Avalanche photodiode

Avalanche photodiodes or avalanche photodiodes ( English avalanche photodiode , APD ) are highly sensitive, fast photodiodes and count among the avalanche diodes . They use the internal photoelectric effect to generate charge carriers and the avalanche effect for internal amplification . They can be regarded as the semiconductor equivalent of the photomultiplier and are used for the detection of very low radiation powers , right down to individual photons, with limit frequencies up to the gigahertz range. The highest spectral sensitivity is, depending on the material used, in a range of approx. 250–1700 nm, whereby one type of diode can only cover a partial range. Hybrid photodetectors are a mixture of photomultipliers and avalanche photodiodes .

Avalanche photodiode

Layout and function

Schematic layer structure of a Si-APD. The color gradients represent the space charge distribution when reverse voltage is applied, with the associated electrical field strength distribution (below).

The avalanche of charge in a Si-APD caused by impact ionization. Photons are absorbed in the completely depleted intrinsic i-layer and generate charge carrier pairs there. In a Si-APD the electrons are accelerated towards the multiplication zone and cause the charge avalanche there.

Avalanche photodiodes are designed for a controlled avalanche breakdown and are similar in structure to pin photodiodes . In contrast to the pin-layer structure of these diodes, the space charge distribution is modeled by an additional narrow and highly doped p- or n-layer (see middle figure) in such a way that, following the intrinsic i- or π-layer, a very high electric field strength distribution is generated (see figure below). This area acts as a so-called multiplication zone and generates the internal amplification of the avalanche photodiodes. A typical Si -APD has a p ⁺ -ipn ⁺ - doping profile , with the weakly p-doped intrinsic i or π layer serving as the absorption region , as in the case of the pin diode . When a reverse voltage is applied, the free electrons generated there by photons drift into the multiplication zone , which is generated by the space charge zone of the pn ⁺ junction. The charge carriers are then strongly accelerated due to the high electrical field strength prevailing there and generate secondary charge carriers through impact ionization , which in turn are accelerated and in turn generate further charge carriers (see figure below). Si avalanche photodiodes are operated with reverse voltages, close to the breakdown voltage , of a few 100 V and achieve a gain of M = 100 ... 500 (multiplication factor). Above the breakdown voltage, the described process progresses like an avalanche (avalanche breakdown), which leads to a (short-term) gain factor of several million.

Multiplication and additional noise factor

The amplification is caused, as described above, by impact ionization of the free charge carriers, whereby, depending on the material, both electrons and holes are used for multiplication in the multiplication zone. The decisive factors are the ionization coefficients of the electrons α _n and the holes α _p , which depend exponentially on the electrical field strength. The charge carriers with the larger ionization coefficient are injected into the multiplication zone in order to achieve optimal and low-noise amplification (e.g. for silicon α _n  > α _p and for germanium and indium phosphide α _n  <α _p ).

That of the applied voltage U _R dependent multiplication factor M is obtained below the breakdown voltage U _BD approximately as follows ( I · R _S is the voltage drop across the series resistance of the diode):

${\displaystyle M={\frac {1}{1-\left({\frac {U_{R}-IR_{S}}{U_{\mathrm {BD} }}}\right)^{n}}}}$ ; with n  <1 depending on the structure and the material of the diode.

Due to the statistical nature of the charge carrier multiplication, the gain is not constant and it comes in addition to the thermal noise (Johnson-Nyquist noise) to an increased shot noise (engl. Shot noise ). This can lead to a deterioration in the signal-to-noise ratio with large amplifications . The excess noise is given with the additional noise factor F ( M ) as follows:

${\displaystyle F(M)=kM+(1-k)\left(2-{\frac {1}{M}}\right)}$,

where k is the ratio of the ionization coefficients of the electrons and holes (for α _n > α _p , k = α _p / α _n or for α _n <α _p , k = α _n / α _p ). It follows from this that the difference in the ionization coefficients should be as large as possible in order to minimize the noise. For Si k ≈ 0.02 and for Ge and III-V compound semiconductors such as InP k ≈ 0.5.

