off by an insulating layer to leave a small area exposed. This
exposed area is then open to become the active area of the diode.
Tunnel diode planar structure
The doping for the area can be introduced by one of a number of means. It can be
introduced by diffusion, alloying or epitaxial growth. Alternatively it is possible to
grow an epitaxial layer over the whole surface and then etch away those areas
that are not required to leave a mesa structure.
All three structures enable high performance diodes to be obtained. Although
these are three popular structures for tunnel diodes, new developments are
occurring using different materials and also involving new structures that offer a
greater variety of characteristics, or they may be tailored to the needs of a
particular material that may be used.
Operation:
According to the classical laws of physics a charged particle in order to cross an
energy barrier should possess energy at least equal to the energy barrier. Hence
the particle will cross the energy barrier if its energy is greater than the barrier
and cannot cross the barrier if its energy is less than the energy barrier. But
quantum mechanically there exists non zero probability that the particle with
energy less than the energy barrier will cross the barrier as if it tunnels across the
barrier. This is called as Tunneling effect. The probability increases with the
decreasing barrier energy.
P α exp (-A*Eb *W)
Where P is the probability that the particle crosses the barrier,
Eb is the barrier energy , W is the barrier width.
In normal PN junction diodes the doping levels will be of the order 1 dopant atom
in 108 atoms of Si (or) Ge. If the doping levels are increased to 1 in 103, the
depletion layer width is of the order of 10 nm. In such a PN junction tunneling
effect is significant, such PN junction devices are called Tunnel diodes.
Current components in a tunnel diode:
The total current in a tunnel diode is given as It = Itun + Idiode + Iexcess. The diode
current in a tunnel diode is same as that for normal PN junction diode which is
given as
Idiode = Ido*(exp (V/ (η *Vt))-1)
Where Ido is reverse saturation current and will be very high in tunnel diode
compared to PN junction diode due to high doping concentrations,
V is voltage applied across diode, Vt is the voltage equivalent of temperature,
η is correction factor 1 for Ge and 2 for Si, For Voltages less than the cut in voltage
of the diode this current is negligible.The excess current is an additional current
due to parasitic tunneling via impurities which determines the valley point. The
tunneling current in given as
Itun = (V/Ro)*exp (-(V/Vo)m)
Where m= 1 to 3 and VO = 0.1 to 0.5 volts, RO is the tunnel diode resistance.
Peak voltage, Peak current of tunnel diode:
The peak voltage of a tunnel diode can be found as follows: At peak voltage
current through the tunnel diode is maximum. Typically the peak voltage will be
less than cut in voltage of tunnel diode and hence the diode current and excess
current can be considered negligible.
Hence for maximum or minimum diode current
V=Vpeak, dItun/dV = 0 which is
Vpeak = ((1/m)(1/m))*Vo , Ipeak = ((1/m)(1/m))*Vo*exp(-1/m).
Characteristics:
For small forward voltages owing to high carrier concentrations in tunnel diode
and due to tunneling effect the forward resistance will be very small. As voltage
increase she current also increases till the current reaches Peak current. If the
voltage applied to tunnel diode is increased beyond the peak voltage the current
will start decreasing. This is negative resistance region. It prevails till valley point.
At valley point the current through the diode will be minimum. Beyond valley
point the tunnel diode acts as normal diode. In reverse biased condition also
Tunnel diode is an excellent conductor due to its high doping concentrations.
Tunnel diodes are made from Germanium or gallium arsenide due to their highest
peak voltage to valley point swing. The ratio of high peak current to valley current
quantifies the maximum voltage swing allowed in negative resistance region.
The IV characteristics of the tunnel diode is shown below
Advantages :
 High speed of operation due to the fact that the tunnelling takes place at
the speed of light.
 Low cost
 Low noise
 Environmental immunity
 Low power dissipation
 Simplicity in fabrication
 Longevity
Disadvantages :
 Low output voltage swing
 Because it is a two terminal device, there is no isolation between input and
output.
Applications :
Some of the applications of Tunnel diode are

Tunnel diodes are used as very high speed switches

Used as high frequency micro wave oscillator
1.2.4 SCOTTKY DIODE
A Schottky barrier diode is a metal semiconductor junction formed by bringing
metal in contact with a moderately doped n type semiconductor material. A
Schottky barrier diode is also called as known as Schottky or hot carrier diode. It is
named after its inventor Walter H. Schottky, barrier stands for the potential
energy barrier for electrons at the junction. It is a unilateral device conducting
currents in one direction (Conventional current flow from metal to
semiconductor) and restricting in the other.
A Schottky barrier diode is a two terminal device with metal terminal acting as
anode and semiconductor terminal acting as anode. The circuit symbol of
Schottky barrier diode is shown in the figure.
Construction :
 It is a metal semiconductor junction diode without depletion layer.
 On one side of junction a metal like gold, silicon, platinum is used and other
side N type doped semiconductor is used.
 For protection purpose metal layer is surrounded by gold or silver layer.
 The metal film forms the positive electrode and semiconductor is the
cathode
A Schottky barrier diode is shown in the figure below
Operation:
 Operation is due to the fact that the electrons in different material have
different potential energy.
 N type semiconductors have higher potential energy as compare to
electrons of metals.
