04 - Semiconductor detectors
Jaroslav Adam
Czech Technical University in Prague
Version 2
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Semiconductor diode detectors
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Semiconductor diode detectors
First used in early 1960s, called crystal counters
Density of solid medium 1000 times bigger than for the gas
Large number of carriers per radiation event improves energy resolution
Electron-hole pair created along the particle trajectory
Compact size, fast timing
Limitation by production of small-size devices, sensitive to radiation damage
Dominantly silicon, germanium for gamma-ray measurements
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Band structure in solids
Electrons in allowed bands in crystalline periodic lattice, bands separated by gaps
Valence band for outer-shell bound electrons confined to the lattice, responsible forinteratomic forces within the crystal
Conduction band of electrons freely migrating across the crystal
Bandgap is separation of these two, classifies material as semiconductor or insulator, 5 eV forinsulators
Crystal electrically neutral, valence band filled unless thermal excitation
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Charge carriers
Bandgap crossed due to thermal energy of the electron
Vacancy (hole) in valence band, represents absence of a negatively charged electron
Electron-hole pair is analog to ion pair in gas
Conductivity my movement of electrons and holes
Probability of thermal electron-hole formation per unit time
p(T ) = CT 3/2 exp(−
Eg
2kT
)(1)
where T is absolute temperature, Eg bandgap energy k Boltzmann constant and C ischaracteristic material constant
Electron-hole pair recombination without external electric field, equilibrium in concentration ofthe pairs
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Diffusion of electrons and holes
Random thermal motion, broadening of carrier distribution with time t according diffusioncoefficient D
σ =√
2Dt (2)
Diffusion coefficient given by mobility µ, Boltzmann constant and absolute temperature
D = µkTe
(3)
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Movement of charges in electric field
Electrons in conduction band moves directly by the electrostatic force
Hole moves in the opposite direction as the electrons fill and leave the vacancy
At moderate field E , drift velocity of electrons and holes is proportional by the mobility
vh = µhE (4)
ve = µeE (5)
Both mobilities of same order in silicon and germanium
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Saturation of drift velocityMost of detectors operated at saturated velocities, about 107 cm s−1, collection at thedistance of 1 mm in 10 nsRlectrons in silicon, holes in silicon, electrons in germanium, holes in germanium
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Spread in arrival position
Spread due to diffusion after drift distance x , 100 µm for small-volume detector
σ =
√2kTxeE
(6)
Also spread in collection time, less than 1 ns in small volume
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Intrinsic semiconductor
All electron-hole pairs created only by thermal excitation
When n is concentration of electrons in conduction band and p concentration of holes invalence band, these are equal in intrinsic semiconductor
ni = pi (7)
At room temperature 1.5× 1010 cm−3 in silicon and 2.4× 1013 cm−3 in germanium
Value of conductivity (inverse to resistivity) given by carrier densities and mobilities, currentsby electrons and holes additive
Maximal resistivity for highest purity materials
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n-type semiconductors
Silicon makes covalent bonds with four nearest atoms
Dopant with five valence electrons, few parts per million
Donor impurity provides electrons to the conduction band without corresponding hole
Density of conducting electrons given by impurity concentration, n ∼= ND
Equilibrium concentration of holes decreased, np = ni pi
Electrons are majority carriers, holes are minority carriers
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p-type semiconductors
Acceptor (trivalent in Si) impurity, unsaturated bond, vacancy represents a hole
Vacancy filled by thermal electrons, number of holes given by acceptor density, p ∼= NA
Holes are majority carriers, equilibrium np = ni pi
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Electron and hole concentrations
Equilibrium between conduction electrons and holes concentrations
Concentrations equal in intrinsic or compensated semiconductor
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Conductivity of semiconductor
Conductivity as a function of acceptor / donor concentration
Any impurity increases conductivity
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Compensated material
Equal concentration of donors and acceptors, behaves as intrinsic
Denoted as type i
Residual imbalance
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Heavily doped material
Denoted as n+ or p+
Thin layer of high concentration of impurity
Used for electrical contacts
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Recombination
Deep impurities at energy levels at the middle of bandgap
Traps for charge carriers
Recombination centers: impurity capturing both minority and majority carriers
Traps and recombination contribute to the loss of charge
