Detection of energetic particles and gamma rays

Semiconductor detectors

Peter Dendooven

KVIdendooven@kvi.nl

Basic Detection Techniques 2009-2010http://www.astro.rug.nl/~peletier/DetectionTechniques.html

2

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

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Semiconductor band structure

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

!

ni = pi = NcN v e"

Eg

2kT = AT3/ 2 e"

Eg

2kT

metalinsulator

>6 eV

semiconductor

~1 eV

electrons

holes

energ

y

occupied

empty

conduction band

valence band

band gap at T=0 K

at T>0 K

kT at 300 K = 0.025 eV

4

Relevant properties of intrinsic Si and Ge

(*) 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)

(*)

2.96

3.62

3.76

ionisation energy (eV) 300 K

77 K

3.6 x 104

4.2 x 104

2.1 x 104

1.1 x 104

mobility (cm2/V/s) at 77 K: electrons

holes

3900

1900

1350

480

mobility (cm2/V/s) at 300 K: electrons

holes

2.4 x 10131.5 x 1010intrinsic carrier density at 300 K (/cm3)

0.665

0.746

1.115

1.165

band gap (eV) 300 K

0 K

1612dielectric constant (relative to vacuum)

4.41 x 10224.96 x 1022atomic density (atoms/cm3)

5.322.33density (g/cm3)

3214atomic number

GeSi

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How to get a signal ?

• 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

300 µm

1 cm

1 cm

Si wafer

!

r E he I

!

I = Ie + Ih = A ni e (ve + vh )

= A ni e E (µe + µh )

A: surface area

v: velocityµ: mobility

6

Doped semiconductors: n-type and p-type

• 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

donor level

acceptor level

• 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

7

8

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)

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Action of radiation

• 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 (")

• " is small: Si, 300 K: 3.62 eV

Ge, 77 K: 2.96 eV

! good energy resolution

semiconductor

~1 eV

electrons

holes

advantages of semiconductor detectors:

• good energy resolution

• high density, relatively high atomic number (Ge):

- good stopping power

- good foton interaction probability

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The Fano factor

statistical variation in the number of e-h pairs

– # of e-h pairs

– if excitations are independent: Poisson statistics

!

N =E

"

": ionisation energy

E: energy deposited by radiation quantum

!

"N

= N =E

#variancestandard deviation

!

F "observed statistical variance

E #

!

"N

2=

E

#

Fano factor:

for Si, Ge: F ~ 0.1-0.15 ~ 3 times better than Poisson statistics

!

"EFWHM

stat

not fully understood phenomenon

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Blocking contact: the p-n junction

n-typemobile electrons

p-typemobile holes

depletion region

diffusion: holes to n-region, electrons to p-region

uncompensated fixed charges build up

emerging “contact” potential stops diffusion

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The depletion region detects ionising radiation

• 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

depletion region

13

increased

depletion region

+ –

Reverse biasing

• 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

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Maximizing the depletion region

• 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)

!

d =2 " V

e N

#

$ %

&

' (

1/ 2

d: depletion region thickness

V: reverse bias voltage

": dielectric constant

e: electronic charge

N: net impurity concentration (atoms/cm3)

15

Germanium detector configurations

$, %: ultrapure n, p type

planar coaxial

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Some examples of Si detectors

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Some examples of Si detectors

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Anatomy of a Ge detector

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Some examples of Ge detectors

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Some examples of Ge detectors

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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

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Properties high-Z semiconductor detectors

4.32.136.480/53HgI2

(300 K)

5.01.64648/30/52Cd0.8Zn0.2Te

(300 K)

4.431.526.0648/52CdTe(300 K)

2.980.725.3232Ge

(77 K)

3.611.122.3314Si

(300 K)

ionisation

energy

(eV)

bandgap

(eV)

density

(g/cm3)

atomic

numbermaterial