Semiconductor radiation detector with enhanced charge collection

• US 5,677,539 A
• Filed: 10/13/1995
• Issued: 10/14/1997
• Est. Priority Date: 10/13/1995
• Status: Expired due to Term

• Current Assignee: Digirad Corporation
• Original Assignee: Digirad Corporation
• Inventors: Pi, Bo, Oganesyan, Ashot, Doty, F. Patrick, Lingren, Clinton L., Conwell, Richard L., Apotovsky, Boris, Friesenhahn, Stanley J., Butler, Jack F.
• Primary Examiner(s): FIELDS, CAROLYN
• Application Number:
  US08/542,883
• Time in Patent Office: 732 Days
• Field of Search: 250/370.13, 250/370.01
• US Class Current: 250/370.13
• CPC Class Codes: G01T 1/241 Electrode arrangements, e.g. continuous or parallel strips or the like constructional or manufacturing details H01L31/00
• CPC Class Codes: H01L 27/14676 X-ray, gamma-ray or corpuscular radiation imagers measuring X-, gamma- or corpuscular radiation G01T1/00

Abstract

A radiation detector for detecting ionizing radiation. The detector includes a semiconductor having at least two sides. A bias electrode is formed on one side of the semiconductor. A signal electrode is formed on a side of the semiconductor and is used to detect the energy level of the ionizing radiation. A third electrode (the control electrode) is also formed on the semiconductor. The control electrode shares charges induced by the ionizing radiation with the signal electrode, until the charge clouds are close to the signal electrode. The control electrode also alters the electric field within the semiconductor, such that the field guides the charge clouds toward the signal electrode when the clouds closely approach the signal electrode. As a result, trapping of charge carrying radiation (i.e., electrons or holes) is minimized, and low-energy tailing is virtually eliminated.

Claims

1. A radiation detector, comprising:

(a) a semiconductor having a plurality of sides and a thickness of at least about 0.5 mm;

(b) a bias electrode formed on at least one side of the semiconductor;

(c) a signal electrode formed on at least one side of the semiconductor; and

(d) at least one control electrode, formed on at least one side of the semiconductor, for directing charge clouds resulting from ionizing events in the semiconductor towards the signal electrode and for substantially reducing the effect on the signal electrode of hole trapping in the semiconductor,

wherein the radiation detector is capable of detecting energies greater than about 20 KeV.

2. The radiation detector of claim 1 wherein the semiconductor has a Z>

34.

3. The radiation detector of claim 1 wherein the semiconductor has a resistivity greater than about 10⁷ ohm-cm.

4. The radiation detector of claim 1 wherein the signal electrode detects an output count rate, the detector including means for applying an actively variable voltage to the control electrode for changing the output count rate.

5. The radiation detector of claim 1 wherein the semiconductor comprises CdTe.

6. The radiation detector of claim 1 wherein the semiconductor comprises CdZnTe.

7. The radiation detector of claim 1 wherein the signal electrode is on the same side of the semiconductor as the control electrode.

8. The radiation detector of claim 1 wherein the signal electrode is on a different side of the semiconductor from the control electrode.

9. The radiation detector of claim 1 wherein the bias electrode is a cathode covering at least a substantial portion of the first side of the semiconductor.

10. The radiation detector of claim 1 wherein the bias electrode is configured to be coupled to a bias voltage source, and wherein the bias electrode is a cathode having a cathode voltage, V_b, and V_b is held at a substantially fixed value.

11. The radiation detector of claim 10 wherein the signal electrode is configured to be coupled to a signal electrode voltage source, and wherein the signal electrode has a voltage, V_a, and V_b -V_a is dependent on the thickness of the semiconductor.

12. The radiation detector of claim 11 wherein the thickness of the semiconductor is approximately 3 mm and V_b -V_a is approximately -500 Volts.

13. The radiation detector of claim 10 wherein the signal electrode is configured to be coupled to a signal electrode voltage source, and wherein the signal electrode is an anode having an anode voltage, V_a, at approximately ground.

14. The radiation detector of claim 10 wherein the control electrode is configured to be coupled to a control electrode voltage source, and wherein the control electrode has a control electrode voltage, V_c, and V_b is at approximately ground.

15. The radiation detector of claim 10 wherein the control electrode is configured to be coupled to a control electrode voltage source, and the signal electrode is configured to be coupled to a signal electrode voltage source, and wherein the control electrode has a control electrode voltage, V_c, and the signal electrode has a voltage, V_a, such that V_c <

V_a.