Collection times much shorter than lifetime in semiconductors
Trapping length defined as a distance traveled before trapping or recombination
May be caused by structural defects (vacancy, interstitial, dislocation)
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Ionizing radiation in semiconductors
Electron-holes pairs by direct ionization or by delta-rays
Ionization energy ε is energy needed to produce one electron-hole pair
Equal number of electrons and holes, independent of p-type or n-type
ε about 3 eV (for gas it is 30 eV)
ε depends on the nature of the radiation, temperature dependence
Depends also on the energy of soft X-rays
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Fano factor
Relation of observed variance in number of electron-hole pairs to Poisson-predicted variance
F ≡observed statistical variance
E/ε(8)
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Pulse formation
Electrons and holes collected at the boundaries of active volumes, transported by electric field
Top plot - all charges formed at a single plot
Two components for different collection times
Collection time similar since mobility differs by 2 - 3, all charges collected
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Electrical contacts
Ohmic contacts at two sides of semiconductor plate
Equilibrium concentration during collection of electrons and holes
p − n junction to suppress leakage current
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Leakage current
Field by voltage of hundreds or thousands volts
Leakage current < 10−9 A, use of blocking contacts
No surface contamination, leakage paths
Use of grooves or guard rings
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The semiconductor junction
Thermodynamic contact of p and n type
Change of doping in a single crystal
Starting with acceptor concentration NA, n-type impurity should diffuse into the crystal,providing donor concentration ND
Charge concentrations p and n altered, charge gradient formed, carrier diffusion
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The semiconductor junction
Diffusion creates space charge ρ(x) and electric field which suppress further diffusionSteady state charge distribution, depletion region is region of charge imbalance, highresistivity here
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Reverse biasing
Voltage applied to the junction, forward / reverse direction
Biased junction with negative voltage on the p side
Large resistance in reverse direction
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Electrical potential across the junction
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Properties of reverse biased junction
Figure : Charge distribution
Figure : Electric field
Depletion layer increasedPartial / full depletion depending on the voltage
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Properties of reverse biased junction
Figure : Electric potential
Maximal voltage below breakdown valueWidth of depletion region gives active volumeActive volume and capacitance vary with voltage
d =
√2εVeN
C =
√eεN2V
(9)
With resistivity ρ = 1/eµN, width of depletion region is
d =√
2εVµρ (10)
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Diffused junction detectors
Historical manufacturing procedure
p-type material exposed to vapors of n-type impurity, surface of heavily doped n-type
Diffused n-type layer up to 2 µm, depletion extends to p-type
Dead layer by non-depleted n-surface
Some incident energy lost before entering the active layer
Not used for spectroscopy
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Surface barrier detectors
Very thin dead layer
High density of electron traps at the surface of n-type crystal, behaves like thin layer of p-type
Etching of the surface, thin evaporated layer of gold for electrical contact
Small oxidation of the surface during evaporation (surface barrier), oxide layer between siliconand gold
Alternatively p-type with evaporated aluminum
Sensitive to light, thin dead layer is transparent
Visible photon has 2 - 4 eV, more than bandgap energy
No light-induced noise in vacuum enclosure
Front surface sensitive to vapors
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Mounting arrangement of surface barrier detector
Outer housing grounded, bias voltage (positive for n-type) through coaxial connector at therear
(a) - coaxial connector (M), silicon wafer (S), ceramic ring (I), electrical contact with metalizedsurfaces of the ring, outer case (C)
(b) - transmission mount, both surfaces accessible
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Ion implantation
Doping impurity introduced by accelerated ion beam
p+ and n+ layers by beams of phosphorus or boron
Monoenergetic ions at about 10 keV, well defined range in semiconductor
Concentration profile of impurity given by varying the beam energy
Annealing to fix the radiation damage, needed temperature <500 C, no thermal diffusion
More stable compared to surface barrier
Entrance window of tens of nm
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Fully depleted detector
With bias voltage high enough, the depletion region extends over entire thickness of the wafer
Preferred in most applications
One side of the junction by heavily doped n+ or p+
Opposite side of high purity material, mild-doped n or p, denoted as ν or π
Entrance window by heavily doped layer, can be very thin
