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TYPES OF DIODES
VARICAP (VARACTOR diode)
In electronics, a Vari-cap diode, varactor diode, variable
capacitance diode, variable reactance diode or tuning diode is a
type of diode which has a variable capacitance that is a function
of the voltage impressed on its terminals.
Applications
Varactor diodes are used as voltage-controlled capacitors. They
are commonly used in parametric amplifiers, parametric oscillators
and voltage-controlled oscillators as part of phase-locked loops
and frequency synthesizers. For example, varactor diodes are used
in the tuners of television sets to electronically tune the
receiver to different stations.
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Operation of a vari-cap
Internal structure of a vari-cap
Varactors are operated reverse-biased so no current flows, but
since the thickness of the depletion zone varies with the applied
bias voltage, the capacitance of the diode can be made to vary.
Generally, the depletion region thickness is proportional to the
square root of the applied voltage; and capacitance is inversely
proportional to the depletion region thickness. Thus, the
capacitance is inversely proportional to the square root of applied
voltage.
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All diodes exhibit this phenomenon to some degree, but specially
made varactor diodes exploit the effect to boost the capacitance
and variability range achieved - most diode fabrication attempts to
achieve the opposite.
In the figure we can see an example of a cross-section of a
varactor with the depletion layer formed of a p-n-junction. But the
depletion layer can also be made of a MOS-diode or a Schottky
diode. This is very important in CMOS and MMIC technology.
Tuning circuits
Generally the use of a vari-cap diode in a circuit requires
connecting it to a tuned circuit, usually in parallel with any
existing capacitance or inductance. Because a D.C. voltage must be
applied reverse bias across the vari-cap to alter its capacitance,
this must be blocked from entering the tuned circuit. This is
accomplished by placing a D.C. blocking capacitor with a
capacitance about 100 times greater than the maximum capacitance of
the vari-cap diode in series with it and applying the D.C. from a
high impedance source to the node between the vari-cap cathode and
the blocking capacitor as shown in the upper left hand diagram,
right.
Since no current flows in the vari-cap, the value of the
resistor connecting its cathode back to the D.C. control voltage
can somewhere in the range of 22 to 150K Ohms and the blocking
capacitor somewhere in the range of 5-100nF. Sometimes, with very
high Q tuned circuits an inductor is placed in series with the
resistor to increase the source impedance of the control voltage so
as not to load the tuned circuit and decrease its Q.
A second circuit, lower left in the image, using two
back-to-back, (cathode to cathode) series connected vari-cap diodes
is another common configuration. Effectively the second vari-cap
replaces the blocking capacitor in the first circuit. This reduces
the overall capacitance by half and the change in capacitance to
half also, but possesses the advantage of reducing the A.C.
component of voltage across each device and symmetrical distortion
should the A.C. component possess enough amplitude to bias the
vari-caps into forward conduction.
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When designing tuning circuits with vari-caps it is usually good
practice to maintain the A.C. component of voltage across the
vari-cap at a minimal level, usually less than 100mV peak to peak,
to prevent this changing the capacitance of the diode too much and
thus distorting the signal and adding harmonics to it.
One remaining circuit, right, depicts two series connected
vari-caps being used in a circuit with separate D.C. and A.C.
signal 'ground' connections. The DC ground is being depicted as the
traditional 'ground' symbol, and the A.C. ground being depicted as
a triangle. Separation of 'grounds' is often done to prevent high
frequency radiation from the low frequency ground node or D.C.
currents in the A.C. ground node upsetting biasing and operating
points of active devices such as vari-caps and transistors.
These circuit configurations are quite common in television
tuners and electronically tuned broadcast A.M. and F.M. receivers
as well as other communications equipment and industrial equipment.
Early vari-cap diodes usually required a reverse voltage range from
0-33v to obtain maximum change in capacitance which was quite
small, from 1-10pF or so. These types were and are still
extensively used in television tuners where only small capacitance
changes are required at the carrier high frequencies of more than
50 MHz used by television. In time vari-cap diodes were developed
which exhibited very large changes in capacitance, 100-500pF, with
relatively small changes in reverse bias of 0-12 or 0-5v. These
types allowed electronically tuned A.M. broadcast receivers to be
realized as well as a multitude of other functions requiring large
capacitance changes at lower frequencies; generally below 10 MHz
some of designs of electronic security tag readers used in retail
outlets require these high capacitance vari-caps in their
voltage-controlled oscillators.
The three leaded devices depicted at the top of the page are
generally two common cathode connected vari-caps in a single
package. In the consumer A.M.-F.M. tuner depicted at the right, a
single dual package vari-cap diode adjusts both the pass band of
the tank circuit, (the main station selector) and the local
oscillator with a single vari-cap for each. This is done to keep
costs down, two dual packages could have been used, and one for the
tank and one for the oscillator, four diodes in all, and this was
what were depicted in the application data for the LA1851N A.M.
radio chip. Two lower capacitance dual varactors are used in the
F.M. section which operates at a frequency about one hundred times
greater and are highlighted by red arrows. In this case four diodes
are used, one dual package each for the tank/band pass filter and
the local oscillator.
Switching
Special types of vari-cap diode exhibiting an abrupt change in
capacitance can often be found in consumer equipment such as
television tuners, which are used to switch radio frequency signal
paths. When in the high capacitance state, usually with low or no
bias, they present a low impedance path to R.F., whereas when
reverse biased their capacitance abruptly decreases and their R.F.
impedance increases. Although they are still slightly conductive to
the R.F. path, the attenuation they introduce decreases the
unwanted signal to an acceptably low level. They are often used in
pairs to switch between two different R.F. sources such as the
V.H.F. and U.H.F. bands in a television tuner by supplying them
with complimentary bias voltages. The fourth device from the left
in the picture at the head of this page is one such device.
