APPLICATION NOTE
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Introduction to the Silicon
Photomultiplier (SiPM)
AND9770/D
The Silicon Photomultiplier (SiPM) is a sensor that addresses thechallenge of sensing, timing and quantifying low−light signals down
to the single−photon level. Traditionally the province of the
Photomultiplier Tube (PMT), the Silicon Photomultiplier now offersa highly attractive alternative that combines the low−light detection
capabilities of the PMT while offering all the benefits of a solid−state
sensor. The SiPM features low−voltage operation, insensitivity tomagnetic fields, mechanical robustness and excellent uniformity of
response. Due to these traits, the onsemi SiPM has rapidly gained
a proven performance in the fields of medical imaging, hazard andthreat detection, biophotonics, high energy physics and LiDAR.
This document provides an introduction to SiPM sensors,
explaining the working principle and primary performanceparameters.
PHOTON DETECTION WITH SPAD AND SIPM
Photon Absorption in Silicon

When a photon travels through silicon, it may be absorbed andtransfer energy to a bound electron. This absorbed energy causes theelectron to move from the valence band into the conduction band,
creating an electron−hole pair. The absorption depth of a photon insilicon depends on its energy (or wavelength) and is shown in Figure 1.
Silicon efficiently absorbs a wide range of wavelengths of light withina depth of a few tens of microns and so is well−suited as a photodetectormaterial. As the photon absorption is wavelength dependent, it follows
that the resulting photon detection efficiency of a silicon photosensor
will also be wavelength dependent.

Figure 1. The Photon Absorption Depth isthe Depth at Which 1−1/e of the Photons
Have Been Absorbed

Silicon as a PhotodiodeA photodiode is formed by a silicon p−n junction that createsa depletion region that is free of mobile charge carriers. Whena photon is absorbed in silicon it will create an electron−hole pair.
Applying a reverse bias to a photodiode sets up an electric field acrossthe depletion region that will cause these charge carriers to beaccelerated towards the anode (holes), or cathode (electrons).
Therefore an absorbed photon will result in a net flow of current ina reverse−biased photodiode.
The Geiger Mode in Single Photon Avalanche Diodes (SPAD)

When a sufficiently high electric field (> 5  105 V/cm) is generated
within the depletion region of the silicon (achieved by the sensor
design and application of a recommended bias), a charge carrier
created there will be accelerated to a point where it carries sufficientkinetic energy to create secondary charge pairs through a process
called impact ionization. In this way, a single absorbed photon can
trigger a self−perpetuating ionization cascade that will spreadthroughout the silicon volume subjected to the electric field. The
silicon will break down and become conductive, effectively
amplifying the original electron−hole pair into a macroscopic current
flow. This process is called Geiger discharge, in analogy to theionization discharge observed in a Geiger−Müller tube.
 Semiconductor Components Industries, LLC, 2011

August, 2023 − Rev. 9

1

Vbias

Figure 2. Breakdown, Quench and Reset
Cycle of a SPAD Working in Geiger Mode

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breakdown, thus quenching the photocurrent and preventing
further Geiger−mode avalanches from occurring. Once thephotocurrent has been quenched, the voltage across thediode recharges to the nominal bias value. The time it takes
for the microcell to recharge to the full operating voltage iscalled the recovery time. It is important to note that theGeiger avalanche will be confined to the single microcell it
was initiated in. During the avalanche process, all othermicrocells will remain fully charged and ready to detectphotons.
Figure 3. “Digital” Pulse Output froma Single SPAD Working in Geiger Mode

A photodiode operated in Geiger mode employs thismechanism of breakdown to achieve a high gain and isreferred to as a Single Photon Avalanche Diode (SPAD). The
application of a reverse bias beyond its nominal breakdown
voltage creates the necessary high−field gradients across thejunction. Once a current is flowing it should then be stoppedor ‘quenched’. Passive quenching (i.e. no active circuitry),is achieved through the use of a series resistor RQ which
limits the current drawn by the diode during breakdown.
This lowers the reverse voltage seen by the diode to a value
below its breakdown voltage, thus halting the avalanche.
The diode then recharges back to the bias voltage, and isavailable to detect subsequent photons. This cycle ofbreakdown, avalanche, quench and recharge of the bias toa value above the breakdown voltage, is illustrated in
Figure 2.
In this way, a single SPAD sensor operated inGeiger−mode functions as a photon−triggered switch, ineither an ‘on’ or ‘off’ state. This results in a binary output
such as illustrated in Figure 3. Regardless of the number ofphotons absorbed within a diode at the same time, it willproduce a signal no different to that of a single photon.
Proportional information on the magnitude of an
instantaneous photon flux is not available.

