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30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 16
MIS capacitor•Elementary device
oxide well matched to silicontransparent to wide rangeexcellent insulator
nitride frequently used in additionlarger
SiO2 Si3N4
Density g.cm-3 2.2 3.1Refractive index 1.46 2.05Dielectric constant 3.9 7.5Dielectric strength V/cm 107 107
Energy gap eV 9 ~5.0DC resistivity at 25C Ω. cm 1014-1016 ~1014
Energy band
diagram
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 17
MOS capacitor characteristics•Apply bias voltage to influence charge under oxide
depletion - potential well which can store chargeinversion - thin sheet of charge with high density
allows conduction in transistorvery close to Si-SiO2 interface
Basis of MOStransistor operation
Basis of MOStransistor operation
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 18
CCD - Charge Coupled Device
1
2
3
drive pulses
polysilicon electrodes
1µm
signal electrons in buried channel
22µm
silicon substrate
gate insulator
column isolation
22µm
φφφ
•2-d array of MOS capacitorselectrode structures isolate pixelsallow to transfer chargethin sensitive regionsignals depend on application low noise, especially if cooled
•Video requirements different toscientific imaging
persistent image smaller area & pixelsReadout time long ms-sall pixels clocked to readout node
•Applicationsastronomy, particle physics, x-raydetection, digital radiography,...
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 19
CCD charge transfer
φ2
φ1
3φ
t1 t2 t3
VG
0V 0V+VG +VG
0V 0V+VG0V
0V 0V+VG 0Vt1
t2
t3
3 1 2 3
•Change voltages on pixels in regular way ("clock")3 gates per pixel3 phases per cycledepletion depth in adjacent regions changesE field transfers charge to next pixel- finally to output register
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 20
Silicon detector radiation damage
•As with all sensors, prolonged exposure to radiation creates some permanent damage- two main effects
Surface damage Extra positive charge collects in oxide
all ionising particles generate such damageMOS devices - eg CCDs - are particularly prone to such damageMicrostrips - signal sharing & increased interstrip capacitance - noise
Bulk damage atomic displacement damages lattice and creates traps in band-gap
only heavy particles (p, n, π, …) cause significant damage
increased leakage currents - increased noisechanges in substrate doping
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 1
Signals•Signal
generalised name for input into instrument system
•Might seem logical to consider signals before sensors but can now seewide range of signal types are possible
depend on sensordepend on any further transformation - eg light to electrical
•Most common types of signalshort, random pulses, usually current, amplitude carries information
typical of radiation sensorstrains of pulses, often current, usually binary
typical of communication systemscontinuous, usually slowly varying, quantity - eg. current or voltage
slow - typical of monitoring instrumentsfast - eg cable TV, radio
•terms like “slow”, “fast” are very relative!
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 2
Typical signals•Some examples
•However, we will find later that speed of signal is not always sufficient to build fastresponding systems
Signal source DurationInorganic scintillator e-t/τ τ ~ few µsOrganic scintillator e-t/τ τ ~ few nsCerenkov ~nsGaseous few ns - µsSemiconductor ~10nsThermistors continuousThermocouple continuousLaser pulse train ~ps rise time
or short pulses ~fs
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 3
Signal formation•Issues in practical applications
durationradiation: depends on transit time through sensor and details of charge inductionprocess in external circuit
linearitymost radiation sensors characterised, or chosen for linearityfor commercial components can expect non-linearity, offset and possiblesaturation
reproducibilityeg. many signals are temperature dependent in magnitude - mobility of chargesother effects easily possible
ageingsensor signals can change with time for many reasonsnatural degradation of sensor, variation in operating conditions, radiationdamage,...
