半导体探测器 semiconductor detector
TRANSCRIPT
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半导体探测器
Semiconductor Detector
周荣 Rong ZHOU
四川大学 核科学与工程技术学院 College of Nuclear Science & Engineering, SCU
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Outline
• Basics of semiconductor
• Principle of semiconductor detector
• Si(Au) surface barrier detector
• HPGe detector
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BASICS OF SEMICONDUCTOR
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Energy Band Theory
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Basic of semiconductor
• Intrinsic semiconductor
Si, Ge
• Doped semiconductor
P-type: Doped with IV-group element B, Al,
Ga
N-type: Doped with VI-group element P, As,
Pb
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Charge Carrier of Semiconductor
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Concentration of Majority Carrier
& Minority Carrier kTEE
nFceCn
/)(
kTEE
pvFeCp
/)(
kTE
pngeCCpn
/
2 2
i i i in p n p n p
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P-N Junction
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Bias of P-N Junction
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PRINCIPLE OF
SEMICONDUCTOR
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How to be a Radiation
Detector?
• Think about gas chamber
No signal when no particles incident
Generate ion-pairs when particles loss energy
in sensitive volume
No ions loss when they are drifting to
anode/cathode in electric field, i.e. the
amplitude of signal could represent primary
ion-pair numbers.
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Intrinsic Semiconductor as
a Detector
• For pure silicon,
resistivity ~105Ω·cm,
1cm thick, 1cm2 area, R=100kΩ
bias voltage 100V, Idark=1mA
We need material of much larger resistivity to build a radiation detector!
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P-N Junction as a Detector
• High reverse resistor
1010Ω·cm
• Easy to generate ion-pairs
(w~3eV)
• Little probability of capture
and combination (carrier life
10-5s >> collecting time 10-7
~ 10-8s
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Ionization Energy of
Semiconductor
• Average ionization energy w has no
connection with incident particles' energy.
Si Ge
300K 3.62eV
77K 3.76eV 2.96eV
/N E w
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Drift of Charge Carrier
• Drift velocity of electrons is similar to that
of vacuum holes.
• Drift velocity increases more slowly with E
when E is very large. Finally drift velocity
would achieve a saturation value of about
107 cm/s
Eu nn
Eu pp
For N-type
semiconductor
For P-type
semiconductor
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Resistivity
• intrinsic resistivity:
Si: 2.3×105Ω·cm
Ge: 50~100 Ω·cm
• cooling with liquid Nitrogen would increase
resistivity obviously
• doping would decrease resistivity
pn pne
1
cmΩ
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P-N Junction and its Current
E
P N
If
IG , IS
GI g W e
SGf III
If : diffusion of majority carrier IG : thermal motion IS : diffusion of minority
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Reverse Bias and Dark
Current
• IL Surface leakage current
• Increase reverse voltage,
IG↑
IS not vary
If ↓
• Dark Current = IL + IG + IS - If
main contribution for dark current
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Charge in P-N Junction
)0(
)0()(
bx
xa
eN
eNx
A
D
n-type p-type
- - - - - - - - - - -
- - - - - - - - - - -
- - - - - - - - - - -
+ + + + +
+ + + + +
+ + + + +
a b0
ND: Donor concentration
NA: Acceptor concentration
ND a=NA b。
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0
4( ) ( )D
eNE x x a
)0( xa
)0( bx
0
4( ) ( )A
eNE x b x
0
2
0
)(2
)( VaxeN
x D
)0( xa
)0( bx 2
0
)(2
)( bxeN
x A
( / )E d dx
Electric Field in P-N Junction
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Depleted Region Width
2
0
2
0
0
22)0( b
eNa
eNV AD
bNaN AD AeN
Vbba
2)( 00
baW Width of depleted region
When NA>>ND, a>>b, W≈a When NA<<ND, a<<b, W≈b
),min( DAi NNN
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Junction Capacitance
• Cd would change with V0, which is different
from the capacity of gas-filled detector。
0
11
VWdV
dQCd
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Two Types of P-N junction
• Diffused junction
• Surface barrier
N-type + Au
P-type + Al
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C
LR
测量仪器
RC
dC)(0 tI dR
SR
SC
Output Circuit
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C
LR
RC
dC)(0 tI dR
SR
SC
0RaC
dC)(0 tI
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Output Signal
• if R0(Cd+Ca) >> tc
ad CC
eNh
number of electron-
hole pairs generated
in sensitive volume
t
)(tV
ad CC
eNh
)(/ 0 ad CCRt
ad
eCC
eN
stc
89 10~10
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Discuss
• h has connection with Cd, while Cd would change with V0.