Figure 4. An SiPM Consists of an Array of Microcells(Photodiode Plus Quench Resistor) with Summed
Output (The Fast Output is Discussed in
the Fast Output Section)

A typical SiPM has microcell densities of between 100
and several 1000 per mm2, depending upon the size of the
microcell. A simplified electric circuit to illustrate theconcept is shown in Figure 4 and a close−up image ofa portion of the surface of the SiPM is shown in Figure 5.
Each microcell detects photons identically andindependently. The sum of the photocurrents from each ofthese individual microcells combines to form a quasi−anlog
output, and is thus capable of giving information on themagnitude of an instantaneous photon flux. The response to
low−level light pulses is shown in Figure 6, and a charge
spectrum of these pulses is shown in Figure 7.

The Silicon Photomultiplier

To overcome this lack of proportionality, the Silicon
Photomultiplier (SiPM) integrates a dense array of small,independent SPAD sensors, each with its own quenching
resistor. Each independently operating unit of SPAD andquench resistor is referred to as a “microcell”. Whena microcell in the SiPM fires in response to an absorbed
photon, a Geiger avalanche is initiated causinga photocurrent to flow through the microcell. This results ina voltage drop across the quench resistor, which in turn
reduces the bias across the diode to a value below the

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Fast Output

onsemi has developed a unique modification to thestandard Silicon Photomultiplier structure that results ina third terminal (in addition to the anode and cathode)carrying an ultra−fast output signal. The microcell structure
schematic in Figure 8, shows how each microcell has
a capacitively coupled output. This fast output signal is thederivative of the internal fast switching of the microcell in
response to the detection of a single photon. The fast output,
like the anode−cathode output, is formed from the sum of all
microcells, as shown in Figure 8, and so proportional
information on the magnitude of the photon flux can beobtained, in addition to the timing information. A typicalfast output pulse is shown in Figure 9. This signal can beused to make ultra−fast timing measurements, using theability to clearly distinguish the arrival time of the first
photon in the pulse, and also allow higher count rates or thecapability for second photon timing. In addition to fast
timing, the fast output also has considerably lower
capacitance which can be beneficial when designing readout
systems for the sensor. The fast output from a 1 mm sensor
has a capacitance of ~1 pF, whereas, reading out from theanode or cathode would have a capacitance of ~100 pF.

Figure 5. Image Showing the Microcell Structure of
the SiPM Surface

Pulse Shape

A typical SiPM anode−cathode output pulse is shown in
Figure 10. The rise time of the SiPM is determined by therise time of the avalanche formation and the variation in thetransit times of signals arriving from different points on thesensor’s active area. Minimizing this transit time spread by
careful design of the tracking can improve the rise time. In
addition, the rise time is affected by the output impedance ofthe sensor and the package.
The recovery time of the sensor, or decay time of the pulse,
is determined by the microcell recharge time constant,which is given by:

Figure 6. Oscilloscope Shot Showing the Discrete
Nature of the SiPM Output when Illuminated by
Brief Pulses of Low−level Lightt RC + C d ǒR q ) R s @ NǓ(eq. 1)

where Cd is the effective capacitance of the microcell, Rq thevalue of the microcell quench resistor, N is the total numberof microcells in the sensor and Rs is any resistance in series
with the sensor (e.g. sense resistance, signal tracks on thesensor).
Since the capacitance of the microcell will depend upon
its area, the reset time will vary for different microcell sizes,
with a 50 mm microcell SiPM having a significantly longer
reset time than a 10 mm microcell SiPM.
It is important to note that the microcell can still fire
during the recovery time before the overvoltage has
recovered to its nominal value, but the gain will be reduced
in proportion to the reduced overvoltage.
Figure 7. Photoelectron Spectrum of the SiPM,
Achieved using Brief, Low−level Light Pulses,
such as those from Figure 6

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Figure 8. Simplified Circuit Schematic of the onsemi SiPM (Left) Showing each Microcell
which is Composed of the SPAD, Quench Resistor and Fast Output Capacitor (Top Right).
The onsemi SiPM Component Symbol is Shown (Lower Right)

