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Avalanche Photo Diode

Author: Apogeeweb
Date: 26 Apr 2019
avalanche photodiode applications

Warm hints: This article contains about 5000 words and reading time is about 15 mins.


The PN junction has unidirectional conductivity, a small forward resistance, and a large reverse resistance. When the reverse voltage increases to a certain value, the reverse current suddenly increases. It is a reverse electrical breakdown. It is divided into avalanche breakdown and Zener breakdown (tunnel breakdown). Avalanche breakdown is when the PN junction reverse voltage increases to a value, the carrier multiplication is like an avalanche, increasing much faster, and the diode fabricated using this characteristic is an avalanche diode. Avalanche breakdown is caused by the electric field, the carrier energy increases, and it collides with the crystal atoms, causing the electrons in the covalent bond to excite to form free electron-hole pairs. The newly generated carriers in turn generate free electron-hole pairs by collision, which is the multiplication effect. 1 born 2, 2 born 4, increased carriers like an avalanche.





Ⅰ Definition of Avalanche Photodiode

Ⅱ Main Features of Avalanche Photodiode

Ⅲ Working Principle of Avalanche Photodiode

Ⅳ Materials of Avalanche Photodiode

Ⅴ Difference between Photo Multiplier Tube and Avalanche Photodiode

5.1 Photo Multiplier Tube

5.2 The Difference Between Photo Multiplier Tube and Avalanche Photodiode

Ⅰ Definition of Avalanche Photodiode

Avalanche photodiode is a p-n junction type photodetecting diode in which the avalanche multiplication effect of carriers is utilized to amplify the photoelectric signal to improve the sensitivity of detection. The basic structure often adopts a Read diode structure (ie, N+PIP+ type structure, P+ side receives light) which is easy to generate avalanche multiplication effect, and a large reverse bias is applied during operation, so that it reaches an avalanche multiplication state; its light absorption. The zone is substantially identical to the multiplication zone (the P zone and the I zone where there is a high electric field).

Avalanche Photodiode Definition: What is Avalanche Photodiode?

The PN junction is coupled with a suitable high reverse bias voltage to accelerate the strong electric field in the depletion layer to obtain a sufficiently high kinetic energy. They collide with the lattice to generate a new electron-hole pair. These carriers It also constantly causes new impact ionization, causing the avalanche of carriers to multiply and gain current gain. In the 0.6 to 0.9 μm band, silicon APD has near-ideal performance. InGaAs (Indium Gallium Arsenide) / InP (Indium Phosphorus) APD is an ideal photodetector for long-wavelength (1.3μn, 1.55μm) fiber-optic communication. The optimized structure is shown in the figure.


The light absorbing layer uses InGaAs material, which has a high absorption coefficient for 1.3μm and 1.55μn light. In order to avoid the InGaAs homojunction tunnel breakdown before the avalanche breakdown, the avalanche region is The absorption is separated, that is, the PN junction is made in the InP window layer. In view of the fact that the hole ionization coefficient in the InP material is larger than the electron ionization coefficient, the avalanche region uses n-type InP, and the n-InP and n-InGaAs heterointerfaces have a large valence band barrier, which easily causes the photo-generated holes to collapse. In the InGaAsP (indium gallium arsenide) transition region of the band gap gradation, SAGM (absorption, classification, and multiplication) structures are formed.


In the manufacture of APD, it is necessary to add a guard ring on the surface of the device to improve the reverse withstand voltage performance; the semiconductor material is superior to Si (used widely for detecting light below 0.9 um), but when detecting long-wavelength light of 1 um or more Commonly used Ge and InGaAs (noise noise and dark current). The disadvantage of this APD is that there is a process of tunnel current multiplication, which will generate large shot noise (reduced p-zone doping, which can reduce the tunnel current, but the avalanche voltage will increase).


An improved structure is the so-called SAM-APD: the multiplication zone uses a wider band gap material (so that it does not absorb light), and the light absorption zone uses a narrow band gap material; here, due to the heterojunction, Decreasing the doping concentration of the multiplication zone without affecting the light absorption region, so that the tunneling current is reduced (if it is a mutant heterojunction, because of the existence of ΔEv, the photogenerated holes will accumulate and affect the device. Response speed, at which point a layer of graded layer can be inserted in the middle of the abrupt heterojunction to reduce the effect of ΔEv).


Ⅱ Main Features of Avalanche Photodiode

(1) Avalanche gain coefficient M (also called multiplication factor), the main characteristics of abrupt junction avalanche diodes

Avalanche Photo Diodes

In the formula, V is a reverse bias voltage, and VB is a body avalanche breakdown voltage; n is a constant with respect to a material, a device structure, and an incident wavelength, and has a value of 1 to 3.


