Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-12-06
2003-03-04
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370120, C250S370130
Reexamination Certificate
active
06528794
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to radiation detecting apparatus of the direct conversion type used in the medical, industrial, nuclear and other fields, and more particularly to a technique of improving the sensitivity for detecting radiation.
(2) Description of the Related Art
Radiation detecting apparatus employing semiconductor detectors include the indirect conversion type which first converts radiation (e.g. X rays) into light and then converts the light into electric signals by photoelectric conversion, and the direct conversion type which converts incident radiation directly into electric signals with a radiation sensitive semiconductor layer. The direct conversion type apparatus has electrodes formed on opposite surfaces of the radiation sensitive semiconductor layer. A predetermined voltage is applied to one of the electrodes (voltage application electrode). The other electrode (carrier collection electrode) collects carriers generated by incident radiation. The carriers are taken out as radiation detection signals, thereby enabling a detection of the radiation.
A conventional radiation detecting apparatus of the direct conversion type employs a single crystal semiconductor as a material for the semiconductor layer. The single crystal semiconductor is used since it has excellent carrier transport characteristics compared with an amorphous or polycrystal semiconductor having numerous localized levels and grain boundaries. The conventional radiation detecting apparatus, with the excellent carrier transport characteristics of the single crystal semiconductor, is used as a radiation energy detector or a radiation pulse counter.
Since only the carriers generated by incident radiation are taken out as signals, the conventional radiation detecting apparatus needs structures on both the opposite electrode sides for inhibiting carrier injection from the electrodes. Thus, the conventional apparatus has the following specific construction.
As shown in
FIG. 1
, the conventional apparatus may include a single crystal semiconductor layer
51
of high specific resistance, which is doped with impurities to make one side p-type and the other side n-type. Structures
52
and
53
are formed on the opposite sides of semiconductor layer
51
to inhibit injection of electrons e and holes h, respectively. Alternatively, as shown in
FIG. 2
, the conventional apparatus may include an n-type (or p-type) single crystal semiconductor layer
61
of relatively low specific resistance, which may be doped with an impurity to make one side p-type (or n-type) or may have a structure
62
in the form of a barrier metal electrode on the one side to inhibit injection of electrons e (or holes h). The other side is guarded against injection of holes h (or electrons e) by the conductivity of the crystal per se. In
FIGS. 1 and 2
, the left electrode acts as a bias voltage application electrode, and the right electrode as a carrier collection electrode.
However, the conventional radiation detecting apparatus has a drawback that it can hardly meet a demand for an enlarged detecting area. In each of the medical, industrial, nuclear and other fields today, there is a strong demand for a semiconductor type radiation detecting apparatus with a large area and high sensitivity to realize a high-speed, highly sensitive and compact radiation detecting system. Under current conditions, a 3-inch diameter is the limit for radiation detecting, single crystal semiconductor materials. It is difficult to realize a semiconductor type radiation detecting apparatus with a large area and high sensitivity.
With an amorphous or polycrystal semiconductor, on the other hand, 1000 cm
2
and larger sizes may be achieved by using a thin film making technique. However, sensitivity is insufficient because of the presence of localized levels and grain boundaries. Here again, it is difficult to realize a semiconductor type radiation detecting apparatus with a large area and high sensitivity.
SUMMARY OF THE INVENTION
This invention has been made having regard to the state of the art noted above, and its object is to provide a semiconductor type radiation detecting apparatus with a large area and high sensitivity.
The above object is fulfilled, according to a first aspect of this invention, by a radiation detecting apparatus for converting incident radiation into electric signals, comprising:
a radiation sensitive semiconductor layer for generating carriers in form of electron-hole pairs in response to the incident radiation;
a pair of electrodes formed on opposite surfaces of the semiconductor layer, respectively, one of the electrodes being a voltage application electrode to which a negative bias voltage is applied, the other electrode being a carrier collection electrode;
a charge storing capacitor connected to the carrier collection electrode for storing charges generated by movement of the carriers in the semiconductor layer;
a switching element connected to the capacitor, the switching element being turned off when the charges accumulate in the capacitor, and turned on when taking the charges out of the capacitor; and
charge-to-voltage converter means for converting the charges taken out of the capacitor through the switching element into voltage signals acting as radiation detection signals;
wherein the semiconductor layer is formed of one of an n-type amorphous semiconductor and an n-type polycrystal semiconductor, both being of high specific resistance, with a &mgr;&tgr; product of holes being larger than a &mgr;&tgr; product of electrons, the &mgr;&tgr; product being a product of mobility &mgr; and mean life &tgr; of the electrons and holes generated by the incident radiation; and
wherein the semiconductor layer has a structure formed on a voltage application electrode side thereof for inhibiting injection of electrons, and a structure formed on a carrier collection electrode side for permitting injection of holes.
The radiation detecting apparatus in the first aspect of the invention, as shown in
FIG. 3
, includes a radiation sensitive semiconductor layer
1
A formed of an n-type amorphous or polycrystal semiconductor of high specific resistance, with the &mgr;&tgr; product of holes h larger than the &mgr;&tgr; product of electrons e among the carriers generated. When radiation is detected by this detecting apparatus, the holes h which are the minority carriers in the semiconductor layer
1
A contribute to the radiation detection. A negative bias voltage −V
A
is applied from a bias voltage supply Ve to one electrode side of semiconductor layer
1
A. Consequently, holes h are promptly injected from a hole injection permitting structure
3
A formed on the carrier collection electrode side of semiconductor layer
1
A, in a quantity corresponding to the quantity of the carriers generated by the incident radiation. The semiconductor layer
1
A has a high specific resistance, whereby the injection of holes h is not inhibited by the conductivity of the semiconductor layer
1
A per se. On the other hand, an injection of electrons e is inhibited by an electron injection inhibiting structure
2
A formed on the voltage application electrode side of semiconductor layer
1
A.
In the prior art, the injection of both holes h and electrons e is inhibited. The first aspect of the invention inhibits only the injection of electrons e which are the majority carriers not contributing to the improvement in sensitivity. Only the injection of holes h which contribute to the improvement in sensitivity is permitted to improve the sensitivity by a degree corresponding to the increase in the holes h. The holes h injected are the minority carriers in the semiconductor layer
1
A, and thus an increase in dark current is suppressed. Of course, the detection area may be enlarged since the semiconductor layer
1
A is formed of an amorphous or polycrystal semiconductor material.
Charges generated by movement of holes h accumulate in a charge storing capacitor Ca while a switching element
4
is turned off
Lee Shun
Porta David
Rader & Fishman & Grauer, PLLC
Shimadzu Corporation
LandOfFree
Radiation detecting apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Radiation detecting apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Radiation detecting apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3044058