Figure 10. Output Pulse Read Out from theAnode−cathode, Showing the Recovery Time of
the Microcell

Figure 9. The Fast Output Pulse from a 6 mm
Sensor. The FWHM of the Fast Output Allows the
Sensor to Resolve High−repetition, Fast Pulses

Fill Factor

The sensor output is a photocurrent, and the total charge
Q generated from an event is given by,
Q + N fired @ G @ q

The fill factor refers to the percentage of the SiPM sensor
surface area that is sensitive to light. Due to the structure of
the SiPM, each microcell needs to be separated from its
neighbor for optical and electrical isolation purposes. In
addition, some surface area is required for the quench
resistor and signal tracks. All of these considerations result(eq. 2)where Nfired is the number of fired microcells, G is the gain
(discussed in Gain section) and q is the electron charge. The
total charge is also equal to the integral of the photocurrent
pulse.

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in a ‘dead space’ around the microcell. The separation
between microcells and the space required for the quench
resistor and tracking is more or less constant, regardless ofthe microcell size. Therefore, larger microcells result ina higher percentage of active surface area (active area/totalarea).
A higher fill factor (larger microcells) results in higher
PDE and gain, but also in higher capacitances, longer
recovery times and lower dynamic range.
A lower fill factor (smaller microcells) results in lower
PDE and gain, but also in lower capacitances, shorterrecovery times and higher dynamic range.

The trade off between fill factor and the typical number ofmicrocells in a 1 mm sensor is shown in Figure 11. The
factors involved in this trade off will be discussed in moredetail in the following sections. Specific values for theseparameters are always in the onsemi datasheets on our
website.
SILICON PHOTOMULTIPLIER PERFORMANCEPARAMETERS
Users of SiPM sensors will typically need to have an
understanding of the following parameters: breakdown
voltage and overvoltage, gain, PDE, dark count rate (noise),
optical crosstalk, afterpulsing and temperature dependency.
The following sections define these parameters, anddescribe how they are measured. All plots used here are
typical and for reference only. Please refer to the current
datasheets on the website for the most up−to−dateinformation on all of the parameters discussed here.
Breakdown Voltage and Overvoltage

The breakdown voltage (Vbr) is the bias point at which theelectric field strength generated in the depletion region is
sufficient to create a Geiger discharge. The Vbr point isclearly identified on a current versus voltage plot by thesudden increase in current, as in Figure 12. The precise
value of the Vbr is determined as the value of the voltage
intercept of a straight line fit to a plot of /I vs V, where I is
the dark current and V is the voltage, as shown in Figure 13.
SiPM sensors are operated at a bias point that is typically
10%−25% higher than the Vbr. The difference between the
Vbr and the bias point is referred to as the ‘overvoltage’(DV). Both the Vbr and the recommended DV range are
given in the onsemi product datasheets. These values allow
the user to calculate the Vbias to apply.

Figure 11. Typical Fill Factors and Mirocell
Numbers for a 1 mm Sensor from the C−Series
Product Family

Vbias + Vbr ) DV

Figure 12. Dark Current vs Voltage Plots for 3 mm
Sensors with 20 mm, 35 mm, and 50 mm Microcells(eq. 3)Figure 13. The Breakdown Voltage is Determined fromthe Intercept of a Linear Fit to the Plot of /I vs. V

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Figure 15. Responsivity as a function of
wavelength for a 3 mm, 35 mm Microcell SiPM,
at +2.5 V and +5 V Overvoltage

Figure 14. Gain as a Function of Over−voltage forDifferent Microcell Size SiPMs(10 mm, 20 mm, 35 mm, and 50 mm)
Gain

structure of the sensor. The PDE is the statistical probability
that an incident photon interacts with a microcell to produce
an avalanche, and is defined as:

The gain of an SiPM sensor is defined as the amount ofcharge created for each detected photon, and is a function of
overvoltage and microcell size, as shown in Figure 14.
Each microcell in the SiPM generates a highly uniformand quantized amount of charge every time an avalanche isgenerated by an absorbed photon in the active volume. The
gain of a microcell (and hence the sensor) is then defined as
the ratio of the chargefrom an activated microcell to thecharge on an electron. The gain can be calculated from theovervoltage DV, the microcell capacitance C, and theelectron charge, q.
G+

