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   b-quarks ~ 1.5 ps  Distance traveled in lab =  c  ~ 4500  m

4 B-quark Tagging  SVT (secondary vertex tagging)  IP (impact parameter) L Primary vertex Secondary 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 E gap (valence – conduction) = 1.12 eV Intrinsic electron density n = hole density p = 1.45 x /cm 3 (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  Consider an Si detector 1 cm x 1 cm x 300  m In this volume there will be 4.5 x 10 8 free charge carriers A mip will produce 3.2 x 10 4 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 / cm 3 Detector grade silicon (10 12 / cm 3 ) Electronics grade silicon (10 17 / cm 3 ) To be compared with silicon density of / cm 3 More heavily doped concentrations ( / cm 3 ) are called p + or n +  In nearly all cases, the impurity concentrations are large compared with the intrinsic carrier concentration (10 10 /cm 3 ) n ~ N D for n-type p ~ N A 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  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 N A > N D (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 / cm 3 ) 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-typen-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