196
G. Lutz and R. Klanner
timing accuracy and 25 ns pulse shaping, are required. From simulations [55] it
is concluded that pixel sensors with 50 μm active thickness and a doping profile
similar to the one shown for APDs in Fig. 5.52 and operated at a gain of ≈ 20 can
reach the required performance. These detectors are called Low-Gain-Avalanche
Detectors, LGADs.
Different to optical photons which generate single electron-hole pairs, minimumionizing produce about 75 charge pairs per micro-meter and a high gain is not
required. In addition to increasing fluctuations, high gain causes also practical
difficulties and increases the shot noise from the dark current. Thin sensors have the
additional advantage of smaller dark currents and a pulse rise time which increases
with decreasing sensor thickness.
The effects which influence the timing accuracy can be grouped in five categories: (1) Position-dependent fluctuations of the charge carriers produced by the
charged particle to be measured, (2) excess noise of the amplification mechanism,
(3) position dependent drift field and coupling of the of the drifting charges to the
readout electrodes, (4) electronics noise, and (5) digitisation error of the time-todigital convertor.
A major issue for LGADs is the control of the gain after irradiation. The change
of the effective doping by dopant removal and defect states, and the decrease of
the mobilities and amplification coefficients of electrons and holes due to radiation
damage appear to present major problems. These are addressed in an extensive R&D
program which started in 2012 and has already given first encouraging results.
5.12 Summary and Outlook
Different concepts of solid silicon sensors and the electronics required for their
readout have been described in this contribution. Although a detailed theoretical
understanding of silicon devices had already been achieved in the 1960s, silicon
detectors remained a niche application, used mainly in Nuclear Physics. This
changed around 1980, when Josef Kemmer adapted the planar technology of microelectronics to sensor fabrication and the ACCMOR Collaboration demonstrated
the reliable long-term operation and excellent physics performance of silicon strip
detectors. Based on these results, many groups started to develop and use silicon
detectors, and today there is hardly a particle physics experiment, which does not
rely heavily on them. The areas covered by silicon detectors in the particle physics
experiments increased from tens of cm 2 to hundreds of m 2 . Large areas of silicon
detectors are even used on satellites for space experiments. In parallel to silicon
detectors, the development of low-noise ASICs and connection technology started.
They are required for reading out the more and more complex silicon sensors. In
addition, a number of industrial producers, in closed collaboration with academia,
developed and fabricated silicon sensors. Today silicon radiation detectors are a
quite big market. Initially developed for Particle Physics, the use of silicon detectors
spread into many different fields of science, medicine and industrial applications.
G. Lutz and R. Klanner
timing accuracy and 25 ns pulse shaping, are required. From simulations [55] it
is concluded that pixel sensors with 50 μm active thickness and a doping profile
similar to the one shown for APDs in Fig. 5.52 and operated at a gain of ≈ 20 can
reach the required performance. These detectors are called Low-Gain-Avalanche
Detectors, LGADs.
Different to optical photons which generate single electron-hole pairs, minimumionizing produce about 75 charge pairs per micro-meter and a high gain is not
required. In addition to increasing fluctuations, high gain causes also practical
difficulties and increases the shot noise from the dark current. Thin sensors have the
additional advantage of smaller dark currents and a pulse rise time which increases
with decreasing sensor thickness.
The effects which influence the timing accuracy can be grouped in five categories: (1) Position-dependent fluctuations of the charge carriers produced by the
charged particle to be measured, (2) excess noise of the amplification mechanism,
(3) position dependent drift field and coupling of the of the drifting charges to the
readout electrodes, (4) electronics noise, and (5) digitisation error of the time-todigital convertor.
A major issue for LGADs is the control of the gain after irradiation. The change
of the effective doping by dopant removal and defect states, and the decrease of
the mobilities and amplification coefficients of electrons and holes due to radiation
damage appear to present major problems. These are addressed in an extensive R&D
program which started in 2012 and has already given first encouraging results.
5.12 Summary and Outlook
Different concepts of solid silicon sensors and the electronics required for their
readout have been described in this contribution. Although a detailed theoretical
understanding of silicon devices had already been achieved in the 1960s, silicon
detectors remained a niche application, used mainly in Nuclear Physics. This
changed around 1980, when Josef Kemmer adapted the planar technology of microelectronics to sensor fabrication and the ACCMOR Collaboration demonstrated
the reliable long-term operation and excellent physics performance of silicon strip
detectors. Based on these results, many groups started to develop and use silicon
detectors, and today there is hardly a particle physics experiment, which does not
rely heavily on them. The areas covered by silicon detectors in the particle physics
experiments increased from tens of cm 2 to hundreds of m 2 . Large areas of silicon
detectors are even used on satellites for space experiments. In parallel to silicon
detectors, the development of low-noise ASICs and connection technology started.
They are required for reading out the more and more complex silicon sensors. In
addition, a number of industrial producers, in closed collaboration with academia,
developed and fabricated silicon sensors. Today silicon radiation detectors are a
quite big market. Initially developed for Particle Physics, the use of silicon detectors
spread into many different fields of science, medicine and industrial applications.