JPH053145B2 - - Google Patents

Description

Claim 1: A substrate having a front surface and a back surface and transparent to radiation in a predetermined frequency range, a back surface detection contact adjacent to the front surface of the substrate, a front surface detection contact covering the back surface detection contact, and the front surface and the back surface. a detection layer interposed between the detection contacts and adjacent to one of the front and rear detection contacts; and a blocking layer interposed between the front and rear detection contacts and adjacent to the other of the front and rear detection contacts. a radiation detector, wherein the detection layer, the back detection contact and the front detection contact are made of a semiconductor material of a first conductivity type, and the detection layer is doped with impurities such that all incident radiation passing through the layer is absorbed. the front-back detection contact has a high conductivity compared to the detection layer, and the radiation detector is sensitive to front-back irradiation radiation; , a radiation detector in which radiation incident on the detector passes through the substrate and the front detection contact and is guided to the detection layer. 2. The radiation according to claim 1, wherein the detection layer has an impurity concentration such that it absorbs substantially all of the radiation in a predetermined frequency range that passes therethrough, and has a thickness that is sufficiently thin so as not to be sensitive to gamma rays. Detector. 3. A radiation detector according to claim 2, wherein the blocking layer is a semiconductor of the first conductivity type having a sufficiently low concentration of impurities to prevent charged carriers from flowing through the impurity band. 4. Radiation according to claim 2 or 3, wherein the blocking layer is a semiconductor material of the second conductivity type having a sufficiently low concentration of impurities to prevent the flow of charged carriers through its impurity band. Detector. 5. The radiation detector of claim 4, wherein the blocking layer is adjacent to the front detection contact and the detection layer is adjacent to the back detection contact. BACKGROUND OF THE INVENTION The present invention relates to a radiation detector, and particularly to a back-illuminated radiation detector that is applicable to the detection of long wavelength infrared radiation (LWIR) and has improved sensitivity. In general, when designing and constructing high quality radiation detectors, it is desired to fabricate a detector that is as sensitive as possible to incident radiation in a desired range of frequencies. What generally limits sensitivity is development, known as dark current. This development is caused by a number of different mechanisms that occur independently but simultaneously at the radiation detector. However, this mechanism is due to the current flowing through the detector regardless of the presence or absence of incident radiation. A dark current is generated by the current that flows when there is no sufficient radiation irradiation. Detection sensitivity decreases in direct proportion to the amount of dark current flowing through the detector. If there is a dark current, noise will be generated in proportion to the current density, and the amount of dark current will reduce the signal-to-noise ratio (SN ratio) of the detector.
Therefore, if the dark current mechanism is eliminated, although not completely, the sensitivity of the radiation detector can be significantly improved. For example, as is commonly known, dark current is generated by thermal charge carriers. In the case of a donor impurity type radiation detector, electrons are ionized from impurity atoms by absorption of thermal energy. These ionized electrons move from the impurity level to the conduction band of the crystal lattice. The electrons are moved by the electric field to the positive contact of the radiation detector. The electric field is generated by a potential difference applied to the radiation detector. Further electrons are injected from the negative contact. A dark current therefore flows through the radiation detector independently of the current generated by the incident radiation. Methods of preventing dark current due to thermal generation mechanisms are well known. Because the mechanism requires thermal energy to operate, the mechanism can be frozen by lowering the temperature of the radiation detector to near absolute zero. Therefore, the proportion of impurity band electrons in the conduction band due to ionization upon radiation absorption increases, resulting in an increase in the sensitivity of the detector to the incident radiation. Gamma radiation, which causes carrier generation, is known as another dark current mechanism. Conventional detectors are designed and constructed to be as insensitive as possible to incident radiation having frequencies outside the desired frequency detection range. However, an incident radiation of a given frequency is absorbed by a practical radiation detector in a certain proportion.