(2) Gain bandwidth product, when the gain is large and the frequency is high,

Avalanche Photo Diodes

In the equation, ω is the angular frequency; N is a constant, which varies slowly with the ionization coefficient ratio; W is the thickness of the depletion region; Vs is the saturation velocity; αn and αp are the ionization coefficients of electrons and holes, respectively, and the gain bandwidth product Is a constant. In order to obtain a high product, large Vs, small W and small αn/αp should be selected (ie, electrons and hole ionization coefficients are different, and carriers with higher ionization coefficients are injected into the avalanche region).


(3) Excessive noise factor F, in the multiplication process, the noise current grows faster than the signal current, and F is used to represent the noise added by the avalanche process plus F≈Mx. Where x is the excess noise index. To choose the right M value, you can get the best signal to noise ratio, so that the system achieves the highest sensitivity.


(4) The temperature characteristics, the carrier ionization coefficient decreases with increasing temperature, resulting in a decrease in the multiplication factor and an increase in the breakdown voltage. The temperature characteristic of the APD is described by the temperature coefficient of the breakdown voltage.

Avalanche Photo Diodes

In the formula, VB and VB0 are breakdown voltages at temperatures T and T0, respectively.

When using, the working point should be temperature-controlled, and a uniform P-N junction should be made to prevent the partial surface from being broken down.



Ⅲ Working Principle of Avalanche Photodiode

An avalanche photodiode is a photovoltaic device with internal gain that utilizes the directional motion of photogenerated carriers in a strong electric field to produce an avalanche effect to obtain the gain of the photocurrent. In the process of avalanche, photo-generated carriers undergo high-speed directional motion under the action of strong electric field, and photogenerated electrons or holes with high kinetic energy collide with the lattice courtyard, causing the lattice atoms to ionize to generate secondary electrons---holes The second electron---hole pair obtains sufficient kinetic energy under the action of the electric field, and the lattice atom ionizes to generate a new electron-hole pair. This process continues like an "avalanche". The number of carriers generated by ionization is much larger than the photogenerated carriers generated by photoexcitation. At this time, the output current of the avalanche photodiode increases rapidly, and the current multiplication factor is defined as:

Avalanche Photo Diode

In the equation, the output current is multiplied, which is the output current before multiplication.


The avalanche multiplication factor is closely related to the impact ionization rate. The impact ionization rate indicates the number of electrons-hole pairs generated by a single unit of drift under the action of an electric field. In fact, the electron ionization rate and the hole ionization rate are not exactly the same, and they are all closely related to the electric field strength. It is determined experimentally that the ionization rate has the following relationship with the electric field strength J:

Avalanche Photo Diode

Avalanche Photo Diode

Where is the width of the depletion layer. The above formula shows that when 

Avalanche Photo Diode

Avalanche Photo DiodeTherefore, the above formula is called the condition for avalanche breakdown. The physical meaning is that under the action of an electric field, avalanche breakdown occurs when each carrier passing through the depletion region can generate a pair of electron-hole pairs on average. At that time, the reverse bias applied to the junction was the avalanche breakdown voltage.


It has been found that the avalanche multiplication phenomenon occurs when the reverse bias voltage is slightly lower than the breakdown voltage, but the value at this time is small, and the change with the reverse bias voltage can be approximated by an empirical formula.

Avalanche Photo Diode

A proper multiplication factor can be obtained by appropriately adjusting the operating bias of the avalanche photodiode. At present, the bias of the avalanche photodiode is divided into low voltage and high voltage, the low voltage is about several tens of volts, and the high voltage is several hundred volts. Avalanche photodiodes have a multiplication factor of hundreds or even thousands of times.


The relationship between the dark current and photocurrent of the avalanche photodiode and the bias voltage is shown in the figure. As can be seen from the figure, when the operating bias is increased, the output bright current (i.e., the sum of the photocurrent and the dark current) increases exponentially. When the bias voltage is low, no avalanche process occurs, that is, no photocurrent multiplication. Therefore, when the optical pulse signal is incident, the generated photocurrent pulse signal is small (such as the A-point waveform).


When the reverse bias rises to point B, the photocurrent produces an avalanche multiplication effect, at which time the photocurrent pulse signal output increases to a maximum (such as the B-point waveform). When the bias voltage approaches the avalanche breakdown voltage, the avalanche current maintains its own flow, causing the dark current to increase rapidly, and the avalanche magnification of the photoexcited carriers decreases. That is, the photocurrent sensitivity decreases as the reverse bias voltage increases, as the pulse signal of the photocurrent decreases at point C.


In other words, when the reverse bias exceeds point B, the useful photocurrent pulse amplitude is reduced due to the faster increase of dark current. So the best working point is near the avalanche breakdown point. Sometimes in order to suppress the dark current, it will move to the left. Although the sensitivity is reduced, the dark current and noise characteristics are improved.

