Particle Physics Booklet - Particle Data Group - Lawrence Berkeley ...

28. Detectors at accelerators 251
28.7. Semiconductor detectors
Updated September 2009 by H. Spieler (LBNL).
28.7.1. Materials Requirements : Semiconductor detectors are essentially
solid state ionization chambers. Absorbed energy forms electron-hole
pairs, i.e., negative and positive charge carriers, which under an applied
electric field move towards their respective collection electrodes, where
they induce a signal current. The energy required to form an electron-hole
pair is proportional to the bandgap. In tracking detectors the energy loss
in the detector should be minimal, whereas for energy spectroscopy the
stopping power should be maximized, so for gamma rays high-Z materials
are desirable.
Measurements on silicon photodiodes [112] show that for photon
energies below 4 eV one electron-hole (e-h) pair is formed per incident
photon. The mean energy Ei required to produce an e-h pair peaks at
4.4 eV for a photon energy around 6 eV. Above ∼1.5 keV it assumes
a constant value, 3.67 eV at room temperature. It is larger than the
bandgap energy because momentum conservation requires excitation of
lattice vibrations (phonons). For minimum-ionizing particles, the most
probable charge deposition in a 300 μm thick silicon detector is about
3.5 fC (22000 electrons). Other typical ionization energies are 2.96 eV in
Ge, 4.2 eV in GaAs, and 4.43 eV in CdTe.
Since both electronic and lattice excitations are involved, the variance
in the number of charge carriers N = E/Ei produced by an absorbed
energy E is reduced by the Fano factor F (about 0.1 in Si and Ge). Thus,
σN = √ FN and the energy resolution σE/E = � FEi/E. However, the
measured signal fluctuations are usually dominated by electronic noise or
energy loss fluctuations in the detector.
A major effort is to find high-Z materials with a bandgap that is
sufficiently high to allow room-temperature operation while still providing
good energy resolution. Compund semiconductors, e.g., CdZnTe, can allow
this, but typically suffer from charge collection problems, characterized
by the product μτ of mobility and carrier lifetime. In Si and Ge
μτ > 1cm2V−1 for both electrons and holes, whereas in compound
semiconductors it is in the range 10−3 –10−8 . Since for holes μτ is typically
an order of magnitude smaller than for electrons, detector configurations
where the electron contribution to the charge signal dominates—e.g., strip
or pixel structures—can provide better performance.
28.7.2. Detector Configurations :Ap-n junction operated at reverse
bias forms a sensitive region depleted of mobile charge and sets up an
electric field that sweeps charge liberated by radiation to the electrodes.
Detectors typically use an asymmetric structure, e.g., a highly doped
p electrode and a lightly doped n region, so that the depletion region
extends predominantly into the lightly doped volume.
In a planar device the thickness of the depleted region is
W = � 2ɛ (V + Vbi)/Ne = � 2ρμɛ(V + Vbi) , (28.1)
where V = external bias voltage
Vbi = “built-in” voltage (≈ 0.5 V for resistivities typically used in Si
detectors)
N = doping concentration
e = electronic charge
ɛ = dielectric constant = 11.9 ɛ0 ≈ 1pF/cminSi
ρ = resistivity (typically 1–10 kΩ cm in Si)
μ = charge carrier mobility
= 1350 cm2 V−1 s−1 for electrons in Si