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Semiconductor particle detectors

PARTICLE PHYSICS 粒子物. Semiconductor particle detectors. Pierpaolo Palestri DPIA, University of Udine ( palestri@uniud.it ) Thanks to: M.Cobal, A.Cristofoli, G.Dalla Betta, A.Dalla Costa, M.P.Giordani, G.Giugliarelli, A.Micelli, A.Pilotto, L.Selmi, A.Vacchi. PARTICLE PHYSICS 粒子物.

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Semiconductor particle detectors

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  1. PARTICLE PHYSICS 粒子物 Semiconductor particle detectors Pierpaolo Palestri DPIA, University of Udine (palestri@uniud.it) Thanks to: M.Cobal, A.Cristofoli, G.Dalla Betta, A.Dalla Costa, M.P.Giordani, G.Giugliarelli, A.Micelli, A.Pilotto, L.Selmi, A.Vacchi

  2. PARTICLE PHYSICS 粒子物 Detectors in high-energy physic • 3 Layers: B-Layer, 286 modules Layer-1, 494 modules Layer-2 676 modules • 6 disks: 144 modules 25 m ATLAS detector Pixel detector 7000 t 46 m 35 cm Pixel detector module 1,3 m

  3. PARTICLE PHYSICS 粒子物 What a detector does • the incoming particle generates free carriers inside a semiconductor • the electric field inside the device separates the charges and creates a current at the terminals • the electrical signal is amplified by an electronic read-out

  4. PARTICLE PHYSICS 粒子物 Outline • Semiconductors • p-n junction • Main figures of merits for detectors • Effect of radiation on performance • From planar to 3D structures

  5. PARTICLE PHYSICS 粒子物 Semiconductors (1) • group IV: Si, Ge • III-V compounds (GaAs, InP, InAs, GaN…) • Alloys (Si1-xGex, In1-xGaxAs) • CdTeSe, ZnTeSe, CdZnTe • behave as conductive or insulating materials by adding impurities a/o changing the device bias

  6. PARTICLE PHYSICS 粒子物 Semiconductors (2) • crystal structure of silicon 8 electrons in the outer shell simplified 2D view

  7. PARTICLE PHYSICS 粒子物 Free carriers: electrons and holes the hole that is left can move too: free carrier with positive charge one electron can break the bond and become a free carrier (negative charge)

  8. PARTICLE PHYSICS 粒子物 Energy bands single atom: energy levels crystal: energy bands free electrons electrons in the outer shell

  9. Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si PARTICLE PHYSICS 粒子物 Doping (1) As B doping with acceptors: B has 3 electrons in the outer shell and this results in a hole that can move in the crystal doping with donors: As has 5 electrons in the outer shell one is not needed for bonding and becomes a free electron

  10. Conduction Band E c E Donor Level d Donor ionization energy Acceptor ionization energy Acceptor Level E a E v Valence Band PARTICLE PHYSICS 粒子物 Doping (2) electron from the donor easily jumps to the conduction band electron in the valence band easily captured by the acceptor a hole is left in the valence band

  11. PARTICLE PHYSICS 粒子物 Doping (3) It can be shown that with free electron concentration hole concentration intrinsic density (1010cm-3 in Si@room temperature) Charge neutrality: Example: Na=1016cm-3 , Nd=0  p=1016cm-3 , n=104cm-3 Note: density of atoms in Si is 51022cm-3

  12. electron E c photons E g photon energy: h v > E g E v hole PARTICLE PHYSICS 粒子物 e-h generation by radiation • the photon promotes an electron in the conduction band • a hole is left in the valence band • if h>> Eg: number of pairsh/Eg • The same happens with many types of particles, not only photons • To measure the generated e-h pairs we need to separate electrons and holes  p-n junction

  13. PARTICLE PHYSICS 粒子物 Outline • Semiconductors • p-n junction • Main figures of merits for detectors • Effect of radiation on performance • From planar to 3D structures

  14. – + V I I N P V Forward bias Reverse bias PARTICLE PHYSICS 粒子物 Structure • N-region: • donor doping; • plenty of free electrons • balanced by fixed positive charge • P-region: • acceptor doping; • plenty of free holes • balanced by fixed negative charge region where the detectors operate

  15. PARTICLE PHYSICS 粒子物 Charge profile (1) at the border between the N and P region we have very few free carriers N P • neutral N-region: • donor doping; • plenty of free electrons • balanced by fixed positive charge • neutral P-region: • acceptor doping; • plenty of free holes • balanced by fixed negative charge depletion region: only fixed charges

