1. Introduction to Silicon Photo-Multiplier
Pankaj Rakshe
* Ref. – Raghunandan Shukla , “Introduction to Silicon Photo-Multiplier”, WAPP2012

2. Why ?
Sensitive Photo-detectors like PMT’s , HPD’s are of great interest to scientific community due to their use in examining processes emitting very low photon flux.
For example,
In GRAPES-3 they are used to detect scintillation light from fibres .
PET scanners
Florescence imaging of the cells

3. Let’s have a look at Classical PMT
Gain ~ 106
Size : Dia 1”, Length 6”
Operating Voltage ~ 2000 V
Quantum Efficiency ~ 30 %
Response time ~ 2 ns
Effected by Magnetic fields
Incoming photon knocks out electron from photo cathode
Emitted electron is focused on electrode
Electrons are multiplied by series of dynodes due to presence of electric field

4. Avalanche Photo Diode :
A solid state Photo-detector
Very small in size
Generally used in reverse bias mode for photo-detection
Gain depends upon biasing voltage
Typical Application Circuit
Typical reverse I-V characteristics

5. APD : Mode of operations
Proportional
Mode
Geiger
Mode
Avalanche Photo Diode (APD) can be used in two modes
Proportional Mode
APD is biased under it’s breakdown voltage
Output pulse height is proportional to the number of incident photons
Gain < 1000
Basically functions as Amplifier
Geiger Mode
APD is slightly above its breakdown voltage (~10%)
Output pulse height is independent of number of incident photons
Gain ~ 106
So, basically in Geiger mode APD functions as a Discriminator

6. Hybrid Photo-Diode
* Photonis HPD Catalog

7. Comparison of Detectors
PMT
APD
HPD
Large Gain (106)
High Cost
Sensitive to Magnetic Field
Bulky
Low Gain (~100)
Low Cost
Insensitive to Magnetic Field
Small Size (Solid State Device)
Small Gain (~1000)
Cost as High as PMT
Insensitive to Magnetic Field
Small Size (Solid State Device)
* S. R. Dugad in WAPP2012

8. Geiger Mode Operation
A very high electric field is created in depletion region of the diode by applying high reverse bias voltage.
Incident photons knocks out a carrier ( electron or hole).
This carrier gains enormous amount of energy accelerating through high electric field
Accelerated carrier imparts energy to more carriers coming in its way (Impact Ionization)
This process goes on and thus an Avalanche of carriers is formed.
This avalanche has to stopped in order to limit the current and avoid damage of the device.

9. Which mode to choose ?? We want.. Solution : Proportional Mode
Output is proportional to the number of input photons.
Large dynamic range
Can not detect single photon
Low gain
Geiger Mode
Output is a digital signal, indicating presence or absence of photon.
Information about input photons is lost.
Can detect single photon
High Gain
We want..
A device with good features from both the modes i.e device with large dynamic range and high gain
Solution :
Silicon Photo Multiplier ( SiPM)

10. Silicon Photo-Multiplier (SiPM)
Compact Device
Operating voltage (30-120V)
Resolution - Single photon detection
Response time – ~100 ps
High gain
High Quantum Efficiency – 90%
High Photon Detection Efficiency – 60%
Immunity to Magnetic Field
1-3 mm
1-3 mm

11. SiPM
SiPM is a 2-D array of Avalanche Photo Diode’s (APD’s) , all resistively coupled together.
SiPM is generally biased above its breakdown voltage , called as Geiger mode.
Each pixel (APD) acts as a binary device, indicating presence or absence of photon. Device as whole gives analog signal indicating number of pixels fired.
Typical size of each APD (pixel) is 50 µm × 50 µm and a typical gain of ~ 106

12. Passive Quenching – Simple !
Once the avalanche develops, it is important to stop the current build-up, so that device is not damaged and it is ready for next detection.
It is achieved with the help of series resistance connected
As the current through APD (pixel) increase, voltage drop across series resistance increases
This effectively decreases voltage across diode and quenches the Avalanche
Current Pulses
Diode Voltage
Geiger
Mode
Proportional
Mode
Quenching
Pulse Output
Recharge

13. Incident Photon knocks out an electron.
SiPM Operation
Incident Photon knocks out an electron.
Electron is accelerated in high electric field at junction generated due high reverse bias
High energy (accelerated) electron imparts energy to electrons in its path and thus due to such multiplication avalanche occurs
This avalanche is quenched by high series resistance and the device is brought back to its operating conditions

14. Typical SiPM pulse..
The signal presents 2 components: 1. Avalanche current reproduced at the output by parasitic capacitor 2. slow component due to the recharge of the diode capacitance (Recovery time ~70ns)