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LED
A light-emitting diode (LED) is a semiconductor light source.
LEDs are used as indicator lamps in many devices and are
increasingly used for other lighting. Appearing as practical
electronic components in 1962, early LEDs emitted low-intensity red
light, but modern versions are available across the visible,
ultraviolet, and infrared wavelengths, with very high
brightness.
When a light-emitting diode is forward-biased (switched on),
electrons are able to recombine with electron holes within the
device, releasing energy in the form of photons. This effect is
called electroluminescence and the color of the light
(corresponding to the energy of the photon) is determined by the
energy gap of the semiconductor. An LED is often small in area
(less than 1 mm2), and integrated optical components may be used to
shape its radiation pattern. LEDs present many advantages over
incandescent light sources including lower energy consumption,
longer lifetime, improved physical robustness, smaller size, and
faster switching. LEDs powerful enough for room lighting are
relatively expensive and require more precise current and heat
management than compact fluorescent lamp sources of comparable
output.
Light-emitting diodes are used in applications as diverse as
aviation lighting, automotive lighting, advertising, general
lighting, and traffic signals. LEDs have allowed new text, video
displays, and sensors to be developed, while their high switching
rates are also useful in advanced communications technology.
Infrared LEDs are also used in the remote control units of many
commercial products including televisions, DVD players, and other
domestic appliances.
Technology
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Inner workings of LED
I-V diagram for a diode
An LED will begin to emit light when the on-voltage is exceeded.
Typical on voltages are 23 volts.
Physics
The LED consists of a chip of semiconducting material doped with
impurities to create a p-n junction. As in other diodes, current
flows easily from the p-side, or anode, to the n-side, or cathode,
but not in the reverse direction. Charge-carriers electrons and
holes flow into the junction from electrodes with different
voltages. When an electron meets a hole, it falls into a lower
energy level, and releases energy in the form of a photon.
The wavelength of the light emitted, and thus its color depends
on the band gap energy of the materials forming the p-n junction.
In silicon or germanium diodes, the electrons and holes recombine
by a non-radiative transition, which produces no optical emission,
because these are
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indirect band gap materials. The materials used for the LED have
a direct band gap with energies corresponding to near-infrared,
visible, or near-ultraviolet light.
LED development began with infrared and red devices made with
gallium arsenide. Advances in materials science have enabled making
devices with ever-shorter wavelengths, emitting light in a variety
of colors.
LEDs are usually built on an n-type substrate, with an electrode
attached to the p-type layer deposited on its surface. P-type
substrates, while less common, occur as well. Many commercial LEDs,
especially GaN/InGaN, also use sapphire substrate.
Most materials used for LED production have very high refractive
indices. This means that much light will be reflected back into the
material at the material/air surface interface. Thus, light
extraction in LEDs is an important aspect of LED production,
subject to much research and development.
Colors and materials
Following table shows the available colors with wavelength
range, voltage drop and material:
Color Wavelength [nm] Voltage drop [V] Semiconductor
material
Infrared > 760 V < 1.63 Gallium arsenide (GaAs) Aluminium
gallium arsenide (AlGaAs)
Red 610 < < 760 1.63 < V < 2.03
Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide
(GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III)
phosphide (GaP)
Orange 590 < < 610 2.03 < V < 2.10
Gallium arsenide phosphide (GaAsP) Aluminium gallium indium
phosphide (AlGaInP) Gallium(III) phosphide (GaP)
Yellow 570 < < 590 2.10 < V < 2.18 Gallium arsenide
phosphide (GaAsP) Aluminium gallium indium phosphide
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(AlGaInP) Gallium(III) phosphide (GaP)
Green 500 < < 570 1.9[56] < V < 4.0
Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide
(AlGaInP) Aluminium gallium phosphide (AlGaP)
Blue 450 < < 500 2.48 < V < 3.7
Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon
carbide (SiC) as substrate Silicon (Si) as substrate under
development
Violet 400 < < 450 2.76 < V < 4.0 Indium gallium
nitride (InGaN)
Purple multiple types 2.48 < V < 3.7
Dual blue/red LEDs, blue with red phosphor, or white with purple
plastic
Ultraviolet < 400 3.1 < V < 4.4
Diamond (235 nm)[57] Boron nitride (215 nm)[58][59] Aluminium
nitride (AlN) (210 nm)[60] Aluminium gallium nitride (AlGaN)
Aluminium gallium indium nitride (AlGaInN) down to 210 nm[61]
Pink multiple types V ~ 3.3[62]
Blue with one or two phosphor layers: yellow with red, orange or
pink phosphor added afterwards,
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or white with pink pigment or dye.[63]
White Broad spectrum V = 3.5 Blue/UV diode with yellow
phosphor
LEDs are produced in a variety of shapes and sizes. The color of
the plastic lens is often the same as the actual color of light
emitted, but not always. For instance, purple plastic is often used
for infrared LEDs, and most blue devices have clear housings.
Modern high power LEDs such as those used for lighting and
backlighting are generally found in surface-mount technology (SMT)
packages, (not shown).
