electrical and optical characteristics of avalanche photo diodes

8/8/2019 Electrical and Optical Characteristics of Avalanche Photo Diodes

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Electrical and Optical characteristics of Avalanche Photodiodes

APDs are high-speed, high sensitivity photodoides utilizing an internal gain mechanism that

functions by applying a reverse voltage. Compared to PIN photodiodes, APDs can measure even

lower level light and are used in a wide variety of applications requiring high sensitivity such as

long-distance optical communications and optical distance measurement.

Avalanche photodiode advantages and disadvantages

The avalanche photodiode has a number of different characteristics to the normal p-n or p-i-n photodiodes, making them more suitable for use in some applications. In view of this it is worth

summarising their advantages and disadvantages..

The main advantages of the avalanche photodiode include:

y Greater level of sensitivity

The disadvantages of the avalanche photodiode include:

y Much higher operating voltage may be required.

y Avalanche photodiode produces a much higher level of noise than a p-n photodiodey Avalanche process means that the output is not linear

Avalanche diode structure and operation

The structure is somewhat more complicated than that of the ordinary p-i-n device. It consists of four layers. There are n+, p, un-doped, and p+ regions. Light absorption takes place in the un-doped region and as before this may be relatively thick. The avalanche region occurs between the

n+ and p regions.

Light enters the un-doped region of the avalanche photodiode and causes the generation of hole-electron pairs. Under the action of the electric field the electrons migrate towards the avalanche

region. Here the electric field causes their velocity to increase to the extent that collisions withthe crystal lattice create further hole electron pairs. In turn these electrons may collide with the

crystal lattice to create even more hole electron pairs. In this way a single electron created bylight in the un-doped region may result in many more being created.

The avalanche photodiode has a number of differences when compared to the ordinary p-i-n

diode. The avalanche process means that a single electron produced by light in the un-dopedregion is multiplied several times by the avalanche process. As a result the avalanche photo

diode is far more sensitive. However it is found that it is not nearly as linear, and additionally theavalanche process means that the resultant signal is far noisier than one from a p-i-n diode. The

structure of the avalanche diode is also more complicated. An n-type guard ring is requiredaround the p-n junction to minimise the electric field around the edge of the junction. It is also



found that the current gain is dependent not only on the bias applied, but also thermalfluctuations. As a result it is necessary to ensure the devices are placed on an adequate heat sink.

Circuit conditions

Avalanche photodiodes require a high reverse bias for their operation. For silicon, a diode willtypically require between 100 and 200 volts, and with this voltage they will provide a current

gain effect of around 100 resulting from the avalanche effect. Some diodes that utilisespecialised manufacturing processes enable much higher bias voltages of up to 1500 volts to be

applied. As it is found that the gain levels increase when higher voltages are applied, the gain of these avalanche diodes can rise to the order of 1000. This can provide a distinct advantage where

sensitivity is of paramount importance.

Photodiode parameters and characteristics

There are a number of parameters that are important in the specification of an avalanche photodiode. These parameters include:

1. Photodiode material

2. Diode size3. Bandwidth

4. Responsivity and gain5. Dark and noise current

6. Excess noise factor

These parameters represent some of the more important items to be specified, but it does notinclude a comprehensive list for all applications. The parameters will be addressed individually:

1. Photodiode material: The materials used have a major effect on determining thecharacteristics of the avalanche diode. The application for the avalanche diode will often

dictate the material used, especially in terms of the wavelength.

Table #. Summary of materials commonly used for avalanche photodiodes and their properties

Material Properties

GermaniumCan be used for wavelengths in the region 800 - 1700 nm.

Has a high level of multiplication noise.

Silicon Can be used for wavelengths in the region between 190 -

1100 nm. Diodes exhibit a comparatively low level of



Material Properties

multiplication noise when compared to those using other

materials, and in particular germanium.

Indium

gallium

arsenide

Can be used for wavelengths to 1600 nm and has a lower

level of multiplication noise than germanium.

2. S ize of the avalanche photodiode: The area over which light is to be collected may

determine the actual size of the photodiode itself. However the larger the diode, thegreater the cost. As a result, it is often more beneficial to utilise optical methods of

focussing the light from a given area onto a smaller avalanche photodiode.

3. Bandwidth: It is important to specify the bandwidth required for the avalanche

photodiode. It is necessary to ensure that the diode can respond to the changes as rapidlyas needed so that data at the required speed can be received. While there is a temptation

to over specify, the required bandwidth should be carefully analysed as there is a penaltyin the signal to noise ratio for choosing a wider bandwidth than is required.

4. Responsivity and gain: The responsivity of a photodiode is measured in amps per wattand is an indication of the current generated for a given excitation in watts. This must begiven for a particular bias voltage as the responsivity varies with the level of bias.

5. Dark and noise current: The darm current is the current that flows in the device when it

is not exposed to any light. Dark current is dominated by surface current, and since thedark and spectral noise current are a strong function of the gain of the avalanche

photodiode, these should be specified at a stated responsivity level.

6. Ex cess noise factor: All avalanche photodiodes generate excess noise due to the

statistical nature of the avalanche process. In data on avalanche photodiodes, this factor isgenerally denoted by the letter F. In essence it can be viewed as the factor by which the

statistical noise on the diode current exceeds that which would be expected from anoiseless multiplier on the basis of statistics (shot noise) alone. Accordingly this factor



gives an indication of the amount of noise a diode introduces above that which would beexpected on the basis of shot noise alone.

.

Baric electrical and optical characteristics of APD

1. Principle of Avalanche multiplication

When light enters a photodiode, electron-hole pairs are generated if the light energy is higher than the band gap energy. Light energy E (eV) and wavelength (nm) have a particular relation

as shown in Equation # below:

Eq#

Therefore, it is sensitive to light wavelengths shorter than 1100 nm. This sensitivity is commonly

expressed by terms called photosensitivity S (A/W) and quantum efficiency QE (%).

