1

Semiconductor Detectors It may be that when this class is taught

10 years on, we may only study semiconductor detectors

In general, silicon provides Excellent energy resolution Excellent charge carrier collection properties Excellent position resolution (EPP) High density (versus gas e.g.) On the negative side, they are subject to

radiation damageSemiconductor detectors are found in

many fields of physical research and industry

2

Semiconductor DetectorsLet’s look at the energy required to

produce a signal Scintillation detectors – 1 photon / 100 eV Ionization detectors – 1 ion pair / 16 eV Silicon detectors – 1 electron-hole pair / 3.6

eV

3

Semiconductor Detectors In EPP, the main use of silicon detectors is

for precision tracking Finn showed an expression for the momentum

resolution of a “tracker” in a magnetic field

Additionally, silicon detectors are used for b-quark tagging b quarks are an indication of interesting

physics b-quarks ~ 1.5 ps Distance traveled in lab = c ~ 4500 m

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4

B-quark Tagging

SVT (secondary vertex tagging)

IP (impact parameter)

L

Primary vertex

Secondary vertex

bbeam

b = distance of closest approach of a

reconstructed track to the true interaction point

5

B-quark Tagging

6

Silicon Intrinsic silicon

Egap (valence – conduction) = 1.12 eV Intrinsic electron density n = hole density p =

1.45 x 1010/cm3 (300K)300K=1/40 eV

7

SiliconOther properties of pure (intrinsic) silicon

There are alternatives to silicon Germanium (Ge), diamond, gallium arsenide

(GaAs), silicon carbide (SiC), …But the silicon’s wide technology base

makes it the usual choice for a detector

8

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9

SiliconConsider an Si detector 1 cm x 1

cm x 300 m In this volume there will be 4.5 x 108

free charge carriers A mip will produce 3.2 x 104 electron-

hole pairs Not a great particle detector!

In order to make a useful detector we need to reduce the number of free charge carriers

10

Dopingn-type

Replace Si with P, As, Sb (donor) Electrons (holes) are majority (minority)

carriers

11

Dopingp-type

Replace Si with B, Al, Ga, In (acceptor) Holes (electrons) are majority (minority)

carriers

12

DopingThe result of doping is to increase the

number of charge carriers by adding impurity levels to the band gap n-type p-type

13

DopingTypical impurity concentrations are 1012-

1018 / cm3

Detector grade silicon (1012 / cm3) Electronics grade silicon (1017 / cm3) To be compared with silicon density of 1022 / cm3

More heavily doped concentrations (1018-1020 / cm3) are called p+ or n+

In nearly all cases, the impurity concentrations are large compared with the intrinsic carrier concentration (1010/cm3) n ~ ND for n-type p ~ NA for p-type

14

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15

p-n Junction Majority carriers diffuse into the boundary Resulting exposed donor (+) and acceptor (-)

atoms build up an E field that halts further diffusion

A thin (< 100 m) depletion region (no free charge carriers) is created at the boundary

No current flows (at equilibrium)

16

p-n Junction

(a)Current flow

(c)Electric field

(b)Charge density

NA > ND

(d)Electrostatic potential

o : built in potential under zero bias

o

17

Forward Bias p-n Junction Positive on p side, negative on n side The electrons can easily overcome the (~1V)

contact potential Current easily flows across the junction even for

small values of forward bias voltage The depletion region becomes smaller

18

Reverse Bias p-n Junction Negative on p side, positive on n side Majority carriers are swept away from the

boundary region and the depletion region becomes larger

Little current flows across the boundary Unless the reverse bias voltage becomes large

enough to overcome the space charge in the depletion region

19

Reverse Bias p-n Junction Most silicon detectors are reversed biased

p-n junctions The charged carrier concentration in the

depletion region is now very low (~<100 / cm3)

Electron-hole pairs created by ionizing particles will be quickly swept out of the depletion region by the electric field

The motion of these electron-hole pairs constitutes the basic signal for particle detection

As in gas detectors, the electrical pulse on the electrodes arises from induction caused by movement of the electrons and holes rather than the actual collection of the charge itself

20

Diodep-n junction is what makes a diode

Note there is a diode “drop” of ~0.7V to get current flowing in the forward bias region

With one exception, the breakdown (Peak Inverse Voltage) region usually destroys a diode

PIV

21

Diode

p-type n-typeanode cathode

22

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23

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24

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25

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26

Depletion DepthThe depletion region acts like a capacitor

It is often the case that electronic noise is the dominant noise source hence it is desirable to have the detector capacitance as small as possible Large V and large d

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27

Semiconductor DetectorsMany varieties

Si strip detector Si pixel detector Si drift chamber CCD (Charged Coupled Device) Surface barrier PIN photodiode Avalanche photodiode a-Se + TFT (Thin Film Transistor)

arrays