Application-specific variations
Flashing LEDs are used as attention seeking indicators without
requiring external electronics. Flashing LEDs resemble standard
LEDs but they contain an integrated multivibrator circuit that
causes the LED to flash with a typical period of one second. In
diffused lens LEDs this is visible as a small black dot. Most
flashing LEDs emit light of one color, but more sophisticated
devices can flash between multiple colors and even fade through a
color sequence using RGB color mixing.
Bi-color LEDs are two different LED emitters in one case. There
are two types of these. One type consists of two dies connected to
the same two leads anti parallel to each other. Current flow in one
direction emits one color, and current in the opposite direction
emits the other color. The other type consists of two dies with
separate leads for both dies and another lead for common anode or
cathode, so that they can be controlled independently.
Tri-color LEDs are three different LED emitters in one case.
Each emitter is connected to a separate lead so they can be
controlled independently. A four-lead arrangement is typical with
one common lead (anode or cathode) and an additional lead for each
color.
RGB LEDs are Tri-color LEDs with red, green, and blue emitters,
in general using a four-wire connection with one common lead (anode
or cathode). These LEDs can have either
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common positive or common negative leads. Others however, have
only two leads (positive and negative) and have a built in tiny
electronic control unit.
Calculator LED display, 1970s
Alphanumeric LED displays are available in seven-segment and
starburst format. Seven-segment displays handle all numbers and a
limited set of letters. Starburst displays can display all letters.
Seven-segment LED displays were in widespread use in the 1970s and
1980s, but rising use of liquid crystal displays, with their lower
power needs and greater display flexibility, has reduced the
popularity of numeric and alphanumeric LED displays.
Advantages
Efficiency: LEDs emit more light per watt than incandescent
light bulbs. Their efficiency is not affected by shape and size,
unlike fluorescent light bulbs or tubes.
Color: LEDs can emit light of an intended color without using
any color filters as traditional lighting methods need. This is
more efficient and can lower initial costs.
Size: LEDs can be very small (smaller than 2 mm2) and are easily
populated onto printed circuit boards.
On/Off time: LEDs light up very quickly. A typical red indicator
LED will achieve full brightness in under a microsecond. LEDs used
in communications devices can have even faster response times.
Cycling: LEDs are ideal for uses subject to frequent on-off
cycling, unlike fluorescent lamps that fail faster when cycled
often, or HID lamps that require a long time before restarting.
Dimming: LEDs can very easily be dimmed either by pulse-width
modulation or lowering the forward current.
Cool light: In contrast to most light sources, LEDs radiate very
little heat in the form of IR that can cause damage to sensitive
objects or fabrics. Wasted energy is dispersed as heat through the
base of the LED.
Slow failure: LEDs mostly fail by dimming over time, rather than
the abrupt failure of incandescent bulbs.
Lifetime: LEDs can have a relatively long useful life. One
report estimates 35,000 to 50,000 hours of useful life, though time
to complete failure may be longer. Fluorescent tubes typically are
rated at about 10,000 to 15,000 hours, depending partly on the
conditions of use, and incandescent light bulbs at 1,000 to 2,000
hours. Several DOE demonstrations have shown that reduced
maintenance costs from this extended lifetime, rather than energy
savings, is the primary factor in determining the payback period
for an LED product.
Shock resistance: LEDs, being solid-state components, are
difficult to damage with external shock, unlike fluorescent and
incandescent bulbs, which are fragile.
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Focus: The solid package of the LED can be designed to focus its
light. Incandescent and fluorescent sources often require an
external reflector to collect light and direct it in a usable
manner.
Disadvantages
High initial price: LEDs are currently more expensive, price per
lumen, on an initial capital cost basis, than most conventional
lighting technologies. As of 2010, the cost per thousand lumens
(kilolumen) was about $18. The price is expected to reach
$2/kilolumen by 2015. The additional expense partially stems from
the relatively low lumen output and the drive circuitry and power
supplies needed.
Temperature dependence: LED performance largely depends on the
ambient temperature of the operating environment. Over-driving an
LED in high ambient temperatures may result in overheating the LED
package, eventually leading to device failure. An adequate heat
sink is needed to maintain long life. This is especially important
in automotive, medical, and military uses where devices must
operate over a wide range of temperatures, and need low failure
rates.
Voltage sensitivity: LEDs must be supplied with the voltage
above the threshold and a current below the rating. This can
involve series resistors or current-regulated power supplies.
Light quality: Most cool-white LEDs have spectra that differ
significantly from a black body radiator like the sun or an
incandescent light. The spike at 460 nm and dip at 500 nm can cause
the color of objects to be perceived differently under cool-white
LED illumination than sunlight or incandescent sources, due to
metamerism, red surfaces being rendered particularly badly by
typical phosphor-based cool-white LEDs. However, the color
rendering properties of common fluorescent lamps are often inferior
to what is now available in state-of-art white LEDs.
Area light source: Single LEDs do not approximate a point source
of light giving a spherical light distribution, but rather a
lambertian distribution. So LEDs are difficult to apply to uses
needing a spherical light field. LEDs cannot provide divergence
below a few degrees. In contrast, lasers can emit beams with
divergences of 0.2 degrees or less.
Electrical polarity: Unlike incandescent light bulbs, which
illuminate regardless of the electrical polarity, LEDs will only
light with correct electrical polarity.
Blue hazard: There is a concern that blue LEDs and cool-white
LEDs are now capable of exceeding safe limits of the so-called
blue-light hazard as defined in eye safety specifications such as
ANSI/IESNA RP-27.105: Recommended Practice for Photo biological
Safety for Lamp and Lamp Systems.