Due to the high energy charge carriers generated by gamma ray absorption, further charge carriers are generated by electron bombardment. Dark current is generated by this multiplication of charged carriers. As a result, gamma rays that generate dark current are of particular importance in the case of radiation detectors operating in environments that are heavily exposed to gamma rays. In the case of most conventional radiation detectors, the sensitivity to this gamma ray is increased. For example, a large volume, low impurity concentration radiation detection region is used to absorb the incident radiation. The low conductivity sensing region has a low impurity concentration to inhibit normal current flow due to the bias voltage applied to the impurity band in the sensing region. The large volume of the detection area compensates for the low impurity concentration needed to maintain radiation absorption efficiency. However, this results in an increase in the gamma sensitivity of the detector. If the detection area has a large volume, the probability that gamma rays will be absorbed increases. As a result, most conventional radiation detectors malfunction in the presence of a certain amount of gamma rays. As mentioned above, there are various mechanisms by which dark current occurs. This mechanism is fairly well known, and methods for inhibiting its operation have also been elucidated. On the other hand, the conduction mechanism of the impurity band is not so well understood. When designing and manufacturing a highly sensitive radiation detector, it is desirable that the detector can be used in a wide range of applications. That is, it is possible to detect the wavelength of a single predetermined radiation and to obtain images of a plurality of radiation sources with high resolution. Thus, radiation detectors must be applicable as separate devices and at the same time usable as high-density focal plane arrays (FPAs). Furthermore, the use of FPAs must be compatible with a wide range of readout mechanisms, including hybrid thin film circuits and monolithic charge-coupled devices (CCDs). Use of a hybrid readout mechanism with an FPA generally requires that the detector be capable of operating in back-illuminated or back-illuminated mode. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a radiation detector that is sensitive to incident radiation of a desired frequency and that can be easily adapted to a wide range of applications. The above object is to provide a rear detection contact close to the surface of a substrate that is substantially transparent to radiation in a predetermined frequency range, a detection layer and an impurity band conduction blocking layer provided on the rear detection contact, and a detection layer and an impurity band conduction blocking layer provided on the rear detection contact. This is achieved by providing a front detection contact provided on the band conduction blocking layer. The front detection contact is further electrically connected to the back detection contact via the radiation detection area of the detection layer. According to the above configuration, the generation of charge carriers due to radiation absorption ionization can be detected. Therefore, the present invention has the advantage of significantly improving the sensitivity to incident radiation within a desired frequency range by sufficiently reducing the dark current flowing through the radiation detector. Detector sensitivity is significantly increased by using a smaller detection area compared to conventional detectors and reducing sensitivity to gamma rays. By sufficiently inhibiting impurity band conduction in the detection region using a blocking layer, radiation sensitivity within a desired frequency range is improved. The detection sensitivity may be significantly improved by using a chopped or pulsed incidence beam to reduce the response time of the detector and extend the operating frequency bandwidth. Another advantage of the present invention is that it can receive incident radiation in both back-illuminated and front-illuminated modes. Yet another advantage of the present invention is that the detector can be manufactured using only conventional manufacturing steps. Furthermore, the present invention has the advantage that a plurality of detectors can be formed as a single device on a common substrate without being separated, thus allowing radiation detectors to be formed as a focal plane array on a monolithic substrate. It is also an advantage of the present invention that highly integrated focal plane arrays for radiation detection can be readily fabricated. The detector structure of the present invention allows the front detection contacts to be electrically isolated from each other without introducing additional manufacturing steps or electrical isolation structures. Additionally, a single metal contact is commonly used for multiple radiation detectors to electrically connect to the backside detection contact of each detector. Therefore, the front surface of the radiation detector can be used to the maximum extent. Similarly, since the detector is used in back-illuminated mode, there is no need to provide a radiation-transparent window on the front surface, and the front surface of the radiation detector can be used effectively. In addition, the monoliths used in focal plane arrays
It is also an advantage of the present application that radiation detection arrays utilizing either CCD or hybrid readout structures can be readily used.