Active volume and capacitance independent of voltage
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Depletion voltage
No electric field in undepleted region(V < Vd ), charge carriers not collected insuch thick dead layer
Partially depleted detector sensitive only infront side
Depletion voltage Vd is voltage needed toextend depletion depth to the back surfaceof the wafer of thickness T
Vd =eNT 2
2ε(11)
At voltage V Vd , electric field is moreuniform across T , the detector isover-depleted
Over-depleted detector is the most commonmode of operation
Thickness which can be fully depletedwithout risk of breakdown depends onpurity, bigger (several cm) with ultrapuregermanium, for silicon several mm
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Fully depleted planar detectors
Rectifying contact = heavily doped layer on high purity wafer of opposite type
Blocking contact = heavily doped layer on the same type substrate, blocks leakage currentfrom minority carrier motion through the junction
Electric field uniform across intrinsic or compensated wafer with rectifying and blockingcontacts
Such p − i − n configuration fully depleted at low voltage
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Transmission detectors
Incident particles pass through the wafer
Measured energy lost during the transit
Wafers of thickness 50 - 2000 µm
Small dead layers at both sides
Empirical test for minimum bias voltage for full depletion
Monoenergetic source incident on both sides should give same amplitude (up to difference incontacts size)
Needed uniform thickness of the wafer
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Johnon noise
Result of finite electrical resistance in undepleted layer
Noise would degrade the energy resolution
Mostly avoided in full depleted detector
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Passivated planar detectors
Adopted fabrication methods for semiconductorintegrated circuits
Ion implantation and photolithography, starting withlarge-area silicon wafer
Complex electrode geometry for position-sensitivedetectors
Start with high purity mild n-type
Surface cleaned, passivated by oxide layer at elevatedtemperature
Photolithographical removal of a given areas of oxide
Junction made by converting thin layers in windowsinto p-type by ion implantation
Blocking contact as n+ layer at the rear
Annealing at elevated temperature to removeradiation damage from ion implantation
Aluminum evaporation for ohmic electrical contacts
Separation and encapsulation of individual detectors
Leakage current suppressed by the oxide layer
Aluminized surface more stable against damage thangold in surface barrier
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Leakage current
In reverse bias, current of fractions of µA observed
Origin in bulk volume and surface of the detector
Small contribution from current minority carriers through the junction, proportional to the areaof the junction
Bigger contribution from thermal generation of electron-hole pairs withing depleted volume,cooling of germanium detectors with lower bandgap energy
Surface leakage at the edge of the junction, depends on encapsulation, humidity andcontamination
Reduction of true bias voltage as the power source is connected through large-value resistor,must be therefore monitored by measuring of the current from the power source
Stability of leakage current is measure of detector performance stability
Radiation damage monitored by long-term behavior of leakage current
Sudden increase of leakage current when increasing bias voltage indicate approach of thebreak-down of the junction
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Detector noise and energy resolution
Fluctuations in bulk and surface leakage currents
Resistance at electrical contacts
The noise results in the broadening of the measured peak
Measured by putting generated stable pulses into the preamplifier with connected detector
Spread in amplitude of test pulses measures the noise contribution
All sources of peak broadening add in quadrature (charge carrier statistics, energy lossfluctuation)
Trapping effects reveal as tails of pulses from monoenergetic source
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Pulse height with detector bias voltage
Recombination and trapping at low field, not all charges collected
Pulse height increases with increasing voltage
Increase with voltage stops when all charges are collected→ saturation region
Multiplication at high electric field, analogy to the gas proportional counter
Further electron-hole pairs from electrons from primary ionization, principle of siliconavalanche detector
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Pulse rise time
Rise time in order of 10 ns, contributions from charge transit time and plasma time
Charge transit time given by migration of electrons and holes through depletion region, sizegiven by wafer thickness, field set by bias voltage
Dependence of transit time on voltage more complicated since field is not uniform and driftvelocity vary
With weakly penetrating particle, all charges created near one boundary, one carrier hasmuch longer collection time than the other
Plasma time observed for heavy charged particles (alpha or fission fragment)
High density plasma-like cloud of electron-hole pairs shields electric field
Charges in the outer region begin to drift sooner than the rest inside
