Download Semiconductor detectors

Basic Detection Techniques 2009-2010
http://www.astro.rug.nl/~peletier/DetectionTechniques.html
Detection of energetic particles and gamma rays
Semiconductor detectors
Peter Dendooven
KVI
[email protected]
Contents
•
Interaction of radiation with matter
– high-energy photons
– charged particles
• heavy charged particles
• electrons
– neutral particles
• neutrons
• neutrinos
•
General radiation detection concepts
–
–
–
–
pulse mode operation
energy spectrum
detector efficiency
timing
•
Radiation detectors
– semiconductor detectors
• operation principle
• examples (silicon, germanium)
• other materials
– scintillation detectors
•
•
•
•
principle
organic scintillators
inorganic scintillators
photosensors
– gas detectors
• ionisation
• proportional
• Geiger
Note: also known as solid-state detectors
2
Semiconductor band structure
energy
conduction band
electrons
>6 eV
band gap
at T=0 K
~1 eV
empty
holes
occupied
valence band
insulator
semiconductor
at T>0 K
!
kT at 300 K = 0.025 eV
ni = pi = N c N v e
metal
"
Eg
2kT
= AT 3/ 2 e
"
Eg
2kT
ni: intrinsic density of electrons in conduction band
pi: intrinsic density of hole in valence band
Nc,v: number of states in conduction, valence band
Eg: band gap at 0 K
A: temperature-independent constant
3
Relevant properties of intrinsic Si and Ge
Si
Ge
atomic number
14
32
density (g/cm3)
2.33
5.32
4.96 x 1022
4.41 x 1022
12
16
1.115
1.165
0.665
0.746
intrinsic carrier density at 300 K (/cm3)
1.5 x 1010
2.4 x 1013
mobility (cm2/V/s) at 300 K: electrons
holes
1350
480
3900
1900
mobility (cm2/V/s) at 77 K: electrons
holes
2.1 x 104
1.1 x 104
3.6 x 104
4.2 x 104
3.62
3.76
(*)
2.96
atomic density (atoms/cm3)
dielectric constant (relative to vacuum)
band gap (eV) 300 K
0K
ionisation energy (eV) 300 K
77 K
(*) due to the small band gap, Ge needs to be cooled to reduce the leakage
current to an acceptable level (usually LN2 cooling, 77 K is used)
4
How to get a signal ?
r
E
e
I
h
!
Si!wafer
1 cm
1
cm
300 µm
I = I e + I h = A n i e (v e + v h )
= A n i e E (µ e + µ h )
A: surface area
v: velocity
µ: mobility
• mimimum ionizing particle
deposits ~400 eV/µm
creates ~3.3 x 104 e-h pairs
• free charge carriers in the same volume
4.5 x 109
! particle signal is drowned
solution: reduce number of free charge carriers,
i.e. deplete the semiconductor !
! doping
! blocking contact
5
Doped semiconductors: n-type and p-type
donor level
acceptor level
• pentavalent elements (group V/15, e.g. P, As, Sb)
have one electron too much to fit in: “donor
impurities”
• extra electrons are lightly bound
- energy level close to the conduction band
- thermally excited into the conduction band
- recombination with holes: ne >> nh
! n-type semiconductors
- electrons are the majority charge carriers
- holes are the minority charge carriers
• trivalent elements (group III/13, e.g. Ga, B, In) have
one electron too little to fit in: “acceptor impurities”
• electrons in missing bond slightly less bound
- energy level close to the valence band
- thermally excited electrons fill the acceptor level,
creating holes
- holes recombine with conduction band
electrons: nh >> ne
! p-type semiconductors
- holes are the majority charge carriers
- electrons are the minority charge carriers
6
7
Conductivity of doped semiconductors
•
•
typical doping levels for detector silicon: 1012 atoms/cm3
heavily doped semiconductors (n+, p+): 1020 atoms/cm3
results in very high conductivity (good for contacts)
8
Action of radiation
electrons
~1 eV
holes
•
•
e-h pairs are produced along track (Ne=Nh)
energy needed per e-h pair is largely independent on
– energy of radiation
– type of radiation
– temperature
! average energy needed = ionisation energy (")
semiconductor
• " is small:
Si, 300 K: 3.62 eV
Ge, 77 K: 2.96 eV
! good energy resolution
advantages of semiconductor detectors:
• good energy resolution
• high density, relatively high atomic number (Ge):
- good stopping power
- good foton interaction probability
9
The Fano factor
statistical variation in the number of e-h pairs
": ionisation energy
E
– # of e-h pairs N =
E: energy deposited by radiation quantum
"
–
if excitations are independent: Poisson statistics
! deviation
standard
Fano factor:
F"
!
for Si, Ge: F ~ 0.1-0.15
!
"N = N =
E
#
variance
" 2N =
E
#
observed statistical variance
!
E #
"E stat
FWHM ~ 3 times better than Poisson statistics
not fully understood phenomenon
!
10
Blocking contact: the p-n junction
n-type
p-type
mobile electrons
mobile holes
diffusion: holes to n-region, electrons to p-region
uncompensated fixed charges build up
emerging “contact” potential stops diffusion
depletion region
11
The depletion region detects ionising radiation
depletion region
• thermally generated charge carriers are quickly swept away
due to the contact potential
! highly suppressed charge carrier density
! relatively small amount of charge carriers created by an
ionising particle is easily detected
• poor performance because:
- small contact potential (~1 V): slow-moving charges can be
trapped, resulting in incomplete charge collection
- depletion layer is thin:
- high capacitance ! large electronic noise
- small sensitive volume cannot detect high-energy radiation
12
Reverse biasing
–
+
increased
depletion region
•
•
•
bias: 100 - 1000 V/cm
V >> contact potential
depletion region thickness increases
– smaller capacitance, smaller electronic noise
– quick and complete charge collection
•
very large electric field: multiplication ! silicon avalanche detector
13
Maximizing the depletion region
# 2 " V &1/ 2
d=%
(
$ eN '
d: depletion region thickness
V: reverse bias voltage
": dielectric constant
e: electronic charge
N: net impurity concentration (atoms/cm3)
•! normal semiconductor purity: depletion 2-3 mm maximum
• problem for “long-range” radiation (e.g. high-energy #-rays)
•
increasing d only by decreasing N
– further refining techniques
• Si: not (yet) possible
• Ge: high-purity germanium (HPGe)
– N ~1010 atoms/cm3 (relative impurity 10-12 !)
– depletion up to a few cm
– compensated material by lithium ion drifting
• Si(Li), Ge(Li): up to ~2 cm (so-called p-i-n structure)
14
Germanium detector configurations
planar
$, %: ultrapure n, p type
coaxial
15
Some examples of Si detectors
16
Some examples of Si detectors
17
Anatomy of a Ge detector
18
Some examples of Ge detectors
19
Some examples of Ge detectors
20
Other semiconductor materials
•
Why ?
– higher atomic number for higher #-ray interaction probability
– room-temperature operation (&Ge)
•
Commercially available detectors
– CdTe
– Cd1-xZnxTe (CZT)
– HgI2
•
Large crystal growth problems cause slow development
– impurities
– defects
! small volumes only
21
Properties high-Z semiconductor detectors
material
Si
atomic
number
density bandgap
(g/cm3)
(eV)
ionisation
energy
(eV)
14
2.33
1.12
3.61
32
5.32
0.72
2.98
48/52
6.06
1.52
4.43
Cd0.8Zn0.2Te
48/30/52
6
1.64
5.0
HgI2
80/53
6.4
2.13
4.3
(300 K)
Ge
(77 K)
CdTe
(300 K)
(300 K)
(300 K)
22