Semiconductor (solid state) detectors

Semiconductor (solid state)detectors

1. Introduction

2. Principle of semiconductors

3. Silicon detectors, p-n junction, depleted region, induced charge

4. energy measurement, germanium detectors

5. position measurement, silicon strip detectors, pixel detectors silicon drift detectors6. DEPFET

7. Photon detectors, APD, SiPM

8. 3D detectors

J. Žáček Experimentální metody jaderné a subjaderné fyziky

1. Introduction

Principle of semiconductors

hole conduction

electron concentrationg(E) - density of electron state in the conduction band f(E) g( E) – electron concentration⦁ lowest energy level in the conduction band g() ≡ density of electron states in the lowest energy level

approximation : f ≈

electron concentration in the lowest energy level hole concentration - density of hole state in the highest energy level of the valence band hole concentration in the highest energy level

Boltzmann constant k ≈ 8.6 ⦁ eV ⦁ E-

μ mobility, E external electric field

Current : J = e + = σ E, σ - conductivity R = 1/σ - resistivity

μ𝑒

i) Direct recombination

Recombination and trapping of the charge carriers

ii) Recombination resulting from impurities in the crystal

iii) Trapping resulting from impurities in the crystal

iv) Structural defects in the lattice

3. Silicon semiconductors, p – n junctionSi:

n- type semiconductor

p- type semiconductor

Approximation of charge densities

Concentration of acceptors

Concentration of donors

Maxwell equations:

Using resistivity of n-type

𝑹𝒏𝑹𝒑

For R≈ 20 000d= 75 μm

For reversed bias V= d ~ 300 μm

R = 1/(e + ) in n-type, = 0

over-dopped p-type

depletion region HV

Ohmic contact : direct metal – p-type not possible, because of the barrier between metal and p-type instead heavily doped p-type and then a metal

Induced charge

Q - charge in the depletion region

but different coordinate frame, zero at the junction

x ⟶ x - , ≡ d, E=-dV/dx

d - thickness of the depletion region

,resistivity R=1/( )

ε ⦁R

𝑡𝑑

𝑞h (𝑡 )=¿

Induced charge at

i.e. If x(t) =0

Ex. /pair

a good preamplifier needed, low noice

DC direct coupling, AC

4. Energy measurementConstruction of p-n junctions

• Diffused junction diode: diffusion of donors to p-type at the temperature 1000 C

• Surface barrier junction: junction between a semiconductor and a metal n-type Si with Au, p-type Si with Al sensitive to light

• Ion-implanted junctions: a substrate is bombarded by ions from an accelerator

Depleted region small ⟹ energy measurement for low energies

Guard ring

Compensating materials developed to increase the depletion region by lithium drifting process known as p-i-n junction

Li diffused to p-type, a narrow n-type is created electrons drifted to p-type, negative space charge application of HV ⟶ positive Li ions drifted to p-type for sufficient time to create

⟹ the same concentration of positive ions and electrons t ⟹ no space charge, i.e. compensated region resistivity up to 100 000 Ω width of compensating region 10-15 mm Si(Li) , the noise is much greater then in normal Si cooling is needed

Energy resolutionFluctuation of energy losses in the depleted region

Landau fluctuation

most probable energy loss

Germanium detectors suitable for γ detection,

Resolution at 1.33 Mev Ge detector 0.15 % NaI 8 %

-- High purity germanium (PHGe), depletion region~ cm, low temperature during - measurement only

Shape of Ge detectors - planar, circular shape, diameter 1-2 cm, volume 10-20 coaxial , volume up to 400

5. Position measurement, silicon strip and pixel detectors

i) Manufacturing of Si strip detectors

ii) Microstrip detectors

iii) Position resolution

iv) Pixel detectors

v) Silicon drift detectors

analog readout

Advantages and disadvantages

Application of strip, pixel and pad detectors

Trackers: precise determination of particle tracks (strips or pixels)Vertex detectors: in collider experiments, detectors situated around the interaction vertex

Topology: sensors mounted on a planar carbon frames or cylindrical carbon frames

Calorimeters: as active layers in sampling calorimeters

forward and backward silicon tracker of the H1 experimentCollider HERA, DESY Hamburg, electrons (~26 GeV) vs protons (920 GeV) several layers of circular planes equipted with strip sensors

Interaction vertex

Beam pipe

electrons

protons

Emitted particle

electronics

Si sensors

sensor

particle

Pad silicon detectors for the readout of the electromagnetic calorimeter CALICE

SiSi wafers 6 x 6 cm, 1 pad 1x1 cm, depletionregion 500 μm

calorimeter: absorber tungsten, active layers from Si wafers electronic layer above active layer

(calorimeter for linear collider)

W - layerSi wafers

readout board

5. DEPFET

Bipolární tranzistor:

Nepřipojený k obvodu Připojený k obvodu

emitor báze kolektor

FET tranzistor

Polem řízené (neboli unipolární či FET) tranzistory spínají/omezují protékající proud na základě toho, jaké napětí je na „drain“

řídicí se nazývá gate a značí se "G",spínaný proud vstupuje do drainu "D" avystupuje z source "S".

Tři jednotky FETu:

drain je zde jako kolektor, source jako emitor a gate jako báze

Proud teče mezi S a D mezi nimiž je napětí.Napětí na D mění vodivost substrátu, tj proud teče/neteče

Zdroj proudu je S, výstupní proud je v D.

DEPFET je FET vytvořený na plně vyčerpanén substrátu. Působí současně jako senzor a zesilovač

Top gate

P –channel FET on a fully depleted n-bulk

𝑛+¿ ¿

electrons from photon are collected at the internal gate the energy deposited by a photon is determined by the change of the FET conductivity

clear mode - change of the FET conductivity,

This difference ~to the total amount of collected charge

7. Semiconductor photon detectors APD - avalanche photodiodereplace e.g. photomultipliers in calorimeters, very small devices,can be connected with fibers

Usual photodiode

avalanche photodiode

HAPD - hybrid APD

SiPM Silicon Photon Multipliers

1156 photodiodes on the area 1.1 x 1.1

depletion region

SiPM detects individual photons, current ~ to the number of fired pixels

Hadron calorimeter

Scintillation light from the tile is collected by a WLS fiber which is directly connected to a SIPM.

WLS fibre

SiPM were first developed for the readout of scintillation light of the hadron calorimeter within CALICE collaboration

(pixel ≡ photodiode)Pedestal ≡ noise

3D detectors

Semiconductor (solid state) detectors

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