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 damage
Semiconductor detectors are found in many fields of physical research and industry

2. Semiconductor Detectors
Let’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
t b-quarks ~ 1.5 ps
Distance traveled in lab = gct ~ 4500 mm

4. to the true interaction point
B-quark Tagging
SVT (secondary vertex tagging)
IP (impact parameter) L
Secondary vertex
Primary vertex b
beam
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. Silicon Other 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. Silicon

9. Silicon Consider an Si detector 1 cm x 1 cm x 300 mm
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. Doping n-type Replace Si with P, As, Sb (donor)
Electrons (holes) are majority (minority) carriers

11. Doping p-type Replace Si with B, Al, Ga, In (acceptor)
Holes (electrons) are majority (minority) carriers

12. Doping
The result of doping is to increase the number of charge carriers by adding impurity levels to the band gap
n-type p-type

13. Doping Typical 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 ( / 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. Doping

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 mm) depletion region (no free charge carriers) is created at the boundary
No current flows (at equilibrium)

16. p-n Junction NA > ND (a)Current flow (b)Charge density
(c)Electric field
(b)Charge density
NA > ND
(d)Electrostatic potential
: built in potential under zero bias

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. Diode p-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-type
anode
cathode

22. Depletion Depth

23. Depletion Depth

24. Depletion Depth

25. Depletion Depth

26. Depletion Depth The 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

27. Semiconductor Detectors
Many 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