Blue pollution: Because cool-white LEDs with high color
temperature emit proportionally more blue light than conventional
outdoor light sources such as high-pressure sodium vapor lamps, the
strong wavelength dependence of Rayleigh scattering means that
cool-white LEDs can cause more light pollution than other light
sources. The International Dark-Sky Association discourages using
white light sources with correlated color temperature above 3,000
K.
Droop: The efficiency of LEDs tends to decrease as current
increases.
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Applications
In general, all the LED products can be divided into two major
parts, the public lighting and indoor lighting. LED uses fall into
four major categories:
Visual signals where light goes more or less directly from the
source to the human eye, to convey a message or meaning.
Illumination where light is reflected from objects to give
visual response of these objects. Measuring and interacting with
processes involving no human vision. Narrow band light sensors
where LEDs operate in a reverse-bias mode and respond to
incident light, instead of emitting light.
For more than 70 years, until the LED, practically all lighting
was incandescent and fluorescent with the first fluorescent light
only being commercially available after the 1939 World's Fair.
In electronics, the basic LED circuit is an electric power
circuit used to power a light-emitting diode or LED. The simplest
such circuit consists of a voltage source and two components
connected in series: a current-limiting resistor (sometimes called
the ballast resistor), and an LED. Optionally, a switch may be
introduced to open and close the circuit. The switch may be
replaced with another component or circuit to form a continuity
tester.
(Although simple, this circuit is not necessarily the most
energy efficient circuit to drive an LED, since energy is lost in
the resistor. More complicated circuits may be used to improve
energy efficiency).
The LED used will have a voltage drop, specified at the intended
operating current. Ohm's law and Kirchhoff's circuit laws are used
to calculate the resistor that is used to attain the correct
current. The resistor value is computed by subtracting the LED
voltage drop from the supply voltage, and then dividing by the
desired LED operating current. If the supply voltage is equal to
the LED's voltage drop, no resistor is needed.
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Series resistor
Series resistors are a simple way to stabilize the LED current,
but energy is wasted in the resistor.
Miniature indicator LEDs are normally driven from low voltage DC
via a current limiting resistor. Currents of 2 mA, 10 mA and 20 mA
are common. Sub-mA indicators may be made by driving ultrabright
LEDs at very low current. Efficiency tends to reduce at low
currents, but indicators running on 100 A are still practical. The
cost of ultrabright LEDs is higher than that of 2 mA indicator
LEDs.
In coin cell powered keyring type LED lights, the resistance of
the cell itself is usually the only current limiting device. The
cell should not therefore be replaced with a lower resistance
type.
LEDs can be purchased with built-in series resistors. These can
save printed circuit board space and are especially useful when
building prototypes or populating a PCB in a way other than its
designers intended. However, the resistor value is set at the time
of manufacture, removing one of the key methods of setting the
LED's intensity. Alphanumeric LEDs use the same drive strategy as
indicator LEDs, the only difference being the larger number of
channels, each with its own resistor. Seven-segment and starburst
LED arrays are available in both common-anode and common-cathode
form.
Series resistor calculation
The formula to calculate the correct resistance to use is
Where power supply voltage (Vs) is the voltage of the power
supply, e.g. a 9 volt battery, LED voltage drop (Vf) is the forward
voltage drop across the LED, and LED current (I) is the desired
current of the LED. The above formula requires the current in
amperes, although this value is usually given by the manufacturer
in mA, such as 20 mA.
Typically, a LED forward voltage is about 1.83.3 volts; it
varies by the color of the LED. A red LED typically drops 1.8
volts, but voltage drop normally rises as the light frequency
increases, so a blue LED may drop around 3.3 volts.
The formula can be explained considering the LED as a
resistance, and applying Kirchhoff's voltage law (KVL) (R is the
unknown quantity):
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Polarity
Unlike incandescent light bulbs, which illuminate regardless of
the electrical polarity, LEDs will only light with correct
electrical polarity. When the voltage across the p-n junction is in
the correct direction, a significant current flows and the device
is said to be forward-biased. If the voltage is of the wrong
polarity, the device is said to be reverse biased and very little
current flows, and no light is emitted. LEDs can be operated on an
alternating current voltage, but they will only light with positive
voltage, causing the LED to turn on and off at the frequency of the
AC supply.
Most LEDs have low reverse breakdown voltage ratings, so they
will also be damaged by an applied reverse voltage above this
threshold. The cause of damage is over current resulting from the
diode breakdown, not the voltage itself. LEDs driven directly from
an AC supply of more than the reverse breakdown voltage may be
protected by placing a diode (or another LED) in inverse
parallel.
PHOTODIODE ( P-N-)
A photodiode is a type of photodetector capable of converting
light into either current or voltage, depending upon the mode of
operation. The common, traditional solar cell used to generate
electric solar power is a large area photodiode. It is PN junction
and the junction is coated with one of the photo sensitive material
(CdS, Se, Zns, PbS etc).
Photodiodes are similar to regular semiconductor diodes except
that they may be either exposed (to detect vacuum UV or X-rays) or
packaged with a window or optical fiber connection to allow light
to reach the photo sensitive part of the device. Many diodes
designed for use specifically as a photodiode use a PIN junction
rather than a p-n junction, to increase the speed of response. A
photodiode is designed to operate in reverse bias.