  16. PARTICLE PHYSICS 粒子物 Charge profile (2) N P n 0 p0 =qND n 0 p0 =-qNA n=ND p0 =0 p=NA n0 =0 • neutral N-region: • donor doping; • plenty of free electrons • balanced by fixed positive charge • neutral P-region: • acceptor doping; • plenty of free holes • balanced by fixed negative charge depletion region: only fixed charges

  17. A a e r a ( x + D x ) E ( x ) E r D x x PARTICLE PHYSICS 粒子物 Poisson equation Gauss’s Law: s: permittivity (~12o for Si) : charge density (C/cm3) Poisson’s equation

  18. PARTICLE PHYSICS 粒子物 Electric field profile (1) N P =qND =-qNA =0 =0 E dE/dx=-qNA /S dE/dx=qND/S x Area=applied potential + built-in different chemical potential of N and P region (1V)

  19. PARTICLE PHYSICS 粒子物 Electric field profile (2) Area=applied potential + built-in E N P • positive electric field  electrons (holes) generated by radiation go to the N (P) region x

  20. PARTICLE PHYSICS 粒子物 Electric field profile (3) N N+ P+ W E dE/dx=qND /S dE/dx=-qNA+ /S dE/dx=-qNA /S x • insertion of a low-doped region: trapezoidal electric field profile  same “area” with lower peak electric field

  21. PARTICLE PHYSICS 粒子物 Electric field profile (4) • doping in the P+ and N+ regions is large so that: W E dE/dx=-qNA /S x • to fully deplete the low doping region • this however requires:

  22. PARTICLE PHYSICS 粒子物 Outline • Semiconductors • p-n junction • Main figures of merits for detectors • Effect of radiation on performance • From planar to 3D structures

  23. PARTICLE PHYSICS 粒子物 Spatial resolution • the structure needs to be split in stripes or pixels pixels stripes cross section

  24. PARTICLE PHYSICS 粒子物 Collected charge • the longer the particle travels in the detector the larger the induced charge • collected charge proportional to WA where A is the pixel area. • large W is required, • but the condition for depletion implies using high bias and very low doping levels (high purity crystals) V W

  25. PARTICLE PHYSICS 粒子物 Dark current depletion width area generation time

  26. PARTICLE PHYSICS 粒子物 Breakdown voltage (1) high electric field results in unwanted e-h pair generation by impact ionization a free electron hits an electron in the valence band obtaining an additional free electron + a hole the same process can be initiated by a hole  the dark current increases significantly

  27. PARTICLE PHYSICS 粒子物 Breakdown voltage (2) dE/dx=-qNA /S E • reduce peak electric field as much as possible  low doping W x • avoid breakdown at the borders  guard rings

  28. PARTICLE PHYSICS 粒子物 Breakdown voltage (3) multiplication by II can be exploited as internal signal amplification (Avalanche Photo Diodes)

  29. PARTICLE PHYSICS 粒子物 Speed • generated e-h pairs need to reach the high-doping regions W carrier velocity 107cm/s

  30. PARTICLE PHYSICS 粒子物 Capacitance • The electronic read-out converts the generated charge into a signal • it adds noise (unwanted random signal) during the process • this noise is amplified by the detector capacitance • C should be as small as possible W area of the pixel

  31. PARTICLE PHYSICS 粒子物 Outline • Semiconductors • p-n junction • Main figures of merits for detectors • Effect of radiation on performance • From planar to 3D structures

  32. PARTICLE PHYSICS 粒子物 Effective doping radiation experienced during the experiments creates defects that act as additional (unwanted) doping higher and higher biases are needed during the detector lifetime to deplete it, since:

  33. PARTICLE PHYSICS 粒子物 Increased dark current radiation experienced during the experiments creates defects assisting unwanted e-h generation the dark current increases reduced when defect density increases

  34. PARTICLE PHYSICS 粒子物 Charge in the oxide • radiation experienced during the experiments creates positive charges in the SiO2 layer on top of the structure • free electrons are induce inside Si and shorts the pixels • additional p-doped regions to maintain isolation SiO2

  35. PARTICLE PHYSICS 粒子物 Outline • Semiconductors • p-n junction • Main figures of merits for detectors • Effect of radiation on performance • From planar to 3D structures

  36. PARTICLE PHYSICS 粒子物 3D detectors (1) W W d • planar structure: trade-off between large W (for collection efficiency) and small W (for high speed and low depletion voltage) • 3D detector: large W for high collection efficiency, small d for high speed and low depletion voltage

  37. PARTICLE PHYSICS 粒子物 3D detectors (2) • sample performance for 3D and planar detectors • vertical electrodes are more difficult to fabricate • p-spray/p-stop needed for isolation after irradiation • 3D are faster and with lower depletion voltage • higher capacitance

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