Avalanche Photo Diode

It can be seen from the volt-ampere characteristic curve in the figure that the current changes with the bias voltage near the avalanche breakdown point is steep, and when the reverse bias voltage changes little, the photocurrent will have a large change. In addition, the reverse bias on the PN junction during avalanche is prone to fluctuations that will affect the stability of the gain. Therefore, after determining the operating point, the stability of the bias voltage is very high.


Since the impact ionization of carriers in an avalanche photodiode is irregular, the direction of motion after collision becomes more random, so its noise is larger than that of a general photodiode. In the case of no multiplication, the noise current is mainly shot noise. When the avalanche multiplies M times, the rms value of the avalanche photodiode's noise current can be approximated by the formula:

Avalanche Photo Diode

Photodetectors are the key components of optical signal conversion in optical fiber communication and photodetection systems, and are an important part of optoelectronic integrated circuit (OEIC) receivers. With the development of integrated circuit computer-aided design technology, through the establishment of PIN avalanche photodiodes ( The mathematical model of APD) and the analysis and research of its characteristics by computer are an important part of OEIC design.


The equivalent circuit model of PIN-APD is usually simulated in PSPICE [1,2,427]. The method can better perform DC, AC and transient analysis. However, it cannot track the changes of carriers and photons in the process of PIN-APD. At the same time, the existence and calculation of some virtual devices in the modeling process make the model characteristics appear error. In this paper, the mathematical model is established by solving the excess carrier rate equation in each region of the reverse bias PIN structure, and the model parameters and devices are modified. The simulation calculation is carried out in Matlab. The simulation results are in good agreement with the actual measurement results.



Ⅳ Materials of Avalanche Photodiode

In theory, any semiconductor material can be used in the multiplication zone:

Silicon materials are suitable for the detection of visible and near-infrared light and have low multiplication noise (excess noise).


The germanium (Ge) material can detect infrared rays having a wavelength of not more than 1.7 & micro; m, but the multiplication noise is large.

InGaAs materials can detect infrared light with wavelengths exceeding 1.6 & micro; m, and the doubling noise is lower than that of tantalum materials. It is generally used as a multiplication zone for heterostructure diodes. This material is suitable for high-speed fiber-optic communication, and the speed of commercial products has reached 10Gbit/s or higher.


Gallium nitride diodes can be used for UV detection.

HgCdTe diodes detect infrared light at wavelengths up to 14 & micro;m, but require cooling to reduce dark current. Very low excess noise can be obtained with this diode.



Ⅴ Difference between Photo Multiplier Tube and Avalanche Photodiode

5.1 Photo Multiplier Tube

5.1.1 Definition of Photo Multiplier Tube

A photo multiplier tube is a vacuum electronic device that converts a weak optical signal into an electrical signal. Photomultiplier tubes are used in optical measuring instruments and spectroscopic instruments. It measures very weak radiated power from 200 to 1200 nm in low-energy photometry and spectroscopy. The appearance of a scintillation counter expands the range of application of photomultiplier tubes. The development of laser detection instruments is closely related to the use of photomultiplier tubes as effective receivers. The transmission and image transmission of television movies is also inseparable from the photomultiplier tube. Photomultiplier tubes are widely used in metallurgy, electronics, machinery, chemical, geological, medical, nuclear, astronomical and space research.

Photo Multiplier Tube


5.1.2 Characteristic of Photo Multiplier Tube

(1) Stability

The stability of the photomultiplier tube is determined by various factors such as the characteristics of the device, the operating conditions and environmental conditions. There are many cases in which the output of the pipe is unstable during the work. The main ones are:

a. Inadequate welding of the electrode inside the tube, loose structure, poor contact of the cathode shrapnel, tip discharge of the pole, flashover, etc., the signal is suddenly large and small.

b. The continuity of the anode output current and the instability of fatigue.

c. The effect of environmental conditions on stability. As the ambient temperature rises, the sensitivity of the tube decreases.

d. Damp environment causes leakage between the pins, causing dark current to increase and instability.

e. Environmental electromagnetic field interference causes work to be unstable.

(2) Extreme working voltage

The limit operating voltage is the upper limit of the voltage that the tube is allowed to apply. Above this voltage, the tube produces a discharge or even a breakdown.


