Semiconductor Detectors
description
Transcript of Semiconductor Detectors
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Semiconductor Detectors It may be that when this class is taught
10 years on, we may only study semiconductor detectors
In general, silicon provides Excellent energy resolution Excellent charge carrier collection properties Excellent position resolution (EPP) High density (versus gas e.g.) On the negative side, they are subject to
radiation damageSemiconductor detectors are found in
many fields of physical research and industry
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Semiconductor DetectorsLet’s look at the energy required to
produce a signal Scintillation detectors – 1 photon / 100 eV Ionization detectors – 1 ion pair / 16 eV Silicon detectors – 1 electron-hole pair / 3.6
eV
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Semiconductor Detectors In EPP, the main use of silicon detectors is
for precision tracking Finn showed an expression for the momentum
resolution of a “tracker” in a magnetic field
Additionally, silicon detectors are used for b-quark tagging b quarks are an indication of interesting
physics b-quarks ~ 1.5 ps Distance traveled in lab = c ~ 4500 m
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B-quark Tagging
SVT (secondary vertex tagging)
IP (impact parameter)
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Secondary vertex
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b = distance of closest approach of a
reconstructed track to the true interaction point
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B-quark Tagging
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Silicon Intrinsic silicon
Egap (valence – conduction) = 1.12 eV Intrinsic electron density n = hole density p =
1.45 x 1010/cm3 (300K)300K=1/40 eV
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SiliconOther properties of pure (intrinsic) silicon
There are alternatives to silicon Germanium (Ge), diamond, gallium arsenide
(GaAs), silicon carbide (SiC), …But the silicon’s wide technology base
makes it the usual choice for a detector
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SiliconConsider an Si detector 1 cm x 1
cm x 300 m In this volume there will be 4.5 x 108
free charge carriers A mip will produce 3.2 x 104 electron-
hole pairs Not a great particle detector!
In order to make a useful detector we need to reduce the number of free charge carriers
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Dopingn-type
Replace Si with P, As, Sb (donor) Electrons (holes) are majority (minority)
carriers
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Dopingp-type
Replace Si with B, Al, Ga, In (acceptor) Holes (electrons) are majority (minority)
carriers
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DopingThe result of doping is to increase the
number of charge carriers by adding impurity levels to the band gap n-type p-type
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DopingTypical impurity concentrations are 1012-
1018 / cm3
Detector grade silicon (1012 / cm3) Electronics grade silicon (1017 / cm3) To be compared with silicon density of 1022 / cm3
More heavily doped concentrations (1018-1020 / cm3) are called p+ or n+
In nearly all cases, the impurity concentrations are large compared with the intrinsic carrier concentration (1010/cm3) n ~ ND for n-type p ~ NA for p-type
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p-n Junction Majority carriers diffuse into the boundary Resulting exposed donor (+) and acceptor (-)
atoms build up an E field that halts further diffusion
A thin (< 100 m) depletion region (no free charge carriers) is created at the boundary
No current flows (at equilibrium)
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p-n Junction
(a)Current flow
(c)Electric field
(b)Charge density
NA > ND
(d)Electrostatic potential
o : built in potential under zero bias
o
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Forward Bias p-n Junction Positive on p side, negative on n side The electrons can easily overcome the (~1V)
contact potential Current easily flows across the junction even for
small values of forward bias voltage The depletion region becomes smaller
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Reverse Bias p-n Junction Negative on p side, positive on n side Majority carriers are swept away from the
boundary region and the depletion region becomes larger
Little current flows across the boundary Unless the reverse bias voltage becomes large
enough to overcome the space charge in the depletion region
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Reverse Bias p-n Junction Most silicon detectors are reversed biased
p-n junctions The charged carrier concentration in the
depletion region is now very low (~<100 / cm3)
Electron-hole pairs created by ionizing particles will be quickly swept out of the depletion region by the electric field
The motion of these electron-hole pairs constitutes the basic signal for particle detection
As in gas detectors, the electrical pulse on the electrodes arises from induction caused by movement of the electrons and holes rather than the actual collection of the charge itself
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Diodep-n junction is what makes a diode
Note there is a diode “drop” of ~0.7V to get current flowing in the forward bias region
With one exception, the breakdown (Peak Inverse Voltage) region usually destroys a diode
PIV
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Diode
p-type n-typeanode cathode
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Depletion DepthThe depletion region acts like a capacitor
It is often the case that electronic noise is the dominant noise source hence it is desirable to have the detector capacitance as small as possible Large V and large d
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Semiconductor DetectorsMany varieties
Si strip detector Si pixel detector Si drift chamber CCD (Charged Coupled Device) Surface barrier PIN photodiode Avalanche photodiode a-Se + TFT (Thin Film Transistor)
arrays