Time response and gain-bandwidth product

The time behavior of an avalanche photodiode is determined by the drift processes in the depletion area and the build-up and breakdown of the charge carrier avalanche in the multiplication zone , which means that APDs are slower than pin or Schottky photodiodes . The transit or transit time of the charge carriers in the multiplication zone largely determines the time constant  Mk, which is directly proportional to the gain M and the ratio of the ionization coefficients k (typical values ​​are in the order of magnitude of  ≈ 1 ps). The constant gain-bandwidth product ( english product gain bandwidth , GBP, GBW od GB.) Gives to himself: ${\displaystyle \tau \!\,\sim }$ ${\displaystyle \tau \!\,}$

${\displaystyle MB=Mf_{\text{c}}={\frac {1}{2\pi \tau }}}$( is the cutoff frequency at which the current decreases by 3 dB). ${\displaystyle f_{\text{c}}}$

A gain-bandwidth product for Si of approx. 200 GHz and for Ge of approx. 30 GHz, as well as for InGaAs-based APDs of> 50 GHz can be achieved. Furthermore, the greatest possible difference in the ionization coefficients of the charge carriers is advantageous here, as well as the narrowest possible multiplication zone.

Materials, structure and spectral sensitivity

VIS-NIR Si and Ge-APD

Schematic cross section of a Si-APD (1 metal contacts, 2 anti-reflective layer made of silicon dioxide or nitride)

Silicon is the most frequently used material because, due to the large difference in the ionization coefficients of the charge carriers, particularly low-noise avalanche photodiodes can be produced. The spectral sensitivity ranges from 300–1000 nm, depending on the version. The highest sensitivity is achieved by NIR-Si-APDs (500–1000 nm) with a maximum spectral sensitivity of approx. 800–900 nm. The types optimized for the short-wave frequency range can up to approx. 300 nm (maximum spectral sensitivity at approx. 600 nm), which is made possible by an absorption zone located near the surface. This is necessary because the penetration depth of the photons decreases with decreasing wavelength. On the other hand, the band gap E _g limits the maximum detectable wavelength, and the limit wavelength λ _g results in:

${\displaystyle \lambda _{g}={\frac {hc}{E_{g}}}={\frac {1{,}24\mathrm {\mu m\cdot eV} }{E_{g}}}}$( is Planck's quantum of action and the speed of light ) ${\displaystyle h}$ ${\displaystyle c}$

for Si (with E _g  = 1.12 eV at 300 K) a value of 1100 nm (for λ> λ _g the corresponding material becomes transparent).

Germanium can be used for avalanche photodiodes in the wavelength range above 1000 nm, as required in fiber optic communications technology. Due to the lower energy of the band gap of E _g  = 0.67 eV (at 300 K), a spectral sensitivity range of 900–1600 nm is achieved. However, the disadvantage of Ge-APDs is the high additional noise factor (the ionization coefficient of the holes α _p is only slightly greater than that of the electrons α _n ) and the high dark current present.

IR-APD with heterostructure made of III-V compound semiconductors

SAM band structure of an InP / GaInAs APD

Avalanche photodiodes made of III-V compound semiconductors have been developed for the fiber optic transmission technology in the 2nd and 3rd window ( 1300 and 1550 nm ) , which have better properties than Ge-APDs, but are significantly more expensive to manufacture. Lower additional noise factors and dark currents are achieved through the combination of III-V compound semiconductors with different band gaps, with InGaAs / InP APDs being the main representatives. In so-called SAM structures ( English separate absorption and multiplication ), indium gallium arsenide (InGaAs) is used as the absorption zone and indium phosphide (InP) as the multiplication zone. Typical layer structure is:

p ⁺ _InP , p _InP , n _InGaAs , n ⁺ _InP or

p ⁺ _InP , n _InP , n  _InGaAs , n ⁺ _InP .

Due to its large band gap of E _g  = 1.27 eV (at 300 K), InP has a lower dark current and, due to a more favorable ratio of the ionization coefficients (α _n <α _p ), a lower- noise gain can be achieved than in InGaAs. The holes serve as primary charge carriers and are injected from the InGaAs absorption zone into the weakly doped n _InP or p _InP multiplication zone. It is crucial that the ratios are chosen so that the electric field strength in the InP layer is high enough for charge carrier multiplication and the InGaAs layer is completely depleted, but at the same time low enough to avoid tunnel currents in the absorption zone . By adjusting the indium and gallium content, the band gap of In _x-1 Ga _x As can be:

${\displaystyle E_{g}=(0{,}36+0{,}63x+0{,}43x^{2})\,\mathrm {eV} }$ Set (at 300 K) between 0.4 and 1.4 eV.