A photodiode is a p-n junction or PIN structure. When a photon
of sufficient energy strikes the diode (at junction), it excites an
electron, thereby creating a free electron (and a positively
charged electron hole). This mechanism is also known as the inner
photoelectric effect. If the absorption occurs in the junction's
depletion region, or one diffusion length away from it, these
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carriers are swept from the junction by the built-in field of
the depletion region. Thus holes move toward the anode, and
electrons toward the cathode, and a photocurrent is produced. This
photocurrent is the sum of both the dark current (without light)
and the light current, so the dark current must be minimized to
enhance the sensitivity of the device.
Photovoltaic mode
When used in zero bias or photovoltaic mode, the flow of
photocurrent out of the device is restricted and a voltage builds
up. This mode exploits the photovoltaic effect, which is the basis
for solar cells a traditional solar cell is just a large area
photodiode.
Photoconductive mode
In this mode the diode is often reverse biased (with the cathode
positive), dramatically reducing the response time at the expense
of increased noise. This increases the width of the depletion
layer, which decreases the junction's capacitance resulting in
faster response times. The reverse bias induces only a small amount
of current (known as saturation or back current) along its
direction while the photocurrent remains virtually the same. For a
given spectral distribution, the photocurrent is linearly
proportional to the illuminance (and to the irradiance).
Although this mode is faster, the photoconductive mode tends to
exhibit more electronic noise. The leakage current of a good PIN
diode is so low (
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Other modes of operation
Avalanche photodiodes have a similar structure to regular
photodiodes, but they are operated with much higher reverse bias.
They are fabricated only with Si. This allows each photo-generated
carrier to be multiplied by avalanche breakdown, resulting in
internal gain within the photodiode, which increases the effective
responsivity of the device. It can handle large signal power when
compared to a photodiode.
A phototransistor is in essence a bipolar transistor encased in
a transparent case so that light can reach the base-collector
junction. It was invented by Dr. John N. Shive (more famous for his
wave machine) at Bell Labs in 1948, but it wasn't announced until
1950. The electrons that are generated by photons in the
base-collector junction are injected into the base, and this
photodiode current is amplified by the transistor's current gain
(or hfe). If the emitter is left unconnected, the phototransistor
becomes a photodiode. While phototransistors have a higher
responsivity for light they are not able to detect low levels of
light any better than photodiodes. Phototransistors also have
significantly longer response times.
Materials
The material used to make a photodiode is critical to defining
its properties, because only photons with sufficient energy to
excite electrons across the material's band gap will produce
significant photocurrents. Materials commonly used to produce
photodiodes include:
Material Electromagnetic spectrum wavelength range (nm)
Silicon 1901100 Germanium 4001700 Indium gallium arsenide
8002600 Lead(II) sulfide
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Critical performance parameters of a photodiode include:
Responsivity
The ratio of generated photocurrent to incident light power is
known as responsivity, typically expressed in A/W when used in
photoconductive mode. The responsivity may also be expressed as
Quantum efficiency, or the ratio of the number of photogenerated
carriers to incident photons and thus a unitless quantity.
Dark current
The current through the photodiode in the absence of light, when
it is operated in photoconductive mode is called Dark current. The
dark current includes photocurrent generated by background
radiation and the saturation current of the semiconductor junction.
Dark current must be accounted for by calibration if a photodiode
is used to make an accurate optical power measurement, and it is
also a source of noise when a photodiode is used in an optical
communication system.
Noise-equivalent power (NEP)
The minimum input optical power to generate photocurrent, equal
to the rms noise current in a 1 hertz bandwidth. NEP is essentially
the minimum detectable power. The related characteristic
"detectivity" (D) is the inverse of NEP, 1/NEP.
There is also the "specific detectivity" ( ) which is the
detectivity multiplied by the square root of the area (A) of the
photodetector, ( ) for a 1 Hz bandwidth. The specific detectivity
allows different systems to be compared independent of sensor area
and system bandwidth; a higher detectivity value indicates a
low-noise device or system.[8] Although it is traditional to give (
) in many catalogues as a measure of the diode's quality, in
practice, it is hardly ever the key parameter.
When a photodiode is used in an optical communication system,
these parameters contribute to the sensitivity of the optical
receiver, which is the minimum input power required for the
receiver to achieve a specified bit error rate.
Applications
P-N photodiodes are used in similar applications to other
photodetectors, such as photoconductors, charge-coupled devices,
and photomultiplier tubes. They may be used to generate an output
which is dependent upon the illumination (analog; for measurement
and the like), or to change the state of circuitry (digital; either
for control and switching, or digital signal processing).
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Photodiodes are used in consumer electronics devices such as
compact disc players, smoke detectors, and the receivers for
infrared remote control devices used to control equipment from
televisions to air conditioners. For many applications either
photodiodes or photoconductors may be used. Either type of
photosensor may be used for light measurement, as in camera light
meters, or to respond to light levels, as in switching on street
lighting after dark.
Photosensors of all types may be used to respond to incident
light, or to a source of light which is part of the same circuit or
system. A photodiode is often combined into a single component with
an emitter of light, usually a light-emitting diode (LED), either
to detect the presence of a mechanical obstruction to the beam
(slotted optical switch), or to couple two digital or analog
circuits while maintaining extremely high electrical isolation
between them, often for safety (optocoupler).
Photodiodes are often used for accurate measurement of light
intensity in science and industry. They generally have a more
linear response than photoconductors.
They are also widely used in various medical applications, such
as detectors for computed tomography (coupled with scintillators),
instruments to analyze samples (immunoassay), and pulse
oximeters.
