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G. Lutz and R. Klanner
storing capability. Before turning to the matrix arrangement the main properties of
the DEPFETs are summarised:
• Combined function of sensor and amplifier;
• Full sensitivity the over complete wafer, low capacitance and low noise, nondestructive repeated readout, complete clearing of the signal charge and thus no
reset noise.
• Continuous (real time) and integrating (charge storage) operating modes can be
chosen.
The signal can be read out either at the source as indicated in the left figure or at
the drain as shown in the linear example. With source readout one compensates the
increase of channel conduction due to the charge in the internal gate by a reduction
of the external gate-source voltage, seen as voltage change of the source. In the
drain readout the source potential is kept constant and the drain-current change can
be directly observed. An important property in pixel detector applications is the fact
that the signal charge collection occurs not only for current carrying DEPFETs but
also for those which have been turned off with the help of the external FET gate.
DEPFET pixel sensors have been developed at the MPI Semiconductor Laboratory in Munich for several purposes, as focal sensors of the proposed European
X-ray observatory XEUS [38] and as vertex detector for the BELLE-II experiment
at KEK in Japan and the proposed International Linear Collider ILC. In XEUS the
combined functions of imaging and spectroscopy are of importance, for the vertex
detectors the measurement of position of charged tracks is of prime interest. This
however has to be done with very high precision (few μm) and at high readout speed.
The position measurement requirement in XEUS is not as stringent; it is matched
to the expected quality of X-ray imaging. However, highest emphasis is given to
spectroscopic quality and quantum efficiency and data readout speed is still large.
As a consequence of these and further requirements circular geometries have
been chosen for XEUS and linear ones for the vertex detectors (see Fig. 5.41). The
excellent spectroscopic capabilities of DEPFETs can be appreciated from the 55 Fe
source spectrum taken with a single circular pixel cell (Fig. 5.42).
The DEPFET with its capability of creating, storing and amplifying signal charge
is an ideal building block for a pixel detector. A large number of DEPFETs can be
arranged in a matrix in such a way as to power selected DEPFETs for reading and
clearing the collected signal charge. Figure 5.43 shows a rectangular arrangement of
DEPFETs. Their drains are connected column wise while gates and clear electrodes
are connected row wise. Each row has its individual readout channel. A row at a
time is turned on with the help of the gate voltage while all other DEPFETs have
zero current. Charge collection does not require a current within the DEPFET.
Readout can be performed in double correlated mode: Turning on the current
with a negative voltage on the gate is followed by a first reading of the current, a
clearing of the signal charge in the internal gate with a positive pulse at the clear
contact and a second current reading before the current is turned down again and
reading is switched to the next row. The difference of first and second current
G. Lutz and R. Klanner
storing capability. Before turning to the matrix arrangement the main properties of
the DEPFETs are summarised:
• Combined function of sensor and amplifier;
• Full sensitivity the over complete wafer, low capacitance and low noise, nondestructive repeated readout, complete clearing of the signal charge and thus no
reset noise.
• Continuous (real time) and integrating (charge storage) operating modes can be
chosen.
The signal can be read out either at the source as indicated in the left figure or at
the drain as shown in the linear example. With source readout one compensates the
increase of channel conduction due to the charge in the internal gate by a reduction
of the external gate-source voltage, seen as voltage change of the source. In the
drain readout the source potential is kept constant and the drain-current change can
be directly observed. An important property in pixel detector applications is the fact
that the signal charge collection occurs not only for current carrying DEPFETs but
also for those which have been turned off with the help of the external FET gate.
DEPFET pixel sensors have been developed at the MPI Semiconductor Laboratory in Munich for several purposes, as focal sensors of the proposed European
X-ray observatory XEUS [38] and as vertex detector for the BELLE-II experiment
at KEK in Japan and the proposed International Linear Collider ILC. In XEUS the
combined functions of imaging and spectroscopy are of importance, for the vertex
detectors the measurement of position of charged tracks is of prime interest. This
however has to be done with very high precision (few μm) and at high readout speed.
The position measurement requirement in XEUS is not as stringent; it is matched
to the expected quality of X-ray imaging. However, highest emphasis is given to
spectroscopic quality and quantum efficiency and data readout speed is still large.
As a consequence of these and further requirements circular geometries have
been chosen for XEUS and linear ones for the vertex detectors (see Fig. 5.41). The
excellent spectroscopic capabilities of DEPFETs can be appreciated from the 55 Fe
source spectrum taken with a single circular pixel cell (Fig. 5.42).
The DEPFET with its capability of creating, storing and amplifying signal charge
is an ideal building block for a pixel detector. A large number of DEPFETs can be
arranged in a matrix in such a way as to power selected DEPFETs for reading and
clearing the collected signal charge. Figure 5.43 shows a rectangular arrangement of
DEPFETs. Their drains are connected column wise while gates and clear electrodes
are connected row wise. Each row has its individual readout channel. A row at a
time is turned on with the help of the gate voltage while all other DEPFETs have
zero current. Charge collection does not require a current within the DEPFET.
Readout can be performed in double correlated mode: Turning on the current
with a negative voltage on the gate is followed by a first reading of the current, a
clearing of the signal charge in the internal gate with a positive pulse at the clear
contact and a second current reading before the current is turned down again and
reading is switched to the next row. The difference of first and second current