For the absorption zone, for. B. In _0.53 Ga _0.47 As used, with a band gap of E _g  = 0.75 eV, with which a spectral sensitivity range similar to that of germanium can be achieved (900–1600 nm). An extension of this range beyond 1600 nm ( L band ) could be achieved by increasing the In content in the absorption zone to In _0.83 Ga _0.17 As, with these APDs an additional In _0.52 Al _0.48 As layer is used as a multiplication zone.

Due to the discontinuity of the energy bands at the boundary of the heterostructure, a potential step arises which leads to the accumulation of holes in the valence band and to a delay in the time behavior and to the limitation of the bandwidth of the APD. So-called SACM structures ( separate absorption, grading and multiplication ), where an InGaAsP (grading) layer is inserted between the absorption and multiplication zone, with a band gap that is between that of InGaAs and InP (0, 75–1.27 eV). A typical layer structure of a SAGM-APD is as follows:

p ⁺ _InP , n _InP , n ⁺ _InGaAsP , n _InGaAs , n ⁺ _InP .

Further developments are SAGCM structures ( separate absorption, grading, charge sheet and multiplication ) and superlattice avalanche photodiodes with further improved noise and amplification properties.

UV-APD made of (Al) GaN and SiC

In recent years, special avalanche photodiodes have been developed for the ultraviolet wavelength range from 250-350 nm, which are based on gallium nitride (GaN) or (4H) silicon carbide . Due to the large band gap of E _g ^GaN  = 3.37 eV or E _g ^4H-SiC  = 3.28 eV, these APDs are relatively insensitive in the solar spectrum ( solar blind ) or in the visible spectral range. You therefore do not need any expensive optical filters to suppress undesired background radiation, as is necessary with the photomultipliers or Si-APDs typically used in this area . Furthermore, they show better properties than PMTs in harsh environments and in high temperature applications, such as. B. the detection or monitoring of flames ( e.g. from gas turbines ) or for gamma ray detection in deep drilling for oil and gas exploration .

With the help of organometallic gas phase epitaxy (MOVPE), APDs in pin and SAM structure made of gallium nitride and aluminum gallium nitride (AlGaN), e.g. B. Al _0.36 Ga _0.64 N as absorption zone , on sapphire substrates (with an AlN interface) can be produced. Quantum efficiencies of up to 45% (at 280 nm) can be achieved and the detection of individual photons in the so-called Geiger mode could be demonstrated.

APDs made of 4H-SiC are far superior in their properties. They are more durable and show little additional noise due to a favorable ratio of the ionization coefficients of the charge carriers of k  ≈ 0.1. In contrast to the direct band gap of GaN, the decrease in sensitivity towards the visible spectral range is not as sharp. Quantum efficiencies of up to 50% (at 270 nm) can be achieved and the detection of individual photons in Geiger mode could also be demonstrated.

Operating modes

Operation proportional to radiation

Below the breakdown voltage, there is a blocking voltage and temperature-dependent gain and avalanche photodiodes can be used to build highly sensitive photodetectors with an output voltage proportional to the output voltage, the APD itself acting as a current source proportional to the radiation output . Silicon APDs have a higher equivalent noise power than, for example, pin photodiodes (since the amplification effect is subject to stochastic mechanisms), but they can still be used to build lower-noise photoreceivers, since with conventional photodiodes the noise contribution of the downstream amplifier is currently much higher than that the pin photodiode . APD amplifier modules are available that compensate for the temperature-dependent gain factor of the APD by adjusting the reverse voltage.

Geiger mode (single photon detection)

Single Photon Avalanche Diode (SPAD)

Avalanche photodiodes (APD) that are specific for operation above the breakdown voltage of the so-called Geiger mode were to be developed as a single photon avalanche diode (short SPAD for engl. Single-photon avalanche diode ) or Geiger-mode APD (G -APD). They achieve a short-term gain of up to 10 ⁸ , because an electron-hole pair generated by a single photon can generate several million charge carriers due to the acceleration in the multiplication zone (caused by the high electric field strength ). Appropriate wiring must prevent the diode from remaining conductive due to the high current (self-preservation of the charge carrier avalanche), which in the simplest case is implemented by a series resistor. The voltage drop across the series resistor lowers the reverse voltage across the APD, which then returns to the locked state ( passive quenching ). The process repeats itself automatically and the current pulses can be counted. With active quenching , special electronics actively lower the reverse voltage when a breakdown current is detected within a few nanoseconds. The SPAD is then reactivated by increasing the reverse voltage above the breakdown voltage. The signal processing of the electronics results in dead times of approx. 100 ns and count rates of approx. 10 MHz can be achieved. In 2011, dead times of 5.4 ns and count rates of 185 MHz with active quenching were achieved experimentally .