Plasma time is time to disperse the cloud to allow standard charge collection
Fixed delay of several ns
Plasma erosion inverse proportional to the field and track position and increase as cube rootof linear carrier density along the track
Short time constant of equivalent circuit to suppress preamplifier influence
Fully depleted detector better for timing since there is no series resistance due to undepletedregion
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Channeling
Energy loss depends on orientation vs. crystal axes
Lower energy loss while traveling parallel to crystal planes than for another arbitrary direction
Can affect pulse height even if the particle is fully stopped
Detector wafers fabricated in the way that channeling would occur parallel to the wafer surface
Less significant for heavy ions
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Radiation damage
Irreversible changes to the crystalline lattice by non-ionizing energy transfers to the atoms
Significant for heavy charged particles
Measurable increase in leakage current
Multiple peaks in measured energy spectrum for monoenergetic radiation after extremedamage
Loss of spatial resolution in position sensitive detectors due to decrease of inter-stripresistance
Bulk and surface effects of radiation damage
Frenkel defect - displacement of atom from normal lattice position, vacancy and interstitialposition, makes trapping sites for charge carriers, point defect
Clusters of damage along the track of heavy particle
Energy to displace silicon atom is 25 eV, done by neutron of kinetic energy of 180 eV
Threshold for electrons 260 keV, little damage with energy below
Minor annealing over time period, but in general the damage is permanent
Leakage current related to surface effects, ionization of the oxide in passivation layer
Radiation-induced charge trapping minimized by electric field as high as possible
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Type inversion
Occurs in high resistivity n-type silicon after long exposure to neutrons or high energyparticles, flux about 1013/cm2
Decrease of effective concentration of donors
Acceptor levels within the bandgap created by the radiation
p+-ν-n+ works after inversion with same polarity, rectifying contact moved from one side tothe other
More voltage for full depletion, risk of breakdown
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Energy calibration
Linear response, independent on the nature of the radiation
1 % discrepancy between protons and alpha particles
Calibration source 241Am, alpha at 5.486 MeV (85 %) and 5.443 MeV (13 %)
Correction for energy loss in the source, material between source and detector and andwindow or dead layer
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Alpha spectrum of 241Am by surface barrier detector
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Pulse height defectPulse height from heavy ion less than from light ion at same energyPulse height effect given as difference between true energy and energy measured withcalibration by alpha particlesDefect of 15 MeV for fission fragment at 80 MeV
Figure : True energy vs. channel number, silicon surface barrier detector
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Pulse height defect
Energy loss at entrance window and dead layer
For heavy ions, largest energy loss at the start of their range, unlike alpha
Energy loss by nuclear collisions resulting in recoil nucleus instead of e-h pair, heavy ions atlower velocity
Recombination of e-h pairs in dense plasma after ionization by heavy ion, depends on anglebetween ionizing particle and electric field, decreases with bias voltage
Pulse height defect increases with radiation damage
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Pulse height defect after radiation damage
The cause is trapping and recombination by radiation damage within the detector
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Fragment spectrum of 252Cf
Standard performance test by fission spectrum of 252Cf, thin source of the isotopeAccurate energy calibrationProbe to energy resolution, low-energy tail and internal multiplication
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Energy calibration for heavy ions
Generalization of E = ax + b with E the energy, x measured pulse height (channel num) anda and b constants
Dependence on the mass m of the ion
E(x ,m) = (a + a′m)x + b + b′m (12)
Constants extracted from Cf spectrum
Periodic measurement of Cf spectrum indicates the onset of radiation damage
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Particle identification by energy loss measurement
Measurement of dE/dx with detector thin vs. particle range, signal proportional to dE/dx(∆E detector)
Number of charge carriers over thickness ∆t is (dE/dx)∆t/ε
Semiconductor wafer 10 µm thick, important thickens uniformity
Mechanical risk to the thin wafer, monolithic combination of ∆E and E detectors
Simultaneous measurement of dE/dx and E by particle identifier telescope
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Particle identifier telescopeSimultaneous measurement of dE/dx and E as coincidence events in standard and thickdetectors, identification by ∆E · E spectrum
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Particle identification by energy loss measurement
Non-relativistic Bethe’s formula
dEdx
= C1mz2
Eln C2
Em
(13)
