04 - Semiconductor detectors - cvut.cz · 2016-05-17 · n-type semiconductors Silicon makes covalent bonds with four nearest atoms ... Electron-holes pairs by direct ionization or

04 - Semiconductor detectors

Jaroslav Adam

Czech Technical University in Prague

Version 2

Jaroslav Adam (CTU, Prague) DPD_04, Semiconductor detectors Version 2 1 / 125

Semiconductor diode detectors


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


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


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


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)


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


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


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


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


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


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


Electron and hole concentrations

Equilibrium between conduction electrons and holes concentrations

Concentrations equal in intrinsic or compensated semiconductor


Conductivity of semiconductor

Conductivity as a function of acceptor / donor concentration

Any impurity increases conductivity


Compensated material

Equal concentration of donors and acceptors, behaves as intrinsic

Denoted as type i

Residual imbalance


Heavily doped material

Denoted as n+ or p+

Thin layer of high concentration of impurity

Used for electrical contacts


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)


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


Fano factor

Relation of observed variance in number of electron-hole pairs to Poisson-predicted variance

F ≡observed statistical variance

E/ε(8)


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


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


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


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


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


Reverse biasing

Voltage applied to the junction, forward / reverse direction

Biased junction with negative voltage on the p side

Large resistance in reverse direction


Electrical potential across the junction


Properties of reverse biased junction

Figure : Charge distribution

Figure : Electric field

Depletion layer increasedPartial / full depletion depending on the voltage


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)


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


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


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


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


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


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


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


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


Johnon noise

Result of finite electrical resistance in undepleted layer

Noise would degrade the energy resolution

Mostly avoided in full depleted detector


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


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


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


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


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


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


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


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


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


Alpha spectrum of 241Am by surface barrier detector


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


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


Pulse height defect after radiation damage

The cause is trapping and recombination by radiation damage within the detector


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


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


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


Particle identifier telescopeSimultaneous measurement of dE/dx and E as coincidence events in standard and thickdetectors, identification by ∆E · E spectrum



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



Alternative approach by the range dependence on energy

R(E) = aEb (14)

Parameter a ∝ 1/mz2, b is constant over similar ion masses


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


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


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)


Germanium gamma-ray detectors


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


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


Germanium ingot and detector elements


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


Lithium drifted detector

Bulk of the wafer compensated, i-type

Contacts by n+ and p+

Used for silicon, replaced by HPGe for germanium


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


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


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


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


Electric field in planar HPGe


Electric field in coaxial geometry

Poisson’s equation in cylindrical coordinates transforms as

d2φ

dr2+

1r

dφdr

= −ρ

ε(17)


Electric field in coaxial HPGe


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


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






Energy resolution

Dominant of germanium detectors for gamma spectroscopy

Separation of close energies, not seen by scintillators


Spectrum by scintillator and Ge(Li)


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


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


Broadening from electronic components W 2E

Measured with test pulses supplied to the preamplifier with connected detector for correctcapacitance


Energy resolution as a function of gamma energy

Better resolution with smaller detector, smaller capacitance and trapping

Large active volume only when needed


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


Shape of the leading edge


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


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


Contributions to full energy peak


Other solid-state detectors


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


The p-i-n configuration


Pulse rise time for beta particles in Si(Li)


Response function

Photoabsorption in Si dominant below gamma energy below 55 keV

Single peak if charge collection is complete


Intensity measurement

Intensity of photons measured as area under the photopeak

Separation of low-energy tail

Result of incomplete charge collection


Linearity and energy resolution


Contributions to width of full-energy peak in Si(Li)


Energy resolution


Energy-dependent efficiency


Intrinsic peak efficiency for low-energy photon spectroscopy


Fluorescence spectroscopy with Si(Li)

Analysis of materials by characteristic X-ray emission


X-ray fluorescence

Emission of characteristic X-rays after electron irradiation


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


Ionization energy vs. bandgap energy


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


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)


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


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


Position sensitive semiconductor detectors


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


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


Silicon strip detector

Independent segments (strips), photolitography and ion implantation

Independent readout of strips, reconstruction by center of gravity


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


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)


Cell in monolithic pixel detector


Monolithic pixel sensor prototype (ULTIMATE) sensor


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


Potential in drift detector

Left: regular part of the detector, right: anode region


Carrier transport in drift detector

Electron trajectories (black), hole trajectories (red)


Cylindrical drift detector


Charge coupled device (CCD)


Transfer channel of CCD


Electrons in partially depleted CCD


Thick film semiconductor X-ray imager


Readout of thin film transistor (TFT) array


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04 - Semiconductor detectors - cvut.cz · 2016-05-17 · n-type semiconductors Silicon makes covalent bonds with four nearest atoms ... Electron-holes pairs by direct ionization or