[Brief explanation of the drawing]

Other advantages of the present application will be clearly understood by reference to the following detailed description in conjunction with the accompanying drawings. In the drawings, the same reference numerals indicate the same parts throughout the drawings. FIG. 1 is a cross-sectional view of a back-illuminated radiation detector according to a preferred embodiment of the present invention. FIG. 2 is a sectional view of a front-to-back illumination type radiation detector according to another embodiment of the present invention. FIG. 3 is a cross-sectional view of a back-illuminated radiation detector using an alternative back-sensing contact connection according to another embodiment of the present invention. FIG. 4 is a partial top view of a monolithic substrate radiation detector focal plane array usable in a hybrid readout structure using a radiation detector of the type of FIG.

[Detailed description of the invention]

The radiation detector according to the invention is suitable for a wide range of applications. In order to describe the invention and understand its operation, a radiation detector configured for use in the mode of operation to which the invention is fundamentally directed will now be described. Various modifications of the invention will be described below. This example description represents the invention and provides the basis for the claims that further define the scope of the invention. FIG. 1 shows a cross-sectional view of a back-illuminated radiation detector, designated by the reference numeral 10. Radiation detector 10 operates in a back-illuminated mode and is particularly sensitive to long wavelength infrared (LWIR) radiation.
Generally, LWIR radiation has frequencies corresponding to a wavelength range of approximately 14-30μ. Arsenic (As) is therefore used as the main impurity in the detector since it has an excitation energy that corresponds to the wavelength energy of the LWIR radiation. The radiation detector 10 includes a detection layer 18, a blocking layer 20,
It consists of front and rear detection contacts 22 and 16 formed on a substrate 12. Front and back metal contacts 26, 28 are provided to make electrical contact to the detector 10. Oxide layer 24 provides electrical isolation between metal contacts 26, 28 and detector 10. A bias reset/sense access circuit 70 is provided for detector 10. LWIR radiation, indicated by arrow 48 , is incident on the back side of detector 10 , through substrate 12 and back side detection contact 16 to radiation detection region 19 of detection layer 18 . There, the presence or absence of LWIR lines is detected. Therefore, the present invention is capable of effective operation at least in backlight mode. To explain the components in more detail, LWIR
A radiation detector 10 is mounted on a substrate 12 that is sufficiently transparent to the radiation.
will be provided. Preferably, substrate 12 is about 500μ
Boron-doped silicon with a thickness of . Although boron absorbs some LWIR radiation, the substrate 12 can be kept substantially transparent by maintaining the impurity concentration below about 1×10 ¹⁴ atoms/cm ³ . The orientation of the crystal lattice structure of the substrate is chosen such that the epitaxial layer formed on the surface of substrate 12 is etched in a conventional anisotropic manner. Preferably, substrate 12 is oriented in a standard <100> mirror crystal lattice orientation. A backside detection contact 16 is formed on the front side of the substrate 12. The rear detection contact 16 must be determined by considering the following two factors. First, the back detection contact 1 has high conductivity and functions as a back contact of the radiation detector 10.
6 must be heavily doped. secondly
Back contact 16 must be thin and lightly doped to allow LWIR radiation to pass through without significant attenuation. Therefore, the back surface detection contact 16 has a thickness of approximately 0.2 μ and a thickness of approximately 5×10 ¹⁸
Preferably, the layer is an ion implantation layer having an impurity concentration of atoms/cm ³ . Arsenic is also a preferred impurity to prevent unnecessary contamination of the radiation detector 10. A backside sense contact grid 14 is formed on the front side of the substrate 12 adjacent to and electrically connected to the backside sense contacts 16. Back detection grid 1
4 acts as a buried wiring and therefore has the same or higher conductivity than the backside detection contact 16. However, contact grid 14 is not limited in its doping concentration or thickness.
This is because there is no need to transmit LWIR radiation. Grid 14 is preferably an ion implanted layer having a thickness of about 0.4 .mu.m and an arsenic concentration of about ^{2.times.10.sup.19} atoms/ ^cm.sup.3 . However, the doping concentration of the sensing contact grid 14 must not be so high as to prevent the formation of an epitaxial layer thereon. Typically, the front side of substrate 12 is annealed and then back side sensing contacts 16 and sensing grid 14 are formed.