C @ DVq

PDE(l, V) + h(l) @ e(V) @ F(eq. 5)where h(l) is the quantum efficiency of silicon, e(V) is theavalanche initiation probability and F is the fill factor of thedevice. The quantum efficiency is a measure of thelikelihood of an incident photon creating an electron−holepair in the sensitive volume of the sensor. The avalanche
initiation probability takes into account the fact that not all
charge carriers in the active volume will go on to initiate anavalanche. The fill factor is the ratio of active to inactive areaon the SiPM, and is a result of the deadspace between the
microcells and therefore depends upon the microcell size.
The PDE is commonly calculated from the responsivity ofthe sensor. The responsivity is defined as the average
photocurrent produced per unit optical power and is givenby:(eq. 4)The unique way in which the SiPM operates can be used
to accurately measure the gain. Each detected photon results
in a highly quantized output pulse, as was shown in Figure 6.
If the charge from each pulse is integrated and a charge
spectrum is formed, the peaks due to successive numbers ofdetected photons will be clearly visible. Such a spectrum
was shown in Figure 7. The separation between each pair ofadjacent peaks (in pC) is constant and corresponds to thecharge generated from a single fired microcell. This cantherefore be used to accurately calculate the gain, using theequation above.

R+

Ip
P op

(eq. 6)

where Ip is the measured photocurrent and Pop is the incident
optical power at a particular wavelength over the sensor
area. The responsivity is typically expressed in units ofAmps per Watt (A/W). Figure 15 shows a typical plot of themeasured SiPM responsivity. When measuring the
responsivity, low intensity light pulses should be used toensure that the sensor is not saturated and care should betaken to have an accurate reference sensor. It should be notedthat the measured responsivity will include contributions
from optical crosstalk and afterpulsing (see Optical
Crosstalk and Afterpulsing sections).

Photon Detection Efficiency and Responsivity

The photon detection efficiency (PDE) is a measure of thesensitivity of an SiPM and is a function of wavelength of theincident light, the aplied overvoltage and microcell fill
factor.
The PDE differs slightly from the quantum efficiency(QE) that is quoted for a PMT or APD, due to the microcell

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Figure 16. PDE as a Function of Wavelength for
Different Overvoltages

Figure 18. Dark Count Rate as a Function of
Overvoltage for a 3 mm, 35 mm Microcell SiPM

Figure 17. PDE as a Function of Overvoltage fora 6 mm, 35 mm Microcell SiPM

Figure 19. DCR as a Function of Threshold for
Different Biases

The PDE can be determined from the responsivity R using
the relation,

crosstalk probabilities that are measured separately and are
discussed in Optical Crosstalk and Afterpulsing sections of
this document respectively. More information on themeasurement of PDE can be found in [1]. A typical PDE
versus wavelength plot is shown in Figure 16. The PDE plot
is used to define the peak wavelength (lp ) and thewavelength range. A plot of the PDE at the peak wavelength
versus overvoltage is shown in Figure 17.

PDE +

R@h@c
@ 100%
l @ G @ e @ (1 ) P AP) @ (1 ) P XT)(eq. 7)

where h is the Planck constant, c is the speed of light, l is the
wavelength of the incident light, G is the gain, and e is theelementary charge. PAP and PXT are the afterpulsing andwww.onsemi.com
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Dark Count Rate

Optical Crosstalk

The main source of noise in an SiPM is the dark count rate
(DCR), which is primarily due to thermal electrons
generated in the active volume. The DCR is a function ofactive area, overvoltage and temperature.
Each dark count is a result of a thermally generated
electron that initiates an avalanche in the high field region.
The signals resulting from the breakdown of the microcell,
due to either photon−generated or thermally−generated
electrons, are identical. Therefore, thermally generated
electrons form a source of noise at the single photon level.
If a threshold can be set above the single photon level thenfalse triggers from the noise can be significantly reduced.
However, the dark counts will contribute to the measured
signal.
Since the dark count is comprised of a series of pulses, its
magnitude is quoted as a pulse rate (kHz), or pulse rate perunit area (kHz/mm2). For continuous or current integration
measurements, it is sometimes more convenient to consider
the contribution as a ‘dark current’ which is also quoted on
the onsemi datasheets.
The DCR can be measured with a simple counting system,
setting a threshold set at the half−photon level. The DCR
increases with increasing bias as shown in Figure 18,
creating an inherent trade−off between increasing PDE and
DCR. The DCR also increases with increasing temperatureand linearly with the active area, so a 3 mm sensor will
typically have 9 times the DCR of a 1 mm sensor.