Silicon photomultiplier (SiPM)

A silicon photomultiplier (SiPM), size of the array approx. 1 mm²

The so-called silicon photomultiplier (SiPM for short) consists of an array of several avalanche photodiodes on a common silicon substrate, which are operated in Geiger mode, i.e. above the breakdown voltage. Each APD cell (size 10… 100 µm) has its own series resistor and all cells (100… 1000) are connected in parallel. The idea is to be able to detect individual photons (high sensitivity) and still be able to measure many photons at the same time. The component works almost analogously up to a certain light intensity, because the pulses of the individual cells add up and each cell still has time to extinguish.

SiPM combine the advantages of PMT and solid-state sensors, they do not require high operating voltages, are insensitive to impacts and magnetic fields and are smaller.

literature

• E. Hering, K. Bressler, J. Gutekunst: Electronics for engineers and natural scientists. Springer, 2005, ISBN 3-540-24309-7 .
• Hari Singh Nalwa: Photodetectors and Fiber Optics. Academic Press, 2001, ISBN 0-12-513908-X .
• Kwok K. Ng: Complete Guide to Semiconductor Devices. 2nd edition, John Wiley & Sons, 2002, ISBN 0-471-20240-1 .
• Simon M. Sze : Physics of Semiconductor Devices. 2nd edition, John Wiley & Sons, 1981. ISBN 0-471-05661-8 .
• MS Tyagi: Introduction to Semiconductor Materials and Devices. John Wiley & Sons, 1991, ISBN 0-471-60560-3 .

Web links

• Technology of KETEK-SiPM
• Avalanche Photodiodes Tutorial (Olympus Microscopy Recource Center - Digital Imaging in Optical Microscopy).
• Detection of the smallest amounts of light with avalanche photodiodes (LASER COMPONENTS GmbH 2009, PDF; 223 kB).