This is to remove surface defects that would prevent uniform epitaxial layer growth. A sensing layer 18 is formed on the front side of the substrate 12 overlying the sensing grid 14 and the back sensing contact 16. The portion of the detection layer 18 covering the rear detection contact 16 substantially forms a radiation detection region 19 of the radiation detector 10 . The doping concentration and thickness of the detection region 19 are dependent on each other, but are determined to achieve maximum radiation absorption efficiency (typically about 85%). Furthermore, detection area 1
It must be taken into consideration that the thickness of 9 is directly proportional to the detection sensitivity of gamma rays. This is because the statistical probability that gamma rays will ionize impurity electrons increases in a thicker detection region than in a thinner but highly doped detection region 19. In particular, it is undesirable to be sensitive to gamma rays because it may cause the detector 10 to malfunction. Taking this into consideration, the detection layer 18 has a thickness of about 7μ (generally 5 to 10μ) and has a thickness of 1×10 ¹⁸ atoms/
Preferably it is an arsenic-doped thin epitaxial layer with an impurity concentration of cm ³ . In that case, the radiation absorption efficiency is about 90% or more. The blocking layer 20 is preferably formed as an epitaxial layer in front of the detection layer 18. A blocking layer 20 is thus provided between the front detection contact 22 and the detection layer 18. In another aspect of the invention, a blocking layer 2 is provided within the radiation detector 10 to substantially prevent the operation of the dark current mechanism known as impurity band conduction.
0 is set. Briefly, this mechanism involves the flow of holes in the impurity band from the impurity band in the crystal lattice toward the negative electrode in response to an applied electric field. Since this conduction occurs within the impurity band, an equal number of holes must be injected from the positive front sensing contact 22 for current to flow. However, if the impurity concentration of the blocking layer 20 is sufficiently lower than the concentration of the detection layer 18, hole injection can be sufficiently prevented by interposing the blocking layer 20 between the positive potential contact and the detection layer 18. This can close the conduction path of the impurity band conduction mechanism and directly reduce the dark current. Therefore, the impurity concentration of the blocking layer 20 is preferably 1×10 ¹⁵ atoms/cm ³ or less. Arsenic is a preferred impurity that can easily prevent unnecessary contamination of the detection layer 18 with foreign impurities.
The thickness of the blocking layer 20 need only be sufficient to prevent direct electrical contact between the front sensing contact and the sensing layer 18. The thickness of the blocking layer is preferably 3μ. The front sensing contact 22 is formed as a thin highly conductive layer adjacent the front side of the blocking layer 20. Therefore, the front detection contact 22 has a density of approximately 1×10 ²⁰ atoms/cm ³
It is preferable to use a low energy ion implantation method so as to have an impurity concentration of . In addition, the detection layer 18
Arsenic is the preferred impurity to reduce the possibility of contamination. An oxide layer 24 having a thickness of about 1000 Å forms the blocking layer 2.
Formed on the entire surface of 0. The provision of this oxide layer 24 allows for the selective processing necessary to form the front and back metal contacts 26 and 28, respectively, of the radiation detector. This allows a specially positioned window to be drilled onto the backside sensing contact grid 14. A window is also drilled over the front detection contact 22. Since the front detection contacts 22 are directly exposed, no separate etching is necessary. The electrically separated front and rear metal contacts 26 and 28 are connected to the radiation detector 1.
It can be deposited by sputtering from the front side of 0 and is in electrical contact with the front sensing contact 22 and the back sensing contact 14. To further simplify the manufacture of radiation detector 10, oxide layer 24 may be used as a mask when forming front detection contact 22 by ion implantation. That is, an oxide layer 24 is formed, and then blocking layer 20 and a portion of backside sensing grid 14 are exposed by ion implantation prior to forming frontside sensing contact 22. Oxide film 24
, the ion implantation layer is formed only on the exposed surfaces of the detection layer 18 and the blocking layer 20. In this way, a V-groove contact layer 30 is formed in addition to the front detection contact 22. The V-shaped groove contact layer 3 is
It is not necessarily necessary to provide the radiation detector 10, and it may be provided as necessary. Also, radiation detection area 1
By arranging the V-groove contact layer 30 at a distance several times the thickness of the detection layer 18 (typically 15 times) from the detection layer 18, the V-groove contact can be made substantially inoperable. To operate the radiation detector 10, an electric field is applied to the detector 10 with the front detection contact at a positive potential relative to the back detection contact 16. This is accomplished by bias reset/sense access circuit 70. This circuit 70 is a typical part of the readout circuit provided in each radiation detector 10.