An additional component of SiPM noise is that of opticalcrosstalk between microcells. Optical crosstalk is a functionof overvoltage and is also affected by the fill factor ofa sensor.
During avalanche, accelerated carriers in the high fieldregion will emit photons that can initiate a secondary
avalanche in a neighboring microcell. These secondary
photons tend to be in the near infrared (NIR) region and can
travel substantial distances through the silicon. Typically
2x10−5 NIR photons are emitted per electron crossing thejunction. As shown in Figure 20, the secondary photons cantravel to neighboring microcells via a number of routes:a) directory to a neighboring microcell, b) reflected from thewindow material on the top of the sensor (usually epoxy or
glass), or c) reflected from the bottom of the silicon
substrate.
The crosstalk is defined as the probability that anavalanching microcell will cause an avalanche in a second
microcell. The process happens instantaneously and asa consequence, single incident photons may occasionallygenerate signals equivalent to 2 or 3 photons, or even higher.
This effect can be seen in Figure 21, which shows the output
of a sensor in the dark, and so all of the signal pulses are due
to dark counts. The central dark count pulse shown in
Figure 21 has a pulse height that is double that of the otherdark counts that are at the single photon level, and so will be
due to a crosstalk event. The optical crosstalk can be
estimated by measuring the ratio of the count rate at thesecond photon level to the count rate at the single photon
level. Figure 19 shows the result of plotting the DCR asa function of threshold position, which produces a step−likeplot. The flat regions of the curve correspond to thefrequency of dark counts at the single, double, triple photon
level etc. Crucially, this plot shows that with a thresholdlevel set at the level of a ~3 photons, the dark count trigger
rate will be negligible.
Figure 22 shows the crosstalk increasing withovervoltage, which is due to the increased number of carriers
crossing the junction due to higher gain. There is an inherent
trade off between PDE (maximized using a highovervoltage) and cross talk (minimized using a lower
overvoltage). Therefore, plots of the type in Figure 23 canbe useful. The subject of optical crosstalk in SPAD arrays is
discussed in detail in by Rech et al. [2] and crosstalkmeasurement in SiPM sensors is also discussed by Eckert etal. [1].

Figure 20. An Illustration of the Various Ways inwhich Secondary Photons can Travel to
Neighboring Microcells to Cause Optical Crosstalk

Figure 21. Oscilloscope Shot Showing Dark Counts

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Afterpulse probability increases with overvoltage (due toincreased avalanche initiation probability and SiPM gain).
Eckert [1] discusses afterpulsing in more detail including its
measurement in SiPM sensors.
Dynamic Range and Linearity

The dynamic range of a given SiPM sensor can be definedas the optical signal range over which the sensor provides
a useful output.
The dynamic range is a function of the total number ofmicrocells, the overvoltage used, and the wavelength of theincident photons. The relationship between the number ofincident photons (Nph ) and the number of fired microcells(Nfired ) is given as,
N fired(M, DV, l) + Mǒǒ

1 * exp *PDE(DV, l) @ N phMǓǓ(eq. 8)where M is total number of microcells, l is the wavelengthof the incident photons, DV is the overvoltage and PDE is thephoton detection efficiency. An example plot generatedusing this equation for 1 mm, 3 mm and 6 mm sensors isshown in Figure 24. It can be seen that the sensor has a linear
response at lower photon fluxes, but as the number ofincident photons increases the sensor output becomesnon−linear. This deviation from linearity is linked to the total
number of microcells in the sensor: more microcells resultin a larger dynamic range. Therefore, for a given sensor size,
smaller microcells will result in a larger dynamic range thanlarger microcells. A general rule of thumb to maintain an
approximately linear response, is to match the maximum
expected power to ~70% of the SiPM range, which is moreclearly seen in Figure 25. Linearity is discussed further in
the application note, Linearity of the SiPM.

