DePFET detector
A DePFET detector is an electronic semiconductor component and represents a special type of field effect transistor with which light and particle radiation can be detected. DePFET stands for English depleted p-channel field-effect transistor , German: "field effect transistor with an impoverished p-channel". The working principle was predicted in 1987 by J. Kemmer and G. Lutz and also proven in 1990. DePFET detectors generally come as arrays of DePFET cells. One DePFET cell corresponds to one pixel .
DePFET cell
The structure of a DePFET cell consists of a completely depleted substrate (mostly silicon ) and a field effect transistor embedded in this substrate. The completely depleted substrate forms the sensitive area for the radiation. The field effect transistor is used for a first pre-amplification of the signal.
Generation of signal charge
Radiation incident on the detector creates electron-hole pairs . In the case of electromagnetic radiation , this happens via the photo effect . Particle radiation generates electron-hole pairs through collisions. These are separated by the sideways impoverishment potential field . The positively charged holes mainly drift to the negatively charged back contact and are absorbed there. The electrons are used to generate signals and are therefore referred to as signal charge. Sideways depletion pushes the signal charge into an area (internal gate) above which a field effect transistor is attached.
Signal measurement
The DePFET cell can be switched on and off via the external gate contact. When switched on, the amount of charge in the internal gate influences the conductivity of the field effect transistor, i.e. the resistance between the source and drain contacts. Therefore, by measuring this conductivity, conclusions can be drawn about the number of electrons and thus about the energy of the incident radiation. Thus, a DePFET detector not only has imaging, but also spectroscopic properties.
Example: In silicon you need an energy of around 3.6 eV to create an electron-hole pair. X-ray light with an energy of 7200 eV can generate 2000 electron-hole pairs when fully absorbed. A DePFET detector has a typical gain of 300 picoamps per electron in the internal gate. The resulting current flow is therefore 600 nanoamps.
Due to the intrinsic preamplification, a DePFET cell has a very low input capacitance. As a result, a noise behavior can be achieved that is only limited by the Fano noise . The indirect measurement of the signal charge enables the cell to be read out without the signal charge being deleted. This enables repeated reading, which in turn reduces statistical fluctuations.
Deletion process
With an additional field effect transistor ( clear FET ), the signal charges can be "sucked off" from the internal gate and the DePFET cell is ready to detect radiation again. The clear contact is charged with a strong positive. The connection between the internal gate and the clear contact is regulated by a further control contact ( clear gate ).
DePFET matrix
construction
In order to interconnect several DePFET cells to form a matrix, all gate, cleargate and clear contacts are connected to one another line by line. The source and drain contacts are connected to one another in columns. The following modes of operation apply:
- Gate: a current flow between source and drain can only be measured after the external gate is switched on. Individual rows of a matrix can thus be "activated" via the gate contact.
- Cleargate: The Cleargate creates a connection between the internal gate and the clear contact.
- Clear: signal charge drain contact.
- Source: Current source of the FET.
- Drain: Current sink of the FET.
Readout cycle
After a row of the matrix has been selected via the gate contact, all cells of this row can be read out. All other cells are switched off at this moment and no current can flow between source and drain. The activated line can be read out either via the source or the drain contact, depending on the design. Then the signal charges are deleted from all pixels in this line via the Cleargate and Clearcontacts. A second measurement, now with an empty internal gate, serves as a reference measurement. The difference between the two signals corresponds to the actual signal. After selecting the next line, the cycle starts over.
Compared to the CCD
Reading out a CCD detector is similar to a chain of buckets. The signal charge of a pixel is shifted to the neighboring pixels until it reaches the readout node. During this charge transport, signal charges, and thus also part of the signal, are always lost. Effects such as blooming or smear can also occur. In contrast, with a DePFET detector, a pixel is read out directly at the corresponding pixel. It is not necessary to shift the signal charge. This enables a fast, lossless and error-free readout.
Areas of application of DePFETs
Currently, DePFET detectors are mainly used within particle accelerators and in the field of X-ray astronomy. Examples are: European XFEL , Belle II , ILC , Athena . DePFET detectors are not yet found in commercially available cameras (as of August 2010).
literature
- Helmut Spieler: Semiconductor Detector Systems . Oxford University Press, 2005, ISBN 0-19-852784-5 .
- Gerhard Lutz: Semiconductor Radiation Detectors . Springer, 1999, ISBN 3-540-64859-3 .
Individual evidence
- ^ J. Kemmer, G. Lutz: New detector concepts . In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment . tape 253 , no. 3 , 1987, pp. 365-377 , doi : 10.1016 / 0168-9002 (87) 90518-3 .
- ^ Emilio Gatti, Pavel Rehak: Semiconductor drift chamber - An application of a novel charge transport scheme . In: Nuclear Instruments and Methods in Physics Research . tape 225 , no. 3 , August 1, 1984, pp. 608-614 , doi : 10.1016 / 0167-5087 (84) 90113-3 .