Bias reset/sense access circuits can be designed in a variety of ways, but operate essentially the same way. A bias reset/sense access circuit used in an embodiment of the present invention will be briefly described. The bias reset/sense access circuit 70 includes a common lead 32 connecting the back metal contact 28 to ground potential, and an output lead 3 interconnecting the metal contact 26 and the bias capacitor 36.
4, a bias reset FET 38, and a detection sense access FET 42. When a detection bias reset signal V _sr is applied that enables the bias reset FET 38 to conduct, the radiation detector 10
voltage is supplied to the A current limited by the small inherent impedance represented by resistor 40 then flows from reference potential V _r on bias input line 46 and charges bias capacitor 36 to the desired radiation detector bias potential. . This bias potential must be sufficient to form a depletion layer over the entire radiation detection area of the detection layer 18. Examples of the invention required a bias potential of 200 mV to 300 mV. The maximum bias potential that can be applied is limited by the thickness of the detection layer 18 of the detector 10. If the bias potential induced depletion layer extends from the front sense contact 22 to the back sense contact 16, punch-through breakdown occurs in the detector 10, thereby limiting the maximum bias potential. Therefore, the depletion layer 18 only needs to expand by approximately the thickness of the radiation detection region 19 of the detection layer 18. It is undesirable for it to spread by exactly that thickness. When bias capacitor 36 is charged to a bias potential, bias reset signal V _sr is removed and the potential difference appearing at radiation detector 10 changes proportionally to the amount of LWIR line 48 incident on detector 10 . That is,
The LWIR line 48 connects the board 12 and the backside detection contact 1.
6 to the radiation detection area of the detection layer 18. Radiation 48 is absorbed by the arsenic impurity atoms and electrons are ionized into the conduction band of the crystal lattice. Holes in the impurity band generated in the depletion layer of the radiation detection region 19 are exposed to the negative potential of the back surface detection contact 1 by the applied electric field.
It is sent in the direction of 6. The potential of the bias capacitor 36 decreases due to the current. The potential drop is proportional to the number of holes in the impurity band generated depending on the intensity of the LWIR ray 48 incident on the radiation detector 10.
The potential drop of the capacitor 36 is detected by the sense potential output FET 42.
is relaxed and transmitted onto the sense output line 44 by. The FET 42 functions as a buffer by being connected as a source follower. That is,
The gate of the FET 42 is connected to the detector output line 34, and the drain is connected to a positive potential V _DD higher than the bias reference potential V _r . Therefore, the sense output line 44,
That is, the potential of the source of the FET 42 is approximately approximate to the potential of the gate of the FET 42. The potential on sense output line 44 can therefore be used to sense the potential of radiation detector 10. A front-to-back illumination type radiation detector indicated by reference numeral 50 will be described with reference to FIG. The radiation detector 50 is slightly different from the detector 10 shown in FIG.
This is another embodiment of the invention in which the difference is essential. A front detection/metal contact structure is used to transmit radiation from the front side of the radiation detector 50 to the radiation detection area of the detection layer 18. In particular, the front detection contact 57 is formed as a thin highly conductive layer close to the front side of the blocking layer 20. Front detection contact 57 enters the front side
It must be substantially transparent to the LWIR line 56. Therefore, the impurity concentration and thickness of the front detection contact 57 are substantially similar to those of the back detection contact 16. The front detection contact grid 58 is adjacent to and connected to the front detection contact 57 in the blocking layer 2.