15. SiPM Specifications 1) Quantum Efficiency :
𝑄𝐸= 𝑁𝑜. 𝑜𝑓 𝑃ℎ𝑜𝑡𝑜𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑁𝑜. 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠
QE for SiPM ( Silicon) is more than 90%
2) Photon Detection Efficiency :
𝑃𝐷𝐸=𝑄𝐸 ×𝐺𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 ×𝐵𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 𝑝𝑟𝑜𝑏𝑎𝑏𝑖𝑙𝑖𝑡𝑦
Due to Resistors used for quenching and other features like Guard rings, some dead area is introduced.
𝐺𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦= 𝐴𝑐𝑡𝑖𝑣𝑒 𝑎𝑟𝑒𝑎 𝑇𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑑𝑒𝑣𝑖𝑐𝑒
Breakdown probability depends upon applied bias voltage and it can be increased to almost %

16. Individual photon peaks well resolved !
SiPM Specifications
3) Linearity SiPM behaves as a perfectly linear device under low light flux. Dynamic Range = 1 to Number of of pixels
1 p.e
2 p.e
4 p.e
5 p.e
3 p.e
0 p.e
Individual photon peaks well resolved !
* Hamamtsu MPPC user manual

17. Timing Resolution
Timing information shown for SiPM includes laser pulse width and delay due to electronics
Timing resolution ~ 100 ps
For comparison timing resolution of one of the best PMT is shown
* P. Buzhan et al. / NIM in Physics Research A 504 (2003) 48–52

18. So far we have seen that SiPM offers exceptionally good features like small size, high dynamic range, high QE, fast response and immunity to magnetic field etc.
But..
SiPM is not perfect !

19. SiPM Noise– Dark Counts ( Primary Noise)
Due to thermal energy, valance band carrier enters into conduction band and gives rise to an avalanche.
Such pulses look exactly like genuine photon event and thus can not be distinguished.
Typical dark count rate ~ few MHz at room temperature
Dark counts increase as ambient temperature increases.
* Claudio Piemonte, FNAL ,October 25th 2006

20. SiPM Noise– After Pulsing (Noise -2)
Carrier Trapping and Delayed Release  Afterpulsing
During avalanche, carriers are trapped due to defects in the crystal and released after some time.
This leads to generation secondary pulse following primary pulse.
At lower temperature trap lifetime increases
* S.Cova, A.Lacaita, G.Ripamonti, IEEE EDL (1991)

21. SiPM Noise- 3 – Cross Talk
Hot-Carrier Luminescence
A. Lacaita et al, IEEE TED (1993)
105 avalanche carriers  1 emitted photon
Counteract:
Optical isolation between pixels
Avalanche charge minimization
F.Zappa et al, ESSDERC (1997)

22. SiPM Biasing 𝑽 𝑺 𝑻 = 𝑽 𝑩𝑹 𝑻 +𝑶𝒗𝒆𝒓 𝑽𝒐𝒍𝒕𝒂𝒈𝒆 Geiger mode: above breakdown
Gain
Geiger mode: above breakdown
VS = VBR + Over-Voltage
Gain dependant on amount of Over-voltage applied
But, Breakdown Voltage is dependent on temperature
Effectively, gain will also change! (3-5%/˚C)
Over-Voltage should be constant for constant gain
𝑽 𝑺 𝑻 = 𝑽 𝑩𝑹 𝑻 +𝑶𝒗𝒆𝒓 𝑽𝒐𝒍𝒕𝒂𝒈𝒆
Biasing Voltage (V)
* Bajarang Sutar
Proportional Mode and Geiger mode
Geiger mode -> high gain and hence single photon detection possible but array of APDs will be able to detect multiple photons.
Gain of the device: ratio of Output electrons and initial electrons
Gain dependant on electric field applied i.e. overvoltage
Breakdown voltage temperature dependant
Considering that SiPM is used For critical applications like medical, nuclear physics expt. Where results should be appropriate the gain variation will cause trouble.
* S. R. Dugad and K. C. Ravindran

23. Solutions to Temperature Dependency
Constant Temperature
Indoor Applications - applicable
Temperature control of detectors in outdoor environment is not possible e.g. GRAPES-3 experiment containing detectors in large area outdoor field of about m2 with temperature variations ˚C
Temperature Dependant Biasing Conditions
The Bias Voltage of SiPM can be controlled for changing the operating point
Temperature dependent Power Supply that will keep over-voltage constant (i.e. Gain constant)
Two solutions to Gain stability
Constant temperature : possible for indoor application
But for outdoor applications: biasing change is option

