Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1998-02-27
2001-06-19
Lee, Eddie C. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S186000, C257S461000, C257S465000
Reexamination Certificate
active
06249033
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a semiconductor apparatus for detecting energy and point of incidence of electromagnetic radiations, particularly but not exclusively X rays, or ionizing particles.
BACKGROUND OF THE INVENTION
The currently known semiconductor apparatuses for detecting energy and point of incidence of electromagnetic radiations are of the following types: the so-called “Charge-Coupled Devices” (CCD), the so-called “Drift Chambers” or “Silicon Drift Detectors” (SDD), the so-called “Microstrip Detectors” and the so-called “Pixel Detectors”.
CCDs essentially include a chip made of a semiconductor, typically silicon, in which a plurality of potential energy wells for the electrons located in one or more successions at predetermined distances are generated by a corresponding plurality of electrodes. The incident radiation generates electron-hole couples in the semiconductor. Holes are immediately collected by a suitable static biasing electric field generated in the semiconductor, while electrons are confined to the potential energy well which is closer to the point of incidence of the ionizing radiation. The potential energy wells are translated by timed signals along the semiconductor to one or more collector electrodes where the electrons generated by the incident radiation are collected and directed to an amplification chain.
The detection of the point of incidence of the ionizing radiation is accomplished in such apparatuses by counting the number of timed pulse signals needed to shift a determined potential well to the collector electrode. A bidimensional measurement of the point of incidence of the ionizing radiation can be made by providing a bidimensional arrangement of potential wells and a plurality of collector electrodes.
In radioastronomy, CCD apparatuses for X ray detection with high resolution in terms of energy due to the low output capacitance of the collector electrode have been developed. The resolution in the detection of the point of incidence depends on the mutual distance between the electrodes which generate the potential energy wells (dimension of the pixels).
The drawback of such apparatuses, however, is given by the need to generate such timing signals. The frequency of such signals can be limited by the need to have a sufficient translation efficiency during the movement of the potential energy wells, the time necessary for processing the signal associated to the electrons in each potential well arriving at the collector electrode, or the allowed power consumption. For example, in the spectroscopic measurements made in astronomy, the frequency of the timing signals is limited to about 100 kHz by the signal processing time. SDD detectors also include a chip made of a semiconductor, typically silicon, in which there are provided a succession of field electrodes (so-called “field strips”) on both surfaces of the semiconductor chip and one or more electrodes for collecting the signal charges only on one surface. Said field strips which are biased by applying voltages that increase the magnitude with the distance from the collector electrodes, generate a static electric field (so-called electric drift field). The incident radiation generates electron-hole couples, the holes being immediately collected as for CCDs by the field strips which are closer to the source thereof (point of incidence), and the electrons drifting in parallel to the surfaces of the chip towards the collector electrodes to which the signal amplification chain is connected because of the electric drift field.
In such detectors the speed of translation of the electrons generated in the semiconductor towards the collector electrode may be greater than that in CCDs. Actually, such a speed is generally proportional to the applied electric drift field because of the lack of the above-mentioned typical limitations of CCDs concerning the charge transfer efficiency and the allowed consumption. Trials have shown, for example, that electric drift fields in the order of 200-1000 V/cm inducing drift speeds between 3 and 14 &mgr;m
s can be applied.
A drawback of such a type of detectors consists in that a reference signal which is synchronous with the radiation arrival time should be generated in order to detect the point of incidence of the ionizing radiation. This further complicates the detector design and the acquisition electronics.
SUMMARY OF THE INVENTION
In view of the state of art described the present invention aims at providing an apparatus for detecting energy and point of incidence of an electromagnetic radiation or an ionizing particle (hereafter generally referred to as “ionizing event”) which does not suffer from the drawbacks of the previously described known detectors.
According to the present invention such an aim is accomplished by an apparatus for detecting energy and point of incidence of an ionizing event comprising at least one semiconductor layer with a first type of conductivity, in which at least one first doped region with the first type of conductivity and a corresponding plurality of second doped regions with a second type of conductivity associated to said at least one first doped region are formed on a first surface of said layer, said at least first doped region and said corresponding plurality of second doped regions defining a respective drift path for charge carriers with the first type of conductivity, and at least one third doped region with the second type of conductivity is formed on a second surface of said layer, and means for biasing said second doped regions and said third doped region which is capable of reversely biasing the junctions between the second doped regions and the semiconductor layer and between the third doped region and the semiconductor layer so as to deplete the semiconductor layer, characterized in that said biasing means is capable of providing two different operating conditions of the detection apparatus, the first operating condition providing the formation of a plurality of potential energy wells for said charge carriers in the semiconductor layer at predetermined distances along said drift path from said first doped region, said wells being able to confine all of the charge carriers generated by an ionizing event essentially to the points of incidence of the ionizing event itself, the second operating condition providing the removal of said potential energy wells so as to cause the charge carriers to drift towards said at least one first doped region along said drift path and keeping the charge carriers confined in the directions which are perpendicular to the drift path.
The detection apparatus according to the present invention does not need any reference signal for detecting the point of incidence of the ionizing event. Actually, after the detector is switched from the first operating condition to the second operating condition, the point of incidence of the ionizing event can be determined by measuring the time interval from said switching to the arrival of the charge carriers. In addition, unlike in CCD detectors, no high-frequency timing signal is needed. The detection apparatus according to the invention is a device with essentially static electric fields like the known SDD detectors, with the difference that the electric fields in the detection apparatus according to the invention are different in either operating condition. In operation, the detection apparatus remains in its first operating condition (otherwise called “acquisition” or “integration”) for any length of time (which is generally inversely proportional to the rate of the incoming ionizing events to be measured but long enough with respect to the reading time); charge carriers generated by an ionizing event are collected in the potential energy well which is the closest to the point of incidence of the radiation. The detection apparatus is then switched to the second operating condition (otherwise called “drift” or “readout” condition) in which the potential energy barriers that prevented the charge carriers from moving along th
Castoldi Andrea
Gatti Emilio
Guazzoni Chiara
Longoni Antonio
Rehak Pavel
Istituto Nazionale Di Fisica Nucleare
Lee Eddie C.
Lipton Roberts S.
Lipton, Weinberger & Husick
Wilson Allan R.
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