Formed in front of 0. Like back sense grid 14, front sense grid 58 functions as a conductive connection between front sense contact 57 and front metal contact 52. Therefore, the impurity concentration and thickness of the front detection grid 58 are the same as those of the back detection grid 1.
It is almost the same as 4. Front metal contact 52 is provided to form an electrically conductive path between radiation detector 50 and bias reset/sense access circuit 70. However, the front metal contact 52 is formed to substantially overlap only the front detection grid 58, and the front surface of the front detection contact 57 is exposed. As a result, the radiation 50 incident on the front side of the detector 50 by the front detection window 54 is guided through the front detection contact 57 and the blocking layer 20 to the radiation detection area 19 of the detection layer 18 . It is therefore possible to operate in front-illuminated or back-illuminated modes or both modes, representing another aspect of the invention. It is clear that in order for radiation detector 50 to maintain satisfactory radiation absorption efficiency, the impurity concentration and/or thickness of detection layer 18 must be increased. In the radiation detector 10 of FIG. 1, the front metal contact 26 functions as a reflector for radiation 48 that initially passes through the radiation detection area of the detection layer 18 without being absorbed. This radiation 48 is reflected back to the detection layer 18 where it is given a second opportunity to be absorbed again. Therefore, the radiation detector 10 of FIG. 1 necessarily exhibits high radiation absorption efficiency. However, in the radiation detector 50 of FIG. 2, the front metal contact 52 cannot function satisfactorily as a radiation reflector. Therefore, the radiation detector 50 has only a single opportunity to absorb radiation 48 incident from the back or radiation 56 incident from the front as it passes through the radiation detection area of the detection layer 18. Therefore, the bias voltage value applied to the bias capacitor 36 is
The change in thickness of 8 must be reflected. FIG. 3 is a sectional view of a back-illuminated radiation detector 60. Detector 60 has a simplified back metal contact structure. This modified radiation detector 60 is substantially the same as the radiation detector 10 of FIG. However, the back metal contact structure has been modified to simplify the manufacture of the detector 60. In particular, the manufacturing process is simplified by completely eliminating V-groove etching. Instead, the oxide layer 24 above the front sensing contact grid 14 is perforated to provide a window. A highly conductive back contact layer 66 is formed on the exposed surface of blocking layer 20 . The impurity concentration and thickness of the contact layer 66 are approximately the same as those of the front detection contact 22. Further, to simplify manufacturing of the detector 60, the back contact layer 66 may be formed at the same time as the front sensing contact 22, and then the back metal contact 64 may be formed at the same time as the front metal contact 26. The contact layer 1 detects the potential of the grid 14 on the back side.
By increasing the potential to a level higher than 6, conduction occurs between the backside detection contact grid 14 and the backside contact layer 66. The transition region formed at the junction of sensing layer 18 and blocking layer 20 in the vicinity of contact layer 66 is constricted, thus allowing current conduction.
Front detection contact 22 and back contact layer 66
The distance between them must be made thicker (typically 15 times thicker) than the thickness of blocking layer 20 to prevent unwanted current from flowing therebetween. FIG. 4 shows a portion of a monolithic substrate focal plane array (FPA). The FPA is provided with a large number of radiation detectors 10 arranged in a matrix. None of the embodiments of the present application described above
Although it can be used in an FPA, for the sake of simplicity, the FPA shown in FIG. 4 will be explained using the embodiment of the back-illuminated radiation detector shown in FIG. 1 of the present invention. Radiation detector 10 uses a common backside detection contact grid 14 that is commonly electrically connected to a number of backside metal contacts 28 that frame the detection array. In particular, FIG. 4 shows the three columns 92, 94, 96 of the radiation detection array and the back metal contacts 28
A surrounding column 90 is shown. The dimensions of the array, including the size and distance separating the radiation detection areas 19 of the detector, are not critical beyond those described above. However, in all cases the front sensing contacts 22 of the detector 10 must be spaced far enough apart that the blocking layer 20 electrically isolates the front sensing contacts 22. A distance of 10μ is usually sufficient. In this case, it must be spaced apart so that a readout structure having a detection output lead 34 and a common lead 32 can be formed on the front side of the FPA. A reset/sense access circuit 70, typically connected to each front metal contact 26, includes a read structure. The resulting hybrid structure is a highly integrated unit with a dense arrangement of radiation detectors 10 operated in back-illuminated mode. Various modifications are possible to optimize the radiation detector 10 for specific applications. Hereinafter, it is assumed that the radiation detector 10 also includes detectors 50 and 60. The impurities typically used in the detection layer 18 can be selected from a variety of ways. For example, indium or gallium can be used as an impurity in order to make the frequency range that the detector 10 can respond to be in the infrared region of 3 to 5 microns. Gallium can also be used in the 8-14μ infrared region. Still other impurities may be used to enable the radiation detector 10 to respond to electromagnetic waves having different frequencies.
