Post on 11-Jan-2022
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
zhourong@scu.edu.cn
Outline
• Basics of semiconductor
• Principle of semiconductor detector
• Si(Au) surface barrier detector
• HPGe detector
BASICS OF SEMICONDUCTOR
Energy Band Theory
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
Charge Carrier of Semiconductor
Concentration of Majority Carrier
& Minority Carrier kTEE
nFceCn
/)(
kTEE
pvFeCp
/)(
kTE
pngeCCpn
/
2 2
i i i in p n p n p
P-N Junction
Bias of P-N Junction
PRINCIPLE OF
SEMICONDUCTOR
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.
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!
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
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
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
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
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
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
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。
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
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
Junction Capacitance
• Cd would change with V0, which is different
from the capacity of gas-filled detector。
0
11
VWdV
dQCd
Two Types of P-N junction
• Diffused junction
• Surface barrier
N-type + Au
P-type + Al
C
LR
测量仪器
RC
dC)(0 tI dR
SR
SC
Output Circuit
C
LR
RC
dC)(0 tI dR
SR
SC
0RaC
dC)(0 tI
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
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
SI(AU) SURFACE BARRIER
DETECTOR
Characteristics
• Cheap and Simple
• Convenient to use
• High energy resolution
• Limited sensitive
volume, for detection of
charged particles
• Fast time response
• Low background
Spectrometer Configuration
Detector Pre-
amplifier
H.V. L.V.
Linear
Amplifier MCA
Vacuum
Pump
Energy Resolution
• Statistic fluctuation
• Noise of detector and circuit
• Infection of injection window
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.
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
Thickness of Injection Window
• Thickness of injection window make
contributions to FWHM
0d
d
Time Response
• Decided by drift velocity of electrons and
vacuum holes.
• 10-9s~10-8s, much faster than gas-filled
detector and scintillation detector
Energy linearity
• Good performance in energy linearity
• has hardly connection with energy and
type of incident particles
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.
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
HIGH PURITY GERMANIUM
DETECTOR
To detect γ ray
• Large sensitivity volume needed.
increase thickness of depleted region
• increase V0
• decrease Ni
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
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.
Structure
• Flat
small volume, thickness
< 2cm, for X-ray and
low energy γ detection.
• Coaxial
Large volume, for γ
detection
Configuration
Refrigerated
by liquid
nitrogen
Refrigerated by
thermoelectric
action
Configuration
HPGe Gamma Spectrometer
Ge Crystal Pre-
amplifier
H.V. L.V.
Spectrometer
Amplifier MCA
Cooling
Computer
(Spectrum
Analysis)
Picture
lead chamber
(detector inside)
liquid nitrogen pot
H.V. supply
Spectrometer
Amplifier
Multi Channel
Analyzer
NIM Case (L.V.
Supply)
Output Signal
Lead edge has connection with the injection place.
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
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
Products of ORTEC and Their
Performances
Other Semiconductor
Detector
• P152~158, teach yourselves
Homework
• P158: 1,3,4,7,8