The photosensitivity is the photocurrent divided by the incident radiant power, expressed inA/W. The quantum efficiency is the ratio of electron-hole pairs generated versus the number of

incident photons.

Avalanche breakdown - is a phenomenon that can occur in both insulating and semiconducting

materials. It is a form of electric current multiplication that can allow very large currents to flowwithin materials which are otherwise good insulators. It is a type of electron avalanche. The

Avalanche process occurs when the carriers in the transition region are accelerated by the electricfield to energies sufficient to free e- h pairs via collisions with bond electrons.

Avalanche breakdown usually destroys regular diodes, but avalanche diodes are designed to

break down this way at low voltages and can survive the reverse current.

The voltage at which the breakdown occurs is called the breakdown voltage. There is ahysteresis effect; once avalanche breakdown has occurred, the material will continue to conduct

if the voltage across it drops below the breakdown voltage.

The number of electron-hole pairs generated during the time that the carriers travel a given

distance is referred to as the ionization rate. Usually, the ionization rate of electrons is defined as and that of holes as . These ionization rates are important factors in determining the avalanche

multiplication mechanism. The ratio k of to is called the ionization ratio and is used as a parameter to indicate device noise.



2.



Applications of Avalanche photodiodes

APDs are high-speed, high sensitivity photodiodes utilizing an internal gain mechanism that

functions by applying a reverse voltage. Compared to PIN photodiodes, APDs can measure even

lower level light and are used in a wide variety of applications requiring high sensitivity such as

long-distance optical communications and optical distance measurement, laser range finders and photon correlation studies.

APDs are also widely used in instrumentation and aerospace applications, offering a combination

of high speed and high sensitivity unmatched by PIN detectors, and quantum efficiencies at >400 nm unmatched by PMTs.

Due to their performance advantages APDs are then used widely in applications such as distancemeasurement, data transmission (over fibre or through free space ), range finding, high speed

industrial inspection ( including colour measurement ) and in various other medical and scientificinstrumentation.

1. Laser rangefinders

A laser rangefinder is a device which uses a laser beam to determine the distance to an object.

The most common form of laser rangefinder operates on the time of flight principle by sending alaser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be

reflected off the target and returned to the sender. Due to the high speed of light, this technique isnot appropriate for high precision sub-millimeter measurements, where triangulation and other

techniques are often used. In addition, APDs used in the application can operate with lower lightlevels and shorter laser pulses, thus making the range finder more eye safe.

Fig. # A long range laser rangefinder is capable of measuring distance up to 20 km; mounted on

a tripod with an angular mount.



Applications of range finder:

y Militaryy 3-DModelling

y Forestryy

Sportsy Industry production processesy Laser measuring tools

2. Positron Emission Tomography

To conduct the scan, a short-lived radioactive tracer isotope is injected into the living subject(usually into blood circulation). The tracer is chemically incorporated into a biologically active

molecule. There is a waiting period while the active molecule becomes concentrated in tissues of

interest; then the subject is placed in the imaging scanner. The molecule most commonly usedfor this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typicallyan hour. During the scan a record of tissue concentration is made as the tracer decays.

Schema of a PET acquisition process Schematic view of a detector block and ring of a PET

scanner

As the radioisotope undergoes positron emission decay (also known as positive beta decay), itemits a positron, an antiparticle of the electron with opposite charge. The emitted positron travels

in tissue for a short distance (typically less than 1 mm, but dependent on the isotope), duringwhich time it loses kinetic energy, until it decelerates to a point where it can interact with an

electron. The encounter annihilates both electron and positron, producing a pair of annihilation

(gamma) photons moving in approximately opposite directions. These are detected when theyreach a scintillator in the scanning device, creating a burst of light which is detected by siliconavalanche photodiodes ( S i AP D). The technique depends on simultaneous or coincident

detection of the pair of photons moving in approximately opposite direction (it would be exactlyopposite in their center of mass frame, but the scanner has no way to know this, and so has a

built-in slight direction-error tolerance). Photons that do not arrive in temporal "pairs" (i.e.within a timing-window of a few nanoseconds) are ignored.



3. Long-distance optical communications

Avalanche photodiodes are used in the receivers or light demodulators in opticalcommunications systems. These optical receivers extract the baseband signal from a modulated

optical carrier signal by converting incident optical power into electric current.

4. Confocal laser scanning microscopy

In a confocal laser scanning microscope, a laser beam passes through a light source aperture and

then is focused by an objective lens into a small (ideally diffraction limited) focal volume withinor on the surface of a specimen. In biological applications especially, the specimen may be

fluorescent. Scattered and reflected laser light as well as any fluorescent light from theilluminated spot is then re-collected by the objective lens. A beam splitter separates off some

portion of the light into the detection apparatus, which in fluorescence confocal microscopy will

also have a filter that selectively passes the fluorescent wavelengths while blocking the originalexcitation wavelength. After passing a pinhole, the light intensity is detected by a photodetectiondevice (usually a photomultiplier tube (PMT) or avalanche photodiode), transforming the light

signal into an electrical one that is recorded by a computer.

Fig #. Principle of confocal microscopy

5. Scintillation counter

A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of atransparent crystal, usually phosphor, plastic (usually containing anthracene), or organic liquid

(see liquid scintillation counting) that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube (PMT) measures the light from the crystal. The PMT is attached to an

electronic amplifier and other electronic equipment to count and possibly quantify the amplitudeof the signals produced by the photomultiplier.



Scintillation counters can be used in a variety of applications.

y Medical imagingy National and homeland security

y Border securityy

Nuclear safety

electrical and optical characteristics of avalanche photo diodes

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