Figure 22. Crosstalk vs Overvoltage for 3 mm SiPM
with 35 mm Microcells

Figure 23. PDE vs Crosstalk for 3 mm SiPM with
20 mm and 35 mm Microcells
Afterpulsing

During breakdown, carriers can become trapped in
defects in the silicon. After a delay of up to several ns, thetrapped carriers are released, potentially initiating anavalanche and creating an afterpulse in the same microcell.
Afterpulses with short delay that occur during the recovery
time of the microcell tend to have negligible impact as the
microcell is not fully charged. However, longer delay
afterpulses can impact measurements with the SiPM if therate is high. onsemi devices have particularly low
afterpulsing due to process defect reduction. The afterpulse
probability is determined by measuring the statistical
distribution of consecutive pairs of dark counts, when
triggered at one half of the single photoelectron amplitude.

Figure 24. The SiPM Photocurrent as a Function of
Incident Power on a Log−log Scale. The SiPM
Maintains a Linear Response at Higher Levels of
Incident Light when there are more Microcells

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effective overvoltage, which in turn affects many of the
SiPMs performance characteristics.
For stable operation in the presence of significant
temperature fluctuations, either bias compensation or
thermal regulation should be considered. Bias compensationis the simpler of the two methods. The bias is automaticallyadjusted in response to a change in the temperature to ensure
a constant overvoltage is maintained.
If a constant overvoltage is maintained, most parameters,such as gain, PDE and timing, will remain the same as atroom temperature. However, regardless of constant
overvoltage, the DCR will be altered by a change intemperature, as shown in Figure 27, due to a change in thegeneration rate of thermal electrons in the sensitive volume.
An increase in temperature will increase the dark countrate, and the converse is also true: for every 10C reduction
in device temperature, there is a 50% decrease in the darkcount rate. Therefore actively cooling an SiPM will
significantly reduce the DCR.
Figure 25. The SiPM Photocurrent as a Function of
Incident Power for a 6 mm Sensor on a Linear
Scale. As a Rule of Thumb, Linearity Drops Off at
about 70% of the Full SiPM Range

Figure 27. The Impact of Temperature Change on
Dark Count Rate (DCR) for Different Bias Values
Figure 26. The Impact of Temperature Change on
the Operating Voltage, for a onsemi Sensor and an
SiPM with a Higher Breakdown Voltage

OTHER USEFUL INFORMATION
Diode Structure

The diode can be fabricated as either a P−on−N type or
N−on−P type, as illustrated in Figure 28. Since electrons
have a higher probability than holes of initiating anavalanche, better efficiency can be achieved when the initial
photon interacts in the p−type layer. This maximizes theflow of electrons through the high−field avalanche region.
Therefore, to maximize sensitivity to shorter wavelength(blue) photons, a P−on−N structure is more efficient, whilean N−on−P structure can give better sensitivity to longer
wavelength photons (green−red).

Temperature Dependency

The primary effects of temperature change on the SiPM
are a change in the breakdown voltage and the dark countrate.
The breakdown voltage changes linearly as a function oftemperature, as shown in Figure 26. The effect for onsemi
SiPM is small (21.5 mV/C) and so in many applications
compensation for this change may not be required.
However, if large temperature fluctuations are experienced
and not compensated for, it will result in changes in the

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P−ON−N
Depletion
RegionN−ON−P
P

N

N

P

Depletion
RegionFigure 28. Different Diode Structures. P−on−N Gives Best Sensitivity to Shorter Wavelengths
and N−on−P for Longer Wavelengths
Direct Detection of Radiation with the SiPM

To date the photomultiplier tube (PMT), a wellestablished and widely available vacuum tube device, hasbeen the sensor of choice for many applications. The
semi−transparent photocathode deposited inside theentrance window inherently limits the QE that can beachieved, with typical PMTs having about 35% at 420 nm.
A gain of 1x106 is achieved at the cost of a high bias voltageof 1−2 kV, which requires the use of costly high−voltage
power supplies. PMTs are generally stable and low noise but
are bulky and delicate due to their vacuum−tube structure.
They can also be adversely affected by magnetic fields
which will limit their suitability for some applications.
Solid state devices have many practical advantages overthe PMT, and this has led to the PIN diode being used inapplications where PMTs were too bulky or delicate, orwhere high voltages are not possible. However, PIN diodes
are severely limited by their complete lack of internal gain.

