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