Introduction to SiPMs

NSSC Summer School

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!

WHAT HAPPENS HERE?

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.