It is possible to use some silicon sensors, such as the PINor APD, for the direct detection of radiation such as X−raysor gamma rays. This is because the photon produces
ionization in the sensitive volume, which is then translated
into a proportional output by the sensor. However, direct
detection of radiation with the SiPM is not generally
possible. This is for two reasons: The sensitive volume in the
SiPM is of the order of a few microns deep, which provides
very poor stopping efficiency for all but the lowest of X−ray
energies. Secondly, the ionization produced by the radiation
would not yield a proportional response. Assuming all of theionization occurs in one microcell, then the signal of ‘1’ isproduced, regardless of the amount of ionization actuallydeposited. Therefore, no proportional (energy) information
could be recorded about the incident radiation.
COMPARISON WITH OTHER SENSORTECHNOLOGIES
Low−light photon sensors constitute the enabling
technology for a diverse and rapidly growing range ofapplications: nuclear medicine, hazard and threat detection,high energy physics, biophotonics, and LiDAR all require
sensors that quantify and/or time−stamp light signals withanywhere from one to several thousands of photons per
event. The ideal photosensor provides a response
proportional to the incident photon flux and incorporates aninternal gain mechanism, yielding signals of sufficientmagnitude as to be easily processed. It should offersub−nanosecond response times and broad spectral
sensitivity, be robust, easy to operate and only generatemanageable amounts of noise or dark count rates.

Figure 30. APD Sensors

The Avalanche Photodiode (APD) is another technology, an
extension of the simple PIN diode. Here the reverse bias is
raised to a point where impact ionization allows for someinternal multiplication, but is below the breakdown bias
where the Geiger mode would take over. In this way, a gainof around a 100 is achieved for a bias of 100−200 V. With
special manufacture, it is possible for gains of several
thousand to be reached using an HV bias of > 1500 V. Whilst
the gain may be lower than that of a PMT, APDs have theadvantage of a QE which can be > 65% and are also
compact, rugged and insensitive to magnetic fields. Theirmain drawbacks are their excess noise (associated with thestochastic APD multiplication process) and in an important
trade−off: The capacitance increases with increasing device

Figure 29. A Typical PMT

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photoelectron spectrum is a feature of the SiPM which is
generally not possible with PMTs due to the variability in thegain, or excess noise. Further technical details on onsemi’sfull range of SiPM sensors can be found atwww.onsemi.com.

area and decreasing thickness, whereas the transit times of
the charge carriers increase with increasing thickness,
implying a performance trade−off between noise andtiming. They are limited in size to ~10 mm diameter.
The SiPM has high gain (1x106) and PDE (>50%)
combined with the physical benefits of compactness,ruggedness and magnetic insensitivity. In addition, the
SiPM achieves its high gain with very low bias voltage(~30 V) and the noise is almost entirely at the single photon
level. Because of the high degree of uniformity between the
microcells the SiPM is capable of discriminating the precise
number of photons detected as distinct, discrete levels at theoutput node. The ability to measure a well resolved

REFERENCES[1] P. Eckert et al., “Characterisation studies of silicon
photomultipliers”, Nucl. Instrum. Methods Phys.
Res. A 620, 217–226 (2010).
[2] I. Rech et al., “Optical crosstalk in single photon
avalanche diode arrays: a new complete model”,
Optics Express Vol. 16, No. 12, 8381 (2008).

onsemi,, and other names, marks, and brands are registered and/or common law trademarks of Semiconductor Components Industries, LLC dba “onsemi” or its affiliates
and/or subsidiaries in the United States and/or other countries. onsemi owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property.
A listing of onsemi’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. onsemi reserves the right to make changes at any time to any
products or information herein, without notice. The information herein is provided “as−is” and onsemi makes no warranty, representation or guarantee regarding the accuracy of theinformation, product features, availability, functionality, or suitability of its products for any particular purpose, nor does onsemi assume any liability arising out of the application or useof any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer is responsible for its products
and applications using onsemi products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information
provided by onsemi. “Typical” parameters which may be provided in onsemi data sheets and/or specifications can and do vary in different applications and actual performance mayvary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. onsemi does not convey any licenseunder any of its intellectual property rights nor the rights of others. onsemi products are not designed, intended, or authorized for use as a critical component in life support systems
or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices intended for implantation in the human body. Should
Buyer purchase or use onsemi products for any such unintended or unauthorized application, Buyer shall indemnify and hold onsemi and its officers, employees, subsidiaries, affiliates,and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or deathassociated with such unintended or unauthorized use, even if such claim alleges that onsemi was negligent regarding the design or manufacture of the part. onsemi is an Equal
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