• if Ca>>Cd, h=Ne/Ca, the infection of Cd could be avoided.
• Use charge sensitive preamplifier instead of voltage or current type preamplifier.
ad CC
eNh
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SI(AU) SURFACE BARRIER
DETECTOR
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Characteristics
• Cheap and Simple
• Convenient to use
• High energy resolution
• Limited sensitive
volume, for detection of
charged particles
• Fast time response
• Low background
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Spectrometer Configuration
Detector Pre-
amplifier
H.V. L.V.
Linear
Amplifier MCA
Vacuum
Pump
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Energy Resolution
• Statistic fluctuation
• Noise of detector and circuit
• Infection of injection window
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Statistic Fluctuation
E
wFv
E
EN
36.236.2
F: Fano factor Si 0.143, Ge 0.129 E: energy loss of incident particles in sensitive volume w: average energy needed to generate one electron-hole pair.
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Noise of Detector and Circuit
• Noise of detector
reverse current of P-N
junction
surface leakage
current
• Noise of circuit
• equivalent noise
charge / equivalent
noise energy
• zero capacitance
noise
• noise slope
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Thickness of Injection Window
• Thickness of injection window make
contributions to FWHM
0d
d
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Time Response
• Decided by drift velocity of electrons and
vacuum holes.
• 10-9s~10-8s, much faster than gas-filled
detector and scintillation detector
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Energy linearity
• Good performance in energy linearity
• has hardly connection with energy and
type of incident particles
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Radiation Damage and Life
• Significant disadvantage of semiconductor
detectors
• easily be damaged by radiation, so
semiconductor detectors have shorter life
than gas-filled and scintillation ones.
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Totally Depleted
Semiconductor Detector
• Giving enough bias voltage, all crystal
volume become depleted region
timing detector
very short rise time (<1ns)
dE/dx detector
particles identification
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HIGH PURITY GERMANIUM
DETECTOR
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To detect γ ray
• Large sensitivity volume needed.
increase thickness of depleted region
• increase V0
• decrease Ni
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HPGe Semiconductor
• Using high purity P-
type germanium crystal
• Donor impurity (P or Li)
are implanted as N+,
and form P-N junction
• metal is evaporated to
the other side for P+
area as particle
injection window.
charge
potential
electric field
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Characteristic
• P region is the space charge region, sensitive volume.
• Usually operating in totally depleted state
• P-N junction detector
• Nonuniform electric field in sensitive volume
• HPGe detector could be stored in room temperature, but must operates in low temperature.
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Structure
• Flat
small volume, thickness
< 2cm, for X-ray and
low energy γ detection.
• Coaxial
Large volume, for γ
detection
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Configuration
Refrigerated
by liquid
nitrogen
Refrigerated by
thermoelectric
action
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Configuration
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HPGe Gamma Spectrometer
Ge Crystal Pre-
amplifier
H.V. L.V.
Spectrometer
Amplifier MCA
Cooling
Computer
(Spectrum
Analysis)
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Picture
lead chamber
(detector inside)
liquid nitrogen pot
H.V. supply
Spectrometer
Amplifier
Multi Channel
Analyzer
NIM Case (L.V.
Supply)
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Output Signal
Lead edge has connection with the injection place.
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Performance
• Energy resolution
fluctuation of number of carrier
leakage current and noise
capture of carrier
• Detection efficiency
relative to 3 inch ×Φ3 inch NaI(Tl) crystal
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Performance
• Peak-to-Compton ratio
ratio of height of total energy peak and
Compton plateau
• Peak shape
FW0.1M/FWHM, FW0.02M/FWHM
• Energy linearity (very good)
• Time characteristic
width of current pulse: 10-9~10-8s
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Products of ORTEC and Their
Performances
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Other Semiconductor
Detector
• P152~158, teach yourselves
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Homework
• P158: 1,3,4,7,8