PIN diodes are much faster and more sensitive than p-n junction
diodes, and hence are often used for optical communications and in
lighting regulation.
P-N photodiodes are not used to measure extremely low light
intensities. Instead, if high sensitivity is needed, avalanche
photodiodes, intensified charge-coupled devices or photomultiplier
tubes are used for applications such as astronomy, spectroscopy,
night vision equipment and laser range finding.
TUNNEL DIODE
A tunnel diode or Esaki diode is a type of semiconductor diode
which is capable of very fast operation, well into the microwave
frequency region, by using the quantum mechanical effect called
tunneling.
It was invented in August 1957 by Leo Esaki when he was with
Tokyo Tsushin Kogyo, now known as Sony. In 1973 he received the
Nobel Prize in Physics, jointly with Brian Josephson, for
discovering the electron tunneling effect used in these diodes.
Robert Noyce independently came
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up with the idea of a tunnel diode while working for William
Shockley, but was discouraged from pursuing it.
These diodes have a heavily doped pn junction only some 10 nm
(100 ) wide. The heavy doping results in a broken band gap, where
conduction band electron states on the n-side are more or less
aligned with valence band hole states on the p-side.
Tunnel diodes were first manufactured by Sony in 1957 followed
by General Electric and other companies from about 1960, and are
still made in low volume today. Tunnel diodes are usually made from
germanium, but can also be made in gallium arsenide and silicon
materials. They are used in frequency converters and detectors.
They have negative differential resistance in part of their
operating range, and therefore are also used as oscillators,
amplifiers, and in switching circuits using hysteresis.
Forward bias operation
Under normal forward bias operation, as voltage begins to
increase, electrons at first tunnel through the very narrow pn
junction barrier because filled electron states in the conduction
band on the n-side become aligned with empty valence band hole
states on the p-side of the p-n junction. As voltage increases
further these states become more misaligned and the current drops
this is called negative resistance because current decreases with
increasing voltage. As voltage increases yet further, the diode
begins to operate as a normal diode, where electrons travel by
conduction across the pn junction, and no longer by tunneling
through the pn junction barrier. Thus the most important operating
region for a tunnel diode is the negative resistance region.
Reverse bias operation
When used in the reverse direction they are called back diodes
and can act as fast rectifiers with zero offset voltage and extreme
linearity for power signals (they have an accurate square law
characteristic in the reverse direction). Under reverse bias filled
states on the p-side become increasingly aligned with empty states
on the n-side and electrons now tunnel through the pn junction
barrier in reverse direction.
I-V curve similar to a tunnel diode characteristic curve
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It has negative resistance in the shaded voltage region, between
v1 and v2.
In a conventional semiconductor diode, conduction takes place
while the pn junction is forward biased and blocks current flow
when the junction is reverse biased. This occurs up to a point
known as the reverse breakdown voltage when conduction begins
(often accompanied by destruction of the device). In the tunnel
diode, the dopant concentration in the p and n layers are increased
to the point where the reverse breakdown voltage becomes zero and
the diode conducts in the reverse direction. However, when
forward-biased, an odd effect occurs called quantum mechanical
tunneling which gives rise to a region where an increase in forward
voltage is accompanied by a decrease in forward current. This
negative resistance region can be exploited in a solid state
version of the dynatron oscillator which normally uses a tetrode
thermionic valve (or tube).
The tunnel diode showed great promise as an oscillator and
high-frequency threshold (trigger) device since it would operate at
frequencies far greater than the tetrode would, well into the
microwave bands. Applications for tunnel diodes included local
oscillators for UHF television tuners, trigger circuits in
oscilloscopes, high speed counter circuits, and very fast-rise time
pulse generator circuits. The tunnel diode can also be used as
low-noise microwave amplifier.[5] However, since its discovery,
more conventional semiconductor devices have surpassed its
performance using conventional oscillator techniques. For many
purposes, a three-terminal device, such as a field-effect
transistor, is more flexible than a device with only two terminals.
Practical tunnel diodes operate at a few milliamperes and a few
tenths of a volt, making them low-power devices. The Gunn diode has
similar high frequency capability and can handle more power.
Tunnel diodes are also relatively resistant to nuclear
radiation, as compared to other diodes. This makes them well suited
to higher radiation environments, such as those found in space
applications.
Longevity
Esaki diodes are notable for their longevity; devices made in
the 1960s still function. Writing in Nature, Esaki and coauthors
state that semiconductor devices in general are extremely stable,
and suggest that their shelf life should be "infinite" if kept at
room temperature. They go on to report that a small-scale test of
50-year-old devices revealed a "gratifying confirmation of the
diode's longevity". As noticed on some samples of Esaki diodes, the
gold plated iron pins can in fact corrode and short out to the
case. This can usually be diagnosed, and the diode inside normally
still works.
A Tunnel Diode is s pn junction that exhibits negative
resistance between two values of forward voltage.
The tunnel diode s basically a pn junction with heavy doping of
p type and n type semiconductor materials .tunnel diode is doped
1000 times as heavily as a conventional diode Heavy doping results
in large no of majority carriers. Because this large no of
carriers, most are not used during
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initial recombination that produces depletion layer. It is very
narrow. Depletion layer of tunnel diode is 100 times narrower.
Operation of tunnel diode depends on the tunneling effect.
TUNNELING
The movement of valence electrons from the valence energy band
to the conduction band with little or no applied forward voltage is
called tunneling.