Therefore, although embodiments of the invention have been focused on detecting LWIR radiation, the invention is applicable not only to infrared detectors but also to radiation detectors in general. It is therefore also possible to use P-type impurities in the detection layer 18 and the blocking layer 20 to obtain a P-type radiation detector. Similar operating principles apply to radiation detectors using P-type impurities as well as N-type radiation detectors. A more important modification of the radiation detector 10 is one in which the blocking layer 20 is formed using an impurity of a conductivity type opposite to that used in the detection layer 18. Although this modification inevitably increases the possibility of impurity contamination of the detection layer 18, the use of a blocking layer of the opposite conductivity type has the advantage of improving the detection reaction time with respect to a tip or pulsed incident beam. . That is, if the incident radiation is chopped with a non-negligible duty cycle, the time required by the detector 10 to obtain an output proportional to the intensity of the incident radiation is significantly reduced. This includes an N-type detection layer 18 as well as a P-type blocking layer 20.
It is thought that this is because the number of trapped electrons that inevitably exist in the N-type blocking layer 20 is reduced by using the N-type blocking layer 20. Similarly, together with the N-type detection layer 18, N
The use of the P-type blocking layer 20 is also achieved by reducing the trapped holes necessarily present in the P-type blocking layer 20. The invention therefore provides a radiation detector 10 with a sufficiently fast response time. Furthermore, a detection layer 18 and a blocking layer 20 are provided to the substrate 12.
A modification can also be considered in which the order is reversed. According to this modification, the radiation detector 10 can operate from a negative reference potential with respect to the ground reference potential applied to the back metal contact 28. However, in this modification, the electrical insulation between the front detection contacts 22 of adjacent radiation detectors 10 formed on the common substrate 12 is impaired. Since the blocking layer 20 has a fairly low impurity concentration, its resistivity is fairly high. Therefore, there is a front detection contact 2 in the blocking layer 20.
2, sufficient electrical insulation can be obtained due to the high and low resistivities and the distance between the front detection contacts 22 of adjacent radiation detectors 10. If the front sensing contact 22 is formed in the sensing layer 18 as suggested in this variation, the electrical insulation will be lost due to the low resistivity of the sensing layer 18. Finally, substrate 12 of radiation detector 10 may be formed using a variety of different materials. This material may be a semiconductor crystal doped with N-type or P-type impurities, as used in embodiments of the invention, or it may be a non-semiconductor material such as glass or sapphire. Such materials can be used to increase the transparency of the substrate in a desired radiation frequency range or to increase the mechanical strength of the radiation detector 10. Thus, a radiation detector has been disclosed that is highly sensitive to incident radiation and is easily applicable to a wide range of applications. Obviously, various modifications are possible in view of the embodiments of the invention described above. In addition to the above-mentioned variations, there are also variations in which impurities are introduced into the detector structure by diffusion, or in which conductively doped polysilicon is used instead of a metal conductor. The manufacturing steps necessary to fabricate the detectors of the various embodiments are all conventional in nature and have not been described in order not to obscure the nature of the invention. It is therefore to be understood that within the scope of the appended claims, the invention may have application other than as specifically described.