Product of E(dE/dx) sensitive to mz2 for different particles at energies not different by largefactor
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Particle identification by energy loss measurement
Alternative approach by the range dependence on energy
R(E) = aEb (14)
Parameter a ∝ 1/mz2, b is constant over similar ion masses
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X-ray spectroscopy with p-i-n diodes
Photoabsorption dominant for soft-X up to 20 keV for silicon of Z=14
High resistivity i-region with p and n contacts on the surface
300 µm enough for 20-30 keV
Diode cooled to minimize noise to detect small number of e-h pairs
Fluorescent X-rays in units of keV
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Photovoltaic mode of operation
Similar to solar cell
No bias voltage, contact potential in the junction (about 1 V in Si)
Measurement of current through the diode
High intensity of incident radiation
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Silicon diodes as personnel monitor
Electronic Personal Dosimeter (EPD)
Usually p-i-n configuration, real-time readout
Replacement for G-M pocket dosimeter
Energy compensation by placing metallic absorber around the detector
Needed uniform sensitivity of pulse-counting over large energy range of X-rays
Neutron exposure with converter material for slow-neutrons and hydrogen in polyethylene forfast neutrons (measuring recoil protons)
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Germanium gamma-ray detectors
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General considerations
More penetrating radiation requires bigger active volume
Limitation by depletion depth d given by voltage V and impurity concentration N
d =
(2εVeN
)1/2(15)
More depletion with lower N
N = 1010 atoms/cm3 makes d = 1 cm at bias V = 1000 V
Correspond to concentration of 1 part per 1012
Can be done with germanium called intrinsic germanium or high-purity germanium (HPGe)
Another approach by compensating with opposite-type impurity, process of lithium ion driftingin silicon or germanium
Thickness up to 2 cm by interstitial donor lithium atoms
Ge(Li) denotes germanium detector with lithium drifting, since 1960s
Replaced since 1980s since Ge(Li) must be continuously cooled, HPGe at room temperaturewith same characteristics
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HPGe fabrication
Start with bulk germanium for industry, local heating and moving the melted zone from oneend to the other
After repetitions, impurity of 109 atoms/cm3
Large single crystals grown
π-type with residual aluminum
Small electrical conductivity - high resistivity material
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Germanium ingot and detector elements
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Planar configuration of HPGe
Disk of high purity π-type
n+ contact by lithium evaporation or with accelerator (phosphorus implantation)
p+ contact by ion implantation (boron) or metal-semiconductor barrier
Radiation damage after implantation makes acceptor sites, annealing
Overdepletion by +V to n+, saturated drift velocities
When operated at 77 K, electron drift velocity saturate at field of 109 V/m
Limit on voltage by breakdown and leakage current at 3-5 kV, holes do not saturate
Fabrication may start with ν-type, situation is symmetrical
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Lithium drifted detector
Bulk of the wafer compensated, i-type
Contacts by n+ and p+
Used for silicon, replaced by HPGe for germanium
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Suppression of leakage current
Conduction of leakage current flowing across the edgesGroove to increase gap surface without reducing active volumeEdge regions excluded preventing trapping at the sidesBottom electrode may be wrapped around the sides (c), also smaller capacitance and lowerelectronic noiseElectric field more complicated compared to planar geometryJaroslav Adam (CTU, Prague) DPD_04, Semiconductor detectors Version 2 66 / 125
Coaxial configuration
Long germanium cylindrical crystal with removed core, active volume of hundreds of cm3
Electrodes on the outer and inner surface
Small capacitance with small inner surface
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Coaxial configuration
Closed-ended geometry preferred for HPGe
Leakage current at front surface avoided
Planar surface is entrance window for weakly penetrating radiation
Corners of the hole rounded to reduce low-field regions near the edges
Small radioisotope can be put into the hole
Rectifying contact (junction) at the outer surface, depletion grows towards inner surface
High voltage at outer surface, inner surface connected to the gate of field effect transistor(FET) in the amplifier
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Field effect transistor (FET)
Conduction through channel from drain to source controlled by electric field from voltageapplied to the gate
High impedance above 1014 Ω
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Electric field and capacitance
Poison’s equation for electric potential φ created by charge density ρ in material with dielectricconstant ε
∇2φ =ρ
ε(16)
Negative charge of filled acceptors in π-type germanium ρ = −eNA
Positive charge for ν-type, ρ = eND by ionized donor sites
Exact compensation in lithium-drifted detector, ρ = 0
