SILICON DETECTORS PART I Characteristics on semiconductors
Semiconductors (I) Z1432 A Density (g/cm 3 ) Atoms/cm x x10 22 rr 1216 E g (300 K) eV Intrinsic carrier density (300 K) cm x x10 13 Intrinsic resistivity (300 K) Ωcm 2.3x e (300 K) cm 2 /(Vs) h (300K) cm 2 /(Vs) SiGe Energy gap Conductivity Two types of charge carriers: Electrons + holes
Semiconductors (II) Si Intrinsic silicon: four covalent bounds, four valence electrons P+ Si n-type silicon: dopant atoms with 5 valence electrons. The fifth electron is loosely bound and even at room temperature it is essentially free to move. Electrons are the majority carriers - B- Si + p-type silicon: dopant atoms with 3 valence electrons. One electron is missing, and this “hole” can be filled by another electron of the lattice, which will leave its position vacant. Hole = positive charge. Holes are the majority carriers EgEg Valence band Conduction band EfEf donor levels Valence band Conduction band EfEf Valence band Conduction band EfEf acceptor levels
tipo p tipo n p-n junction (I) depletion layer Electric field
Reverse biased p-n junction (I) +V V p n d -x p xnxn N A >>N D x x p+n E V -x p xnxn
Reverse biased p-n junction (II) Depletion voltage: voltage necessary to deplete all the junction thickness How to know the depletion voltage of a diode? Measurement of the capacitance
Leakage current The main sources of leakage current in a silicon sensor are: 1) Diffusion of charge carriers from undepleted regions of the detector to the depleted region. Generally well controlled, small contribution ~few nA/cm 2 2) Thermal generation of electron-hole pairs in the depleted regions. Temperature dependent, contribution ~ A/cm 2 3) Surface currents depending on contamination, surface defects from processing.. It may be the dominant contribution, but it can be reduced processing guard rings
p-n junction as detector Metal contact photonCharged particle n+-type implant n-type bulk -V +V electron hole P+ Energy lost by a m.i.p. in 1 mm of silicon is ~ 300 KeV. The typical thickness of detectors is ~300 m. Energy necessary for a m.i.p. to produce a pair of electron-hole in Si: 3.6 eV A m.i.p. produces ~25000e-≈ 4fC
Why a reverse-biased diode? The amount of charge deposited in the typical 300 m of thickness of a silicon detector is very small an therefore it would be masked by the fluctuations of the current which the applied field makes flow even in high resistivity, hyper-pure silicon. If we reverse-bias the diode, we will have the necessary electric field and only a very small current. -V +V depleted region Increasing the polarization voltage, it is possible to extend the depletion layer down to the backplane. To have full efficiency, the polarization voltage must be high enough to deplete the full detector thickness (typically 300 m) junction
What is the signal? When do we see the signal? The signal arises immediately the charge carriers start to move. The current signal induced on the diode is due to the movement of charge carriers in the electric field (Ramo’s theorem) Signal
Fabrication N-type silicon SiO 2 B B As n+ P+ n-type wafers are oxidized at 1030 o C to have the whole surface passivated. Using photolithographic and etching techniques, windows are created in the oxide to enable ion implantation. Different geometries of pads and strips can be achieved using appropriate masks. The next step is the doping of silicon by ion implantation. Dopant ions are produced from a gaseous source by ionisation using high voltage.The ions are accelerated in an alectric field to energy in the range of 10 keV-100 keV and then the ion beam is directed to the windows in the oxide. P+ strips are implanted with boron, while phosphorous or arsenic are used for the n+ contacts. An annealing process at 600 o C allows partial recovery of the lattice from the damage caused by irradiation. The next step is the metallisation with aluminium, required to make electrical contact to the silicon. The desired pattern can be achieved using appropriate masks. Al The last step before cutting is the passivation, which helps to maintain low leakage currents and protects the junction region from mechanical and ambient degradation.