5 Solid State Detectors
145
5.5 Semiconductor Detector Principles
The very basic and most common detector type, the reverse-biased diode, has
already been sketched in Sect. 5.2. Here we will give some more information on this
device and also present some more sophisticated principles, the semiconductor drift
chamber and the DEPFET detector-amplification structure, while detectors based
on the avalanche mechanism will be discussed in a later chapter.
5.5.1 Reverse Biased Diode (as Used in Strip and 3-D
Detectors)
The principle of a reverse biased diode has already been sketched in Sect. 5.2. Here
a more detailed discussion is given. Even without applying a bias the p-n junction
develops a space charge region due to the diffusion of electrons and holes across
the junction leading to a surplus of negative charge on the p-side and of positive
charge on the n-side of the junction. This creates an electric field, a drift current and
a space charge region on both sides of the junction. At any point of the device drift
and diffusion currents cancel each other in equilibrium without external bias. Such
a device can already be used as radiation detector since electron–hole pairs created
in the space charge region will be separated by the electric field thus create a current
across the junction.
Reverse biasing will increase the space charge region and therefore the electric
field. For a strongly asymmetric, but in each region uniformly doped p + n junction
(as shown in Fig. 5.1) the depth of the space charge and therefore the sensitive region
increases with the square root of the applied voltage.
Reverse biased diodes have been used as energy sensitive radiation detectors
in Nuclear Physics for quite some time. The real breakthrough came with strip
detectors in Particle Physics used for particle tracking with micro-meter accuracy.
Many small strip-like diodes were integrated on the same wafer and each one
connected to its own readout channel (Fig. 5.3). The particle position was given by
the channel giving the signal. More sophisticated strip detectors will be described
in Sect. 5.6.
Planar pixel detectors are obtained by shortening the individual strips so that
they do not reach anymore the detector edge and form a two-dimensional pattern.
Detectors with pixel sizes down to 15 × 15 μm 2 have been built. The main difficulty
of such detectors is their readout. Different realisations will be discussed later.
A different concept of diode detectors, the so called 3-D detectors [11], is
shown in Fig. 5.4: Holes with diameters of a few micro-meters are etched into
the crystal orthogonal to its surface, and alternate holes are n + - and p + -doped. A
voltage difference between the n + - and p + -doped columns generates an electric
field parallel to the crystal surface. The number of electron-hole pairs produced by
a charged particle traversing the detector at large angles to the surface is given by
145
5.5 Semiconductor Detector Principles
The very basic and most common detector type, the reverse-biased diode, has
already been sketched in Sect. 5.2. Here we will give some more information on this
device and also present some more sophisticated principles, the semiconductor drift
chamber and the DEPFET detector-amplification structure, while detectors based
on the avalanche mechanism will be discussed in a later chapter.
5.5.1 Reverse Biased Diode (as Used in Strip and 3-D
Detectors)
The principle of a reverse biased diode has already been sketched in Sect. 5.2. Here
a more detailed discussion is given. Even without applying a bias the p-n junction
develops a space charge region due to the diffusion of electrons and holes across
the junction leading to a surplus of negative charge on the p-side and of positive
charge on the n-side of the junction. This creates an electric field, a drift current and
a space charge region on both sides of the junction. At any point of the device drift
and diffusion currents cancel each other in equilibrium without external bias. Such
a device can already be used as radiation detector since electron–hole pairs created
in the space charge region will be separated by the electric field thus create a current
across the junction.
Reverse biasing will increase the space charge region and therefore the electric
field. For a strongly asymmetric, but in each region uniformly doped p + n junction
(as shown in Fig. 5.1) the depth of the space charge and therefore the sensitive region
increases with the square root of the applied voltage.
Reverse biased diodes have been used as energy sensitive radiation detectors
in Nuclear Physics for quite some time. The real breakthrough came with strip
detectors in Particle Physics used for particle tracking with micro-meter accuracy.
Many small strip-like diodes were integrated on the same wafer and each one
connected to its own readout channel (Fig. 5.3). The particle position was given by
the channel giving the signal. More sophisticated strip detectors will be described
in Sect. 5.6.
Planar pixel detectors are obtained by shortening the individual strips so that
they do not reach anymore the detector edge and form a two-dimensional pattern.
Detectors with pixel sizes down to 15 × 15 μm 2 have been built. The main difficulty
of such detectors is their readout. Different realisations will be discussed later.
A different concept of diode detectors, the so called 3-D detectors [11], is
shown in Fig. 5.4: Holes with diameters of a few micro-meters are etched into
the crystal orthogonal to its surface, and alternate holes are n + - and p + -doped. A
voltage difference between the n + - and p + -doped columns generates an electric
field parallel to the crystal surface. The number of electron-hole pairs produced by
a charged particle traversing the detector at large angles to the surface is given by