 When these two are brought together in contact, there a flow of electron in
both direction across the metal-semiconductor interface when contact is
first made.
 A voltage is applied to the schottky diode such that the metal is positive
with respect to semiconductor.
 The voltage will oppose the built in potential and makes it easier to current
flow.
Current components in Schottky diode:
In a Schottky barrier diode current conduction is through majority carriers which
are electrons in N type semiconductor. The current in Schottky barrier diode
current is
IT = IDiffusion+ITunneling+IThermionic emission
Where IDiffusion is diffusion current due to concentration gradient and Diffusion
current density Jn= Dn*q* dn/dx for electrons, where Dn is the diffusion constant
for electrons, q is electronic charge = 1.6*10-19 Coulombs, dn/dx is the
concentration gradient for electrons.
ITunneling is the tunneling current due to quantum mechanical tunneling through the
barrier. This current is directly proportional to the probability of tunneling. The
probability of tunneling increases with the decreasing barrier or built in potential
and decreasing depletion layer width.
IThermionic emission is current due to thermionic emission. Due to thermal agitation
some of carriers which have energy equal to or larger than the conduction band
energy at the metal-semiconductor interface, contribute to the current flow. This
is termed as thermionic emission current.
It is suitable for high-speed switching applications because the forward voltage is
low and the reverse recovery time is short. The reverse recovery time is solely
dependent on junction capacitance which will be of the order of Pico farads.
Characteristics:
The VI characteristics of Schottky barrier diode is shown below
From the VI characteristics it is obvious that the VI characteristics of Schottky
barrier diode is similar to normal PN junction diode with the following exceptions
The forward voltage drop of Schottky barrier diode is low compared to normal PN
junction diode. The forward voltage drop of Schottky barrier diode made of silicon
exhibits a forward voltage drop of 0.3 volts to o.5 volts.
The forward voltage drop increases with the increasing doping concentration of n
type semiconductor.
The VI characteristics of Schottky barrier diode is Steeper compared to VI
characteristics of normal PN junction diode due to high concentration of current
carrier.
Advantages:
Schottky diodes are used in many applications where other types of diode will not
perform as well. They offer a number of advantages:
 Low turn on voltage: The turn on voltage for the diode is between 0.2 and
0.3 volts for a silicon diode against 0.6 to 0.7 volts for a standard silicon
diode. This makes it have very much the same turn on voltage as a
germanium diode.
 Fast recovery time: The fast recovery time because of the small amount of
stored charge means that it can be used for high speed switching
applications.
 Low junction capacitance: In view of the very small active area, often as a
result of using a wire point contact onto the silicon, the capacitance levels
are very small.
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:
 RF mixer and detector diode: The Schottky diode has come into its own
for radio frequency applications because of its high switching speed and
high frequency capability. In view of this Schottky barrier diodes are used in
many high performance diode ring mixers. In addition to this their low turn
on voltage and high frequency capability and low capacitance make them
ideal as RF detectors.
 Power rectifier: Schottky barrier diodes are also used in high power
applications, as rectifiers. Their high current density and low forward
voltage drop mean that less power is wasted than if ordinary PN junction
diodes were used. This increase in efficiency means that less heat has to be
dissipated, and smaller heat sinks may be able to be incorporated in the
design.
 Power OR circuits: Schottky diodes can be used in applications where a
load is driven by two separate power supplies. One example may be a
mains power supply and a battery supply. In these instances it is necessary
that the power from one supply does not enter the other. This can be
achieved using diodes. However it is important that any voltage drop across
the diodes is minimised to ensure maximum efficiency. As in many other
applications, this diode is ideal for this in view of its low forward voltage
drop.
Schottky diodes tend to have a high reverse leakage current. This can lead
to problems with any sensing circuits that may be in use. Leakage paths
into high impedance circuits can give rise to false readings. This must
therefore be accommodated in the circuit design.
 Solar cell applications: Solar cells are typically connected to rechargeable
batteries, often lead acid batteries because power may be required 24
hours a day and the Sun is not always available. Solar cells do not like the
reverse charge applied and therefore a diode is required in series with the
solar cells. Any voltage drop will result in a reduction in efficiency and
therefore a low voltage drop diode is needed. As in other applications, the
low voltage drop of the Schottky diode is particularly useful, and as a result
they are the favoured form of diode in this application.
 Clamp diode - especially with its use in LS TTL: Schottky barrier diodes
may also be used as a clamp diode in a transistor circuit to speed the
operation when used as a switch. They were used in this role in the 74LS
(low power Schottky) and 74S (Schottky) families of logic circuits. In these
chips the diodes are inserted between the collector and base of the driver
transistor to act as a clamp. To produce a low or logic "0" output the
transistor is driven hard on, and in this situation the base collector junction
in the diode is forward biased. When the Schottky diode is present this
takes most of the current and allows the turn off time of the transistor to
be greatly reduced, thereby improving the speed of the circuit.
An NPN transistor with Schottky diode clamp
In view of its properties, the Schottky diode finds uses in applications right
through from power rectification to uses in clamp diodes in high speed logic
devices and then on to high frequency RF applications as signal rectifiers and in
mixers.Their properties span many different types of circuit making them almost
unique in the variety of areas and circuits in which they can be used.