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VI CHARACTERISTICS
As the forward voltage is first increased, the tunnel diode is
increased from zero, electrons from the n region tunnel through the
potential barrier to the potential barrier to the p region. As the
forward voltage increases the diode current also increases until
the peak to peak is reached. Ip = 2.2 mA. Peak point voltage
=0.07V
As the voltage is increased beyond Vp the tunneling action
starts decreasing and the diode current decreases as the forward
voltage is increased until valley point V is reached at valley
point voltage Vv= 0.7V between V and P the diode exhibits negative
resistance i.e., as the forward bias is increased , the current
decreases. When operated in the negative region used as
oscillator.
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 wireless can
be considered primitive Schottky diodes.
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When current flows through a diode, there is small voltage drop
across the diode terminals. A normal silicon diode has a voltage
drop between 0.61.7 volts, while a Schottky diode voltage drop is
between approximately 0.150.45 volts. This lower voltage drop can
provide higher switching speed and better system efficiency.
Construction
A metalsemiconductor junction is formed between a metal and a
semiconductor, creating a Schottky barrier (instead of a
semiconductorsemiconductor junction as in conventional diodes).
Typical metals used are molybdenum, platinum, chromium or tungsten;
and the semiconductor would typically be N-type silicon.[1] 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 the p-n and Schottky diode
is reverse recovery time, when the diode switches from conducting
to non-conducting state. Where in a p-n diode the reverse recovery
time can be in the order of hundreds of nanoseconds and less than
100 ns for fast diodes, Schottky diodes do not have a 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 in high-power
semiconductors brings increased EMI noise. With Schottky diodes
switching essentially instantly with only slight capacitive
loading, this is much less of a concern.
It is often said that the Schottky diode is a "majority carrier"
semiconductor device. This means that if the semiconductor body is
doped n-type, only the n-type carriers (mobile electrons) play a
significant role in normal operation of the device. The majority
carriers are quickly injected into the conduction band of the metal
contact on the other side of the diode to become free moving
electrons. Therefore no slow, random recombination of n- and p-
type carriers is involved, so that this diode can cease conduction
faster than an ordinary p-n rectifier diode. This property in turn
allows a smaller device area, which also makes for a faster
transition. This is another reason why Schottky diodes are useful
in switch-mode power converters; the high speed of the diode means
that the circuit can operate at frequencies in the range 200 kHz to
2 MHz, allowing the use of small inductors and capacitors with
greater efficiency than would be possible with other diode types.
Small-area Schottky diodes are the heart of RF detectors and
mixers, which often operate up to 50 GHz.
Limitations
The most evident limitations of Schottky diodes are the
relatively low reverse voltage ratings for silicon-metal Schottky
diodes, typically 50 V and below, and a relatively high reverse
leakage current. Some higher-voltage designs are available; 200V is
considered a high reverse voltage. Reverse leakage current, because
it increases with temperature, leads to a thermal instability
issue. This often limits the useful reverse voltage to well below
the actual rating.
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While higher reverse voltages are achievable, they would be
accompanied by higher forward voltage drops, comparable to other
types; such a Schottky diode would have no advantage.[2]
Silicon carbide Schottky diode
Schottky diodes constructed from silicon carbide have a much
lower reverse leakage current than silicon Schottky diodes, and
higher reverse voltage. As of 2011 they were available from
manufacturers in variants up to 1700 V.[3]
Silicon carbide has a high thermal conductivity, and temperature
has little influence on its switching and thermal characteristics.
With special packaging silicon carbide Schottky diodes can operate
at junction temperatures of over 500 K (about 200 C), which allows
passive radiative cooling in aerospace applications.[3]
Applications
Voltage clamping
While standard silicon diodes have a forward voltage drop of
about 0.7 volts and germanium diodes 0.3 volts, Schottky diodes
voltage drop at forward biases of around 1 mA is in the range 0.15
V to 0.46 V (see the 1N5817[4] and 1N5711[5] datasheets found
online at manufacturer's websites), which makes them useful in
voltage clamping applications and prevention of transistor
saturation. This is due to the higher current density in the
Schottky diode.
Reverse current and discharge protection
Because of a Schottky diodes low forward voltage drop, less
energy is wasted as heat making them the most efficient choice for
applications sensitive to efficiency. For instance, they are used
in stand-alone ("off-grid") photovoltaic (PV) systems to prevent
batteries from discharging through the solar panels at night, and
in grid-connected systems with multiple strings connected in
parallel, in order to prevent reverse current flowing from adjacent
strings through shaded strings if the bypass diodes have
failed.
Power supply
They are also used as rectifiers in switched-mode power
supplies; the low forward voltage and fast recovery time leads to
increased efficiency.
Schottky diodes can be used in power supply "OR"ing circuits in
products that have both an internal battery and a mains adapter
input, or similar. However, the high reverse leakage current
presents a problem in this case, as any high-impedance voltage
sensing circuit (e.g. monitoring the battery voltage or detecting
whether a mains adaptor is present) will see the voltage from the
other power source through the diode leakage.
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PIN DIODE
A PIN diode is a diode with a wide, lightly doped 'near'
intrinsic semiconductor region between a p-type semiconductor and
an n-type semiconductor region. The p-type and n-type regions are
typically heavily doped because they are used for ohmic
contacts.
The wide intrinsic region is in contrast to an ordinary PN
diode. The wide intrinsic region makes the PIN diode an inferior
rectifier (one typical function of a diode), but it makes the PIN
diode suitable for attenuators, fast switches, photodetectors, and
high voltage power electronics applications.
