Download Intro to SiPM

Introduction to SiPMs
NSSC Summer School
What are we Doing Here?
Focus of the talk
Preamp/Filter
FPGA-based data
data acquisition
system
Digital Processing
Hardware toolchain for a generic (light-detection) experiment
Silicon
• 28Si (92%), 29Si(5%), 30Si(2%)
• Z=14, so [Ne]3s23p2
• The two 3s and two 3p electrons are lightly bound
• Tetrahedral structure
similar to diamond.
Not isotropic, solution of
S.W.E. for lattice leads
to bands.
Conduction Band
E
DE Band Gap
Valence Band
Filled Core Bands
3s23p2 electrons
Behavior of electrons and holes in bands
• electrons in conduction band will lose energy and
sink to bottom of band
• holes in valence band will gain energy and rise to
the top of the band
• at T=0 all levels are filled up the the Fermi Level
Insulators and Conductors
k = 8.6 x 10-5 eV/K, so at 300 K kT ~ 1/40 eV
DE >> 1 eV means material is an insulator
DE << 1 eV means material is a conductor
DE ~ 1 eV means material is an semiconductor
Intrinsic Carriers
• At operating temperatures, for silicon the Fermi
level is roughly in the middle of the band gap region
• Let Ec = energy of bottom of conduction band
• Let Ef = Fermi level
• Let Eg = band gap energy
• Then Ec – Ef = Eg/2
Intrinsic carrier density
• ni = T3/2 exp(-Eg/2kT)
• for pure silicon:
ni = ne = nh
But not for DOPED silicon….
Phosphorus Doping
• P is [Ne]3s23p3 - an "extra" electron in 3p
• This "extra" electron has an energy just below the
bottom of the conduction band
• Thermal agitation can take the 3p electron into the
conduction band
• Ef is effectively higher -> closer to conduction band
•
n-type doping
Gallium, Boron, Indium
• empty levels just above the valence band
• Ef is moved closer to the valence band
• "p-type doping"
Number of carriers now changed
• ne = ni exp(Ef – Ei)/kT
• nh = ni exp(Ei – Ef)/kT
• Ei is the intrinsic Efermi and Ef is the new effective
Efermi
• Putting a p-type material in contact with an n-type
material creates a junction
p-n junction
n-type
p-type
E
Carriers migrate into other material, creating
an electric field and depletion region
Depletion region
• The electric field sweeps out any free charge in
that region – hence the name
• The region can be forward or reverse biased.
• Reverse biasing leads to a particle detector!
WHAT HAPPENS HERE?
Below breakdown
Generation due to Ionization due to charged
light absorption
high energy particles
Above breakdown
Impact ionization and avalanche multiplication of electrons
and holes in the presence of a large electric field.
Impact ionization occurs in the depletion region of the diode.
Outside of the depletion region, carriers recombine without separating.
Avalanche Breakdown
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Carriers gain kinetic energy and generate additional electron-hole
pairs through impact ionization.
Multiplication Factor (empirical):
M=
1
Va n
1−
V br
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Single photon sensitivity devices possible by biasing past
breakdown (current gain up to 106)
● Requires quenching circuit to stop the avalanche
1 of 2 breakdown mechanisms. The other is Zener breakdown
Avalanche Photodiode (APD)
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Two-terminal p-n junction device
operated past breakdown
Impact ionization causes an
avalanche of carriers
Can be operated in either proportional
mode or Geiger mode
Semiconductor analog to
photomultiplier tubes
Silicon photomultipliers (SiPMs)
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Arrays (Microcells) of Geigermode operated APDs coupled
by a quenching resistor
Each microcell is of order 10
microns allowing for compact,
robust design
Low breakdown voltage
compared to PMTs
The signal parameters are
practically independent of
external magnetic fields, in
contrary to vacuum PMTs
Single-photon sensitive!
Dark Counts
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Spurious output current pulses
produced in the absence of light
Due to thermal excitation of carriers
from the valence to the conduction
band
Indistinguishable from a photogenerated event
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Primary source of noise
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Very temperature dependent
Optical Crosstalk
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Occurs when an avalanche in one microcell
causes adjacent microcells to fire
Limits practical setting of the gain
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Primarily a function of overvoltage
Afterpulsing
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A release of a trapped charge in a
pixel experiencing an avalanche can
trigger a secondary avalanche while
the pixel is recovering from the
primary avalanche. This is
afterpulsing.
Increases the recovery time (RC
time constant of the quenching
resistor and the junction
capacitance) of the fired pixel, which
degrades the time resolution
characteristic of the SiPM.