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What are we Doing Here?
Preamp/FilterFPGA-based datadata acquisition
systemDigital Processing
Hardware toolchain for a generic (light-detection) experiment
Focus of the talk
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 BandConduction Band
Valence BandValence Band
Filled Core BandsFilled Core Bands
DE Band GapE
3s23p2 electrons
Behavior of electrons and holes in bands
• electrons in conduction band will lose energy andsink to bottom of band
• holes in valence band will gain energy and rise tothe 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 Fermilevel 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 thebottom of the conduction band
• Thermal agitation can take the 3p electron into theconduction 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-typematerial creates a junction
p-n junction
n-typen-type p-type p-type
ECarriers migrate into other material, creatingan electric field and depletion region
Depletion region
• The electric field sweeps out any free charge inthat region – hence the name
• The region can be forward or reverse biased.
• Reverse biasing leads to a particle detector!
Generation due tolight absorption
Ionization due to chargedhigh energy particles
Impact ionization and avalanche multiplication of electronsand holes in the presence of a large electric field.
Below breakdown
Above breakdown
Impact ionization occurs in the depletion region of the diode.Outside of the depletion region, carriers recombine without separating.
Avalanche Breakdown● Carriers gain kinetic energy and generate additional electron-hole
pairs through impact ionization.
● Multiplication Factor (empirical):
● Single photon sensitivity devices possible by biasing pastbreakdown (current gain up to 106)● Requires quenching circuit to stop the avalanche
● 1 of 2 breakdown mechanisms. The other is Zener breakdown
M= 1
1−|V a
V br|n
Avalanche Photodiode (APD)
● Two-terminal p-n junction deviceoperated past breakdown
● Impact ionization causes anavalanche of carriers
● Can be operated in either proportionalmode or Geiger mode
● Semiconductor analog tophotomultiplier tubes
Silicon photomultipliers (SiPMs)● Arrays (Microcells) of Geiger-
mode operated APDs coupledby a quenching resistor
● Each microcell is of order 10microns allowing for compact,robust design
● Low breakdown voltagecompared to PMTs
● The signal parameters arepractically independent ofexternal magnetic fields, incontrary to vacuum PMTs
● Single-photon sensitive!
Dark Counts● Spurious output current pulses
produced in the absence of light
● Due to thermal excitation of carriersfrom the valence to the conductionband
● Indistinguishable from a photo-generated event
● Primary source of noise
● Very temperature dependent
Optical Crosstalk● Occurs when an avalanche in one microcell
causes adjacent microcells to fire
● Limits practical setting of the gain
●
● Primarily a function of overvoltage
Afterpulsing
● A release of a trapped charge in apixel experiencing an avalanche cantrigger a secondary avalanche whilethe pixel is recovering from theprimary avalanche. This isafterpulsing.
● Increases the recovery time (RCtime constant of the quenchingresistor and the junctioncapacitance) of the fired pixel, whichdegrades the time resolutioncharacteristic of the SiPM.