Operation
A PIN diode operates under what is known as high-level
injection. In other words, the intrinsic "i" region is flooded with
charge carriers from the "p" and "n" regions. Its function can be
likened to filling up a water bucket with a hole on the side. Once
the water reaches the hole's level it will begin to pour out.
Similarly, the diode will conduct current once the flooded
electrons and holes reach an equilibrium point, where the number of
electrons is equal to the number of holes in the intrinsic region.
When the diode is forward biased, the injected carrier
concentration is typically several orders of magnitude higher than
the intrinsic level carrier concentration. Due to this high level
injection, which in turn is due to the depletion process, the
electric field extends deeply (almost the entire length) into the
region. This electric field helps in speeding up of the transport
of charge carriers from P to N region, which results in faster
operation of the diode, making it a suitable device for high
frequency operations.
Characteristics
A PIN diode obeys the standard diode equation for low frequency
signals. At higher frequencies, the diode looks like an almost
perfect (very linear, even for large signals) resistor. There is a
lot of stored charge in the intrinsic region. At low frequencies,
the charge can be removed and the diode turns off. At higher
frequencies, there is not enough time to remove the charge, so the
diode never turns off. The PIN diode has a poor reverse recovery
time.
The high-frequency resistance is inversely proportional to the
DC bias current through the diode. A PIN diode, suitably biased,
therefore acts as a variable resistor. This high-frequency
resistance may vary over a wide range (from 0.1 ohm to 10 k in some
cases; the useful range is smaller, though).
The wide intrinsic region also means the diode will have a low
capacitance when reverse biased.
In a PIN diode, the depletion region exists almost completely
within the intrinsic region. This depletion region is much larger
than in a PN diode, and almost constant-size, independent of
the
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reverse bias applied to the diode. This increases the volume
where electron-hole pairs can be generated by an incident photon.
Some photodetector devices, such as PIN photodiodes and
phototransistors (in which the base-collector junction is a PIN
diode), use a PIN junction in their construction.
The diode design has some design tradeoffs. Increasing the
dimensions of the intrinsic region (and its stored charge) allows
the diode to look like a resistor at lower frequencies. It
adversely affects the time needed to turn off the diode and its
shunt capacitance. PIN diodes will be tailored for a particular
use.
Applications
PIN diodes are useful as RF switches, attenuators, and
photodetectors.
RF and Microwave Switches
Under zero or reverse bias, a PIN diode has a low capacitance.
The low capacitance will not pass much of an RF signal. Under a
forward bias of 1 mA, a typical PIN diode will have an RF
resistance of about 1 ohm, making it a good RF conductor.
Consequently, the PIN diode makes a good RF switch.
Although RF relays can be used as switches, they switch very
slowly (on the order of 10 milliseconds). A PIN diode switch can
switch much more quickly (e.g., 1 microsecond).
The capacitance of an off discrete PIN diode might be 1 pF. At
320 MHz, the reactance of 1 pF is about 500 ohms. In a 50 ohm
system, the off state attenuation would be about 20 dB -- which may
not be enough attenuation. In applications that need higher
isolation, switches are cascaded to improve the isolation.
Cascading three of the above switches would give 60 dB of
attenuation.
PIN diode switches are used not only for signal selection, but
they are also used for component selection. For example, some low
phase noise oscillators use PIN diodes to range switch
inductors.
RF and Microwave Variable Attenuators
By changing the bias current through a PIN diode, it's possible
to quickly change the RF resistance.
At high frequencies, the PIN diode appears as a resistor whose
resistance is an inverse function of its forward current.
Consequently, PIN diode can be used in some variable attenuator
designs as amplitude modulators or output leveling circuits.
PIN diodes might be used, for example, as the bridge and shunt
resistors in a bridged-T attenuator.
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Limiters
PIN diodes are sometimes used as input protection devices for
high frequency test probes. If the input signal is within range,
the PIN diode has little impact as a small capacitance. If the
signal is large, then the PIN diode starts to conduct and becomes a
resistor that shunts most of the signal to ground.
Photodetector and photovoltaic cell
The PIN photodiode was invented by Jun-ichi Nishizawa and his
colleagues in 1950.
PIN photodiodes are used in fibre optic network cards and
switches. As a photodetector, the PIN diode is reverse biased.
Under reverse bias, the diode ordinarily does not conduct (save a
small dark current or Is leakage). A photon entering the intrinsic
region frees a carrier. The reverse bias field sweeps the carrier
out of the region and creates a current. Some detectors can use
avalanche multiplication.
The PIN photovoltaic cell works in the same mechanism. In this
case, the advantage of using a PIN structure over conventional
semiconductor junction is the better long wavelength response of
the former. In case of long wavelength irradiation, photons
penetrate deep into the cell. But only those electron-hole pairs
generated in and near the depletion region contribute to current
generation. The depletion region of a PIN structure extends across
the intrinsic region, deep into the device. This wider depletion
width enables electron-hole pair generation deep within the device.
This increases the quantum efficiency of the cell.
Typically, amorphous silicon thin-film cells use PIN structures.
On the other hand, CdTe cells use NIP structure, a variation of the
PIN structure. In a NIP structure, an intrinsic CdTe layer is
sandwiched by n-doped CdS and p-doped ZnTe. The photons are
incident on the n-doped layer unlike a PIN diode.
A PIN photodiode can also detect X-ray and gamma ray
photons.