24. Approaches To Temperature Compensation
Method Used
Author
Year
Remarks
Limitations
Dark Current Control
Miyomoto et al.
2009
Dark Current and Temperature relation approximated to exponential function and use of Thermistor (similar relation) for compensation
Uses Expensive Commercial Power Supply
Limited to one/two channels
External control required for temperature compensation
Li et al.
2012
Dark current and Bias Voltage relation used for designing the voltage controlled current sink that changes bias condition
Bias Voltage Control
Bencardino et al.
Use of Temp. to voltage converter and a Op-amp based circuit for changing the bias return potential w.r.t. temperature variations
Licciulli et al.
2013
Blind SiPM used as a temperature sensor and amplitude of Dark pulses of Blind SiPM is maintained constant using Op-amp based feedback circuit for constant gain of other SiPM in parallel
Gil et al.
2011
External Input of the power supply is controlled by a micro-controller based system to change the output voltage w.r.t. temp.
Dorosz et al.
LabVIEW based feedback system for controlling the power supply output
Explain the approaches used till now
Commercial power supply is used. --- costly, single channel, no temperature compensation provision, high current (not necessary)
SiPM is in still research stage. New structures are coming everyday.
Hence only one or two channels are sufficient for purpose of research.
But if one needs to replace the PMT with SiPM, then for large scale experiment (~1000 detectors), mutiple commercial power supplies are not an option.
Very costly and not feasible (3.5 Lacs cost) Hence need of multichannel power supply with temperature compensation at low cost

25. A Temperature Compensated Power Supply for Silicon-Photomultiplier

26. Specifications Sr. No. Parameter Value 1. Output Voltage 0 to 100V 2.
Temperature Reading Resolution
0.1 ˚C 3.
Temperature Compensation Factor
10 to 100 mV/˚C 4.
Number of Channels 8 5.
Output Voltage Resolution
10 mV 6.
Maximum Current Limit per Channel
100 µA 7.
Full Scale Leakage Current per Channel
40 µA

27. Block Diagram High Voltage Generation (Voltage Multiplier Chain) PC
USB
Control Unit (Micro-controller)
Voltage Regulation Scheme
Temperature Sensors
DAC
Divided into following parts:
High Voltage generation
High Voltage Regulation
Control Unit: microcontroller and PC
Current Sense
Current Sense
SiPM

28. Prototype Power Supply (SiPM-PPS-v1)

29. Features of SiPM Power Supply
Programmable Output Voltage ( V) with resolution of ~12 mV
Programmable Temperature Compensation Factor (~12 – 100 mV/˚C)
In-built Data Acquisition System for recording Temperature and Leakage Current via USB

30. Ripple = 5 mVP-P 0.04113% for 10% change in line voltage
No load to Full load (100uA) regulation %
Ripple = 5 mVP-P
Stable within 10 mV for 0 – 100 V

31. Compensation Algorithm Verification
Comp. factor = 240 mV/˚C
Comp. Factor = (0.029x2-1.4x) /˚C

32. SiPM Experimental Setup

33. SiPM Test: Without Compensation
Gain Variation of about 4 %/˚C

34. SiPM Test: With Compensation
Gain Variation of 0.8 %

35. In Conclusion Without compensation gain variation nearly 4 %/˚C
With compensation gain variation %/˚C
Output Ripple of 5 mV with the Resolution of 12.5 mV in 100 V with temperature correction at each 0.2 ˚C change

36. Compared to Keithley 6487 Unit
SiPM Programmable Power Supply
Temperature Compensation Feature
Small Size and Light Weight (portable)
Multi-channel (8/16 channels)
Cost – Inexpensive !!!
(₹ 750/channel) which is 400 times less
Voltage Source and Pico-ammeter

37. SiPM-PPS-v2 Number of channels = 16 Better output resolution = 6.25 mV
DAC resolution = 14 bits
Better current sensing resol. = 1 nA
ADC resolution = 16 bits
Provision for expandability
I2C for multiple boards

38. Specifications Comparison
Sr. No.
Parameter
Required Value
Prototype Board
SiPM_PPS v2 1.
Output Voltage
0 to 100V
0 to 100 V 2.
Number of Channels 8 16 3.
Output Voltage Resolution
10 mV
12.5 mV
6.25 mV 4.
Maximum Current Limit per Channel
100 µA 5.
Full Scale Leakage Current per Channel
40 µA
35 µA 6.
Temperature Reading Resolution
0.1 ˚C 7.
Temperature Compensation Factor
12.5 to 100 mV/˚C
6.25 to 100 mV/˚C

39. Summary
SiPM has some outstanding features that in some respect could replace PMT in near future.
Newer versions of SiPM are being developed to overcome its limitations.
The temperature dependence is one of the major limitation preventing the use of SiPM in outdoor applications. The programmable temperature compensated power supply is useful for operating the SiPM in different environmental conditions.

40. Acknowledgment Prof. S. R. Dugad (TIFR) Prof. P. D. Khandekar (VIIT)
Mr. Sergey Los (FNAL)
Mr. Raghunandan Shukla (TIFR)
Ms. Sarrah Lokhandwala (TIFR)
Prof. C. S. Garde (VIIT)
Prof. S. K. Gupta (TIFR)