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Electric field in planar HPGe
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Electric field in coaxial geometry
Poisson’s equation in cylindrical coordinates transforms as
d2φ
dr2+
1r
dφdr
= −ρ
ε(17)
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Electric field in coaxial HPGe
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Surface dead layer
Dead layer by n+ contact, thickness of hundreds of µm with lithium evaporation
Negligible effect for gamma above 200 keV
Ion implantation for soft X-rays, layer in fractions of µm
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Detector cryostat and dewar
Small bandgap (0.7 eV), cooling needed to suppress leakage current from thermallygenerated carriers
77 K by liquid nitrogen
Ge(Li) must be cooled continuously to prevent redistribution of drifted lithium
Vacuum tight cryostat against heat transfer from surrounding air
Charcoal or other cryopump within sealed volume
Thin end window near the crystal
Performance of coaxial detector fine up to 130 K
Mechanical cooling for smaller portable spectrometer
Longer lifetime with continues cooling also for HPGe, leakage of gases and water vapor intovacuum system not absorbed by cryopump at the room temperature
Surface contamination increases leakage current
High voltage only when cooled to protect preamplifier from high leakage current
Preamplifier close to the detector to minimize the capacitance
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Detector cryostat and dewar
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Detector cryostat and dewar
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Energy resolution
Dominant of germanium detectors for gamma spectroscopy
Separation of close energies, not seen by scintillators
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Spectrum by scintillator and Ge(Li)
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Energy resolution
FWHM for monoenergetic source, WT given by statistics in number of carriers, chargecollection efficiency and electronic noise
W 2T = W 2
D + W 2X + W 2
E (18)
Statistical fluctuation in number of carriers W 2D = (2.35)2FεE
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Statistical contribution WD
FWHM bigger than statistical limitJaroslav Adam (CTU, Prague) DPD_04, Semiconductor detectors Version 2 81 / 125
Incomplete charge collection W 2X
Estimate as FWHM vs. bias voltage
Extrapolation to infinite voltage
Effect of velocity saturation
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Broadening from electronic components W 2E
Measured with test pulses supplied to the preamplifier with connected detector for correctcapacitance
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Energy resolution as a function of gamma energy
Better resolution with smaller detector, smaller capacitance and trapping
Large active volume only when needed
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Charge collection process
Shaping time of electronics larger than pulse rise to avoid the ballistic deficit
Pulse shape important for timing, also event-to-event variation
Leading edge of signal pulse given by charge collection
Saturation electron drift velocity 105 m/s in Ge at 77 K with field 105 V/m
Saturation of holes velocity requires field 3 times bigger
Time for charge collection hundreds of ns
Event-to-even fluctuation by e-h position in active volume
Charge carriers created at one point with short-range particle, separate collection times forelectrons and holes
Distribution of collection times for larger range particle given by spatial distribution of e-h pairs
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Shape of the leading edge
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Leading edge shape for coaxial drifted detector
Radial position can be determined by pulse shape analysis, position sensing or correction ofradius-dependent effects
Two- or three-dimensional position sensing with segmented electrodes
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Gamma-ray spectroscopy with germanium detectors
Spectrum of 137Cs with 662 keV
Higher resolution but smaller intrinsic peak efficiency compared to scintillator, due to lower Zof germanium
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Contributions to full energy peak
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Other solid-state detectors
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Lithium-drifted silicon detectors Si(Li)
Compensated silicon of thickness 5-10 mm
Limit to thickness by maximal Li drifting distance
Higher purity than Ge(Li)
Very soft X-rays (lower Z), transparent for high-energy gamma (background to soft-X)
Beta electrons, lower backscattering compared to Ge
Larger bandgap preventing thermal leakage current, same statistical limit to resolution
Cooling by liquid nitrogen to suppress leakage current and to prevent redistribution of Li
Room temperature allowed for some versions
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The p-i-n configuration
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Pulse rise time for beta particles in Si(Li)
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Response function
Photoabsorption in Si dominant below gamma energy below 55 keV
Single peak if charge collection is complete
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Intensity measurement