5.1.3 Application of Photo Multiplier Tube

Due to its high gain and short response time, the photomultiplier tube is widely used in celestial photometry and celestial spectrophotometry because its output current is proportional to the number of incident photons. The advantages are: high measurement accuracy, measurement of relatively weak celestial bodies, and measurement of rapid changes in celestial luminosity. Among astronomical photometry, the application of the multiplier tube of the neon cathode, such as RCA1P21. The quantum efficiency of this photomultiplier tube is around 4,200 angstroms, which is about 20%. There is also a photomultiplier tube of a double alkali photocathode, such as GDB-53. Its signal-to-noise ratio is an order of magnitude larger than the RCA1P21, and the undercurrent is very low. In order to observe the near-infrared region, a photomultiplier tube of a multi-alkali photocathode and a gallium arsenide cathode is commonly used, and the latter has a quantum efficiency of up to 50%. A normal photomultiplier can only measure one message at a time, that is, the number of channels is one. matrix.

Since the number of channels is limited by the fine wire at the end of the anode, only a hundred channels are achieved.


5.2 The Difference Between Photo Multiplier Tube and Avalanche Photodiode

In simple terms, the photodiode cannot amplify the signal, and the photomultiplier tube can amplify the signal, so it is generally used for the detection of weak light.

5.2.1 Different Principle

The photodiode is the energy band theory of the semiconductor. When the light illuminates the photodiode, when the energy of the light is greater than the band gap energy, the electrons of the valence band are excited by the conduction band, and the original valence electrons leave holes. Thus, electron-hole pairs are generated in the P region, the N region, and the depletion layer. Under the electric field of the depletion layer, the electrons accelerate toward the N region and the holes to the P region, so that the P region is positively charged, the N region is negatively charged, and each moves (drifts) toward the opposite electrode, thus generating a current. Then, by detecting this current, you can get the information of the light, or you can amplify this current to generate electricity. This is the solar cell.


There are two main principles of photomultiplier tubes: photoelectric effect and secondary electron emission theory. First of all, the photoelectric effect is understood by everyone. When light is incident on the surface of an alkali metal, electrons are emitted. When the kinetic energy of the electron exceeds the work function of the alkali metal, it will escape from the metal surface. If a very sensitive current is used. The current signal can be detected. The second part is amplification, and the escaped electrons are amplified by the dynode, which will increase the current signal and finally output a large current signal at the anode. In contrast to the photomultiplier tube, the photomultiplier tube not only generates current but also amplifies the current, but the principle of generating current is completely different.


5.2.2 The Scope of Application Is Different

This is very clear, where the use of weak light detection generally uses photomultiplier tubes, such as some gamma cameras, detection of radioactive sources, and so on. The application of photodiodes has been briefly explained in the principle and will not be described again.


5.2.3 Manufacturer

Photodiode is a very common optoelectronic component, so there are many manufacturers, you can google, but the production of photomultiplier tube is very few, the world's photomultiplier tube is mainly produced by a company: Japan's coast Song Guangzi, if you are doing photoelectric detection, you should be very familiar with this company, followed by photonis in the UK, but this company has been discontinued after the financial crisis.


Frequently Asked Questions about Avalanche Photo Diode

1.What is avalanche photodiode?
An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode that exploits the photoelectric effect to convert light into electricity. It provides very sensitive light detection.  From a functional standpoint, it can be regarded as the semiconductor analog of photomultiplier.


2. How does avalanche photodiode work?
An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of photomultipliers.


3. What is the difference between PIN photodiode and avalanche photodiode?
The main advantage of the APD is that it has a greater level of sensitivity compared to PIN. The avalanche action increases the gain of the diode many times, providing much higher sensitivity. ... Some diodes that use specialized manufacturing processes enable much higher bias voltages of up to 1,500 V to be applied.


4. In what applications are avalanche photodiodes APD most likely to be used?
Typical applications of avalanche photodiodes include receivers in optical fiber communications, range finding, imaging, high-speed laser scanners, laser microscopy, and optical-time domain reflectometers (OTDR).


5. What are the drawbacks of avalanche photodiode?
Avalanche photodiode disadvantages:
Much higher operating voltage may be required.
Avalanche photodiode produces a much higher level of noise than a PN photodiode.
Avalanche process means that the output is not linear.


6. What are the characteristics of photodiode?
All photodiode characteristics are affected by changes in temperature. They include shunt resistance, dark current, breakdown voltage, responsivity and to a lesser extent other parameters such as junction capacitance.


7. What are the advantages of photodiode?
Advantages of photodiode :
The photodiode is linear
Low resistance
A very good spectral response
Better frequency response
Low dark current
Fastest photodetector
Long lifetime
Low noise


8. What is photodiode and its application?
Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices 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.


9. What is the difference between zener diode and avalanche diode?
The main difference between Zener breakdown and avalanche breakdown is their mechanism of occurrence. Zener breakdown occurs because of the high electric field whereas, the avalanche breakdown occurs because of the collision of free electrons with atoms. Both these breakdowns can occur simultaneously.


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