Semiconductor Detectors

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. b = distance of closest approach of a reconstructed track to the true interaction point b beam B-quark Tagging • SVT (secondary vertex tagging) • IP (impact parameter) L Secondary vertex Primary vertex

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

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. anode cathode p-type n-type Diode

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