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

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

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

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

B-quark Tagging

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

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

Silicon

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

Doping n-type Replace Si with P, As, Sb (donor) Electrons (holes) are majority (minority) carriers

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

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

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

Doping

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)

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

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

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

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

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

Diode p-type n-type anode cathode

Depletion Depth

Depletion Depth

Depletion Depth

Depletion Depth

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

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