•all these effects mean one should always be checking or calibrating measurementsintended for accuracy as best one can
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 4
Optical transmitters
Eg
•Semiconductor lasers most widely usedNow dominate telecomms industry
>> Gb/s operation•Principle
Forward biased p-n diode=> population inversion
direct band gap materialGaAs ~850nmGaAlAs ~ 600-900nmIn, Ga, As, P ~0.55-4µm
•+ polished optical facets=> Fabry-Perot cavity
optical oscillatorlase at I > Ithreshold
photon losses from cavity or absorptionoften very linear
8
6
4
2
02520151050
Current (mA)
un-irradiated after 2x10
14n/cm
2
1nsec
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 5
Modern semiconductor lasers
•Quantum well structuresconfine charge carriers to active layer
refractive index difference=> waveguide confines light
minimise lateral dimensions for efficiency& low Ithreshold
=>low power (~mW), miniature deviceswell matched for optical fibre transmission
•VCSELs Vertical Cavity Surface Emitting Laseremit orthogonal to surfaceultra-low power
cheap to make (test on wafer)can be made in arrays
non-linear L-I characteristicbut very suitable for digital applications
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 6
Passage of radiation through matter•Need to know a few elementary aspects of signal formation whether interested inlight or other radiation
How far does radiation penetrate?How much of incident energy is absorbed?
•Signal current - duration and magnitudeconsequence of charge carriers generated
electrons + holes (semiconductor) or ions (gases, liquids)current duration depends on
distance over which charge depositedrapid absorption or thin sensor give fast signals
electric fieldonly charges in motion generate currents
current in external circuit is induced
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 7
Light•I ~ I0exp(-L/Labs)
1/Labs = Natomσ Natom = ρNAvogadro/A = no. atoms per unit volume
•PhotoabsorptionE ~ eV- 100keV atom ionised in single process, all photon energy transferredat low energies depends on atomic properties of materialat higher energies σpa ~ Z4-5/Eγ
3 above K-shell edge
•Compton scattering~MeV quantum collision of photon with charged particle, usually e-
transfer of part of photon energy, often small
•Pair production>> MeVall energy transferred to e+e- pairto conserve momentum and energy, needs recoil
must take place in field of nucleus or electron
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 8
Light absorption-•Low energies
see consequence of atomic behavioureg silicon bandgap
NB strong dependence on wavelength in near-visible regions
•High energiesatomic shell structurevisible
then electrons appearas quasi-free
Compton scatteringstarts to dominate
at ~60keV - not shown10
-2
100
102
104
106
108
Abs
orpt
ion
leng
th [ µm
]
0.01 0.1 1 10 100 1000Photon energy [keV]
Silicon
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 9
Light absorption•Far UV to x-ray energies
atomic shell structurephoto-absorption
coherent = Rayleigh scatteringatom neither ionised nor nor excited
incoherent = Compton = Zf(E )
pair production Eγ > 2mecontributions from nucleus (~Z2)and atomic electrons (~Z)
small contribution from nuclear interactions
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 10
Charged particles•Ionisation dominates Units: x = density x thickness = [g.cm-2]
Stopping power = dE/dx scales in similar way for all particles with p/m = βγdominated by interactions withatomic electrons
•low energiesslow particles lose energy rapidly
dE/dx increases with to maximum
Bragg peak
•relativistic energiesdecline ~ 1/β2
to minimum valuefurther slow rise ~ log(p/m)
•most cosmic rays and high energyparticles approximately MIPs
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 11
dE/dx•Measured energy loss can provide another way of identifying particles
gas detectors with multiple samples of ∆E from same particl emomentum measurement is needed - from bending in B fieldaccompanied by good calibration of p and dE/dx
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 12
Electrons•are special because of their low mass
classically accelerated charge radiates
•brehmstrahlung radiation in matteracceleration in nuclear field
•synchrotron radiation in acceleratorsgenerates beams of low energy x-rays
typical E ~ 1-10keVwidely used for studying atomic properties, eg protein crystallography
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 13
Other neutral particles•neutrons
do not generate ionisation directly so hard to measure
•at low energiesmostly elastic collisions with atoms in materialsimple kinematics determines energy transfer
∆T max = 4ATinc/(1+A)2
low Z materials favoured to absorb neutron energyC, D2O moderators in nuclear reactorshydrogenous or boron compounds used as detectors
30 October, [email protected] www.hep.ph.ic.ac.uk/~hallg/ 14
Sensor equivalent circuits•Many of the sensors considered so far can be modelled as
current source + associated capacitancetypical values ~ few pF
but can range from~100fF semiconductor pixel~10-20pF gas or Si microstrip, PM anode~100pF large area diode~µF wire chamber
usually there is some resistance associated with the sensor, eg leads ormetallisation but this has little effect on signal formation or amplification
•Notable exception: microstrips - gas or siliconthe capacitance is distributed, along with the strip resistanceforms a dissipative transmission line
Cdet
isignal
ZL