Individual evidence

1. ↑ ^{a b c d e f} E. Hering, K. Bressler, J. Gutekunst: Electronics for engineers and natural scientists . Springer, 2005, ISBN 978-3-540-24309-0 , pp. 266-279 .
2. ↑ ^{a b c} Avalanche Photodiodes - A User Guide. PerkinElmer 2010 (PDF; 80 kB).
3. ^ ^{A b c d e} Kwok K. Ng: Complete Guide to Semiconductor Devices (Mcgraw-Hill Series in Electrical and Computer Engineering) . McGraw-Hill Inc., US, 1995, ISBN 978-0-07-035860-7 , pp. 425-432 .
4. ↑ ^{a b c} Changlin Ma: Characterization and modeling of SAGCM InP / InGaAs Avalanche Photodiodes Dissertation SIMON FRASER Univ. 1995, pp. 22-44 (PDF file; 5.1 MB)
5. ↑ ^{a b c d} Hari Singh Nalwa: Photodetectors and Fiber Optics . Academic Press, 2001, ISBN 978-0-12-513908-3 , pp. 334-351 .
6. ↑ ^{a b} D. Haško, J. Kováč, F. Uherek, J. Škriniarová, J. Jakabovič and L. Peternai: DESIGN AND PROPERTIES OF InGaAs / InGaAsP / InP AVALANCHE PHOTODIODE. (PDF file; 480 kB) Journal of ELECTRICAL ENGINEERING, VOL. 58, NO. 3, 2007, 173-176
7. ↑ ^{a b c} Takashi Mikawa, Masahiro Kobayashi, Takao Kaneda: Avalanche photodiodes: present and future. Proceedings of SPIE Vol. 4532 - Active and Passive Optical Components for WDM Communication, pp.139-145
8. ↑ ^{a b} Functionality and areas of application of Avalanche photodiodes ( Memento from October 17, 2006 in the Internet Archive ) LASER COMPONENTS GmbH 2006 (PDF file, 165 kB)
9. ↑ ^{a b} Datasheets APD Modules ( Memento from February 2, 2014 in the Internet Archive ) LASER COMPONENTS GmbH (PDF file, 229 kB)
10. ↑ Characteristics and use of Si APD (Avalanche Photodiode) Hamamatsu Photonics KK 2004 (PDF file, 340 kB)
11. ↑ D. Gustedt, W. Wiesner: Fiber optic transmission technology . Franzis' Verlag GmbH, 1998, ISBN 978-3-7723-5634-6 , pp. 54 .
12. ↑ Basic Parameters of In _x-1 Ga _x As at 300 K NSM data archive from the Ioffe Institute , St. Petersburg
13. ↑ JK Park, I. Yun: Modeling of Avalanche Gain for High-speed InP / InGaAs Avalanche Photodiodes. Electron Devices and Solid-State Circuits, 2008, 1-4
14. ↑ ^{a b} Joe C. Campbell et al. a .: Recent advances in avalanche photodiodes . In: IEEE Journal of Selected Topics in Quantum Electronics . tape 10 , no. 4 , July 2004, p. 777–787 , doi : 10.1109 / JSTQE.2004.833971 ( ece.ucsb.edu [PDF; accessed February 1, 2014]).
15. ^ P. Friedrichs, T. Kimoto, L. Ley, G. Pensl: Silicon Carbide . tape 2 : Power Devices and Sensors . Wiley-VCH Verlag GmbH & Co. KGaA, 2010, ISBN 978-3-527-40997-6 , pp. 467-480 .
16. ↑ GA Shaw et al. (MIT Lincoln Laboratory), S. Soloviev et al. (GE Global Research): Deep UV Photon-Counting Detectors and Applications. ( Memento of February 1, 2014 in the Internet Archive ) (PDF; 1019 kB) Proc. of SPIE - Advanced Photon Counting Techniques III, Volume 7320, 2009.
17. ↑ P. Sandvik et al .: Harsh Environment Solid-State Gamma Detector for Down-hole Gas and Oil Exploration, Phase II - Final Technical Report. GE Global Research, DOE Award Number: DE-FC26-04NT42107, 2007
18. ^ R. McClintock et al .: Solar-blind avalanche photodiodes. In: Proc. of SPIE - Quantum Sensing and Nanophotonic Devices III. Vol. 6127, 2006.
19. ↑ Ryan McClintock, Jose L. Pau, Kathryn Minder, Can Bayram, Manijeh Razeghi .: III-Nitride photon counting avalanche photodiodes. ( Memento from January 16, 2014 in the Internet Archive ) (PDF file; 510 kB) In: Proc. of SPIE - Quantum Sensing and Nanophotonic Devices V. Volume 6900, 2008, 69000N, doi: 10.1117 / 12.776265 .
20. ^ Joe C. Campbell et al .: Low-Dark-Current SiC Avalanche Photodiodes. 210th Meeting of The Electrochemical Society, Meet. Abstr. 2006, Volume MA2006-02, No. 28, Pages 1320. ( ma.ecsdl.org PDF).
21. ↑ M. Stipčević, H. Skenderović, and D. Gracin: Characterization of a novel avalanche photodiode for single photon detection in VIS-NIR range. In: Optics Express. Volume 18, No. 16, 2010, pp. 17448-17459.
22. ↑ A. Eisele et al .: 185 MHz count rate, 139 dB dynamic range single-photon avalanche diode with active quenching circuit in 130 nm CMOS technology. Intl. Image Sensor Workshop (IISW'11), Hokkaido, Japan; 2011, R43 (PDF; 298 kB).
23. ↑ Introduction to silicon photomultipliers (SiPMs). Company brochure of First Sensor AG (Germany), PDF, 6 pages, English, accessed on July 14, 2017.
24. ↑ Introduction to SiPM. Corporate font of the company SensL Technologies Ltd. (Ireland), PDF, 16 pages, English, accessed July 14, 2017.
25. ^ Slawomir Piatek: A technical guide to silicon photomultipliers (SiPM). ( Memento from July 8, 2017 in the Internet Archive ) Website of Hamamatsu Photonics , English.