Intensity of photons measured as area under the photopeak
Separation of low-energy tail
Result of incomplete charge collection
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Linearity and energy resolution
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Contributions to width of full-energy peak in Si(Li)
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Energy resolution
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Energy-dependent efficiency
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Intrinsic peak efficiency for low-energy photon spectroscopy
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Fluorescence spectroscopy with Si(Li)
Analysis of materials by characteristic X-ray emission
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X-ray fluorescence
Emission of characteristic X-rays after electron irradiation
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Semiconductors other than Si or Ge
Slow development of compound semiconductors
First demonstration with radiation signals in AgCl
Issues with electrically active impurities and defects
Temporal instability due to polarization
Requirements on active volume, high-Z, operation at room temperature
Small bandgap to reduce ionization energy
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Ionization energy vs. bandgap energy
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Diamond detectors
Better radiation hardness
Higher bandgap (less pairs but lower leakage current and still good signal-to-noise ratio)
Comparison with Si:
Synthetical production by chemical vapor deposition (CVD)
Fast response because of high mobility (fast charge collection), fields of kV/mm
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Mobility vs. temperature in diamond detectors
No strong dependence around room temperature, unlike Si
Varies with concentration of phosphorus impurity, 100 ppm (rectangles), 500 ppm (dots),1000 ppm (open circles)
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Design of a diamond detector
Low leakage current, no need for pn-junction
Metal contacts on the side, high bias voltage to read detector output
Prototypes for accelerator experiments, synchrotron monitoring and neutron detection
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Avalanche detectors
Charge multiplication in solid-state detector, gains of several hundreds
High electric field, migrating electrons create secondary ionization, pulse rise of few ns
Convenient geometry for high field inside but low at the surface (risk of breakdown)
Detection of low energy X-rays of light from scintillator
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Position sensitive semiconductor detectors
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Resistive charge division
Position along one direction by signals of two amplifiers
Signal in E is proportional to the total energy deposition
Resistive contact works as a charge divider
Signal in P measures x/L
Ratio P/E gives position independent of deposited charge
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Fission fragments of 252Cf detected by resistive division detectorHorizontal: E signal, vertical: P signal
A collimator of 9 equidistant slits in front of the detector
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Silicon strip detector
Independent segments (strips), photolitography and ion implantation
Independent readout of strips, reconstruction by center of gravity
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Hybrid pixel detector
Individual electrically isolated electrodesPad detector for segments larger than mm, pixel detector for smallerSmall capacitance and leakage current, smaller noisePixels are connected to readout chip by flip-chip bonding
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Monolithic pixel detector
Detection element and readout in the same layer, layer thickness µm, min signal of 100electrons
One pixel cell has collecting element, preamplifier and readout structure
Detecting element is reverse bias diode
Also PMOS transistors with quadruple well technology (right)
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Cell in monolithic pixel detector
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Monolithic pixel sensor prototype (ULTIMATE) sensor
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Silicon drift detector
Charges from ionization are transported through detector over ∼cm distances
2D position, one coordinate from anodes and one from time of drift
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Potential in drift detector
Left: regular part of the detector, right: anode region
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Carrier transport in drift detector
Electron trajectories (black), hole trajectories (red)
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Cylindrical drift detector
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Charge coupled device (CCD)
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Transfer channel of CCD
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Electrons in partially depleted CCD
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Thick film semiconductor X-ray imager
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Readout of thin film transistor (TFT) array
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