16. The radiation detector of claim 10 wherein the control electrode is configured to be coupled to a control electrode voltage source, and the signal electrode is configured to he coupled to a signal electrode voltage source, and wherein the control electrode has a control electrode voltage, V_c, and the signal electrode has an anode voltage, V_a and V_c is approximately equal to (V_a +V_b)/2.

17. The radiation detector of claim 10 wherein the control electrode is configured to be coupled to a control electrode voltage source, and wherein the control electrode has a control electrode voltage, V_c, such that V_c is approximately equal to V_b.

18. The radiation detector of claim 17 wherein V_c =V_b.

19. The radiation detector of claim 17 wherein the control electrode is connected to the bias electrode.

20. The radiation detector of claim 17 wherein the control electrode is part of the bias electrode.

21. The radiation detector of claim 10 wherein the control electrode is coupled to a first terminal of a capacitor, a second terminal of the capacitor being coupled to ground.

22. The radiation detector of claim 21 wherein the signal electrode is configured to be coupled to a signal electrode voltage source, and wherein the signal electrode is an anode having an anode voltage V_a, and wherein a first resistance exists between the bias electrode and the control electrode and a second resistance exists between the control electrode and the signal electrode, whereby the capacitor and the control electrode charge to a voltage level determined by V_b -V_a.

23. The radiation detector of claim 1 wherein the control electrode is configured to be coupled to a control electrode voltage source, and wherein the signal electrode is configured to be coupled to a signal electrode voltage source, and wherein the control electrode has a control electrode voltage, V_c, and the signal electrode has a voltage, V_a, such that V_c <

V_a.

24. The radiation detector of claim 1 wherein the control electrode is coupled to a constant voltage source.

25. The radiation detector of claim 1 wherein the bias electrode is a cathode having a cathode surface area and the signal electrode has a substantially smaller surface area than the cathode surface area.

26. The radiation detector of claim 25 wherein the signal electrode is an anode located on an opposing surface of the semiconductor from the first side having the cathode.

27. The radiation detector of claim 26 wherein the control electrode is located on the opposing surface and surrounds the anode.

28. The radiation detector of claim 27 wherein the control electrode comprises a substantially circular ring surrounding the anode.

29. The radiation detector of claim 1 wherein the bias electrode, the signal electrode, and the control electrode are formed on the surface of the semiconductor.

30. The radiation detector of claim 29 wherein the bias electrode, the signal electrode, and the control electrode comprise gold film.

31. The radiation detector of claim 1 wherein at least one of the bias electrode, the signal electrode, and the control electrode is buried under the surface of the semiconductor.

32. The radiation detector of claim 1 wherein the signal electrode is segmented into a plurality of segments.

33. The radiation detector of claim 1 wherein the control electrode is segmented into a plurality of segments.

34. The radiation detector of claim 1 wherein the radiation detector includes a plurality of control electrodes.

35. A radiation detector for detecting ionizing radiation, comprising:

(a) a semiconductor responsive to ionizing radiation having a plurality of sides;

(b) a cathode formed on a first side of the semiconductor, the cathode having a cathode surface area and wherein the cathode is coupled to a cathode voltage source, the cathode having a cathode voltage, V_b ;

(c) an anode formed on a second side of the semiconductor, the second side opposing the first side and having a center, the anode comprising a contact located near the center of the second side and having a substantially smaller surface area than the cathode surface area, wherein the anode is coupled to an anode voltage source, the anode having an anode voltage, V_a ; and

(d) a control electrode formed on the second side of the semiconductor, the control electrode forming a single ring surrounding the anode, wherein the control electrode is coupled to a control voltage source, the control electrode having a control voltage, V_c, such that V_c <

V_a.

36. The radiation detector of claim 35 wherein the semiconductor is formed from CdZnTe.

37. The radiation detector of claim 35 wherein the semiconductor has a thickness and V_b -V_a is dependent on the thickness.

38. The radiation detector of claim 37 wherein the thickness of the semiconductor is approximately 3 mm and V_b -V_a is approximately -400 Volts.

39. The radiation detector of claim 38 wherein V_a is approximately ground and V_c is approximately -300 Volts.

40. The radiation detector of claim 35 wherein the control electrode is coupled to a constant voltage source.

41. The radiation detector of claim 35 wherein the control electrode is coupled to a first terminal of a capacitor, a second terminal of the capacitor being coupled to ground.

42. The radiation detector of claim 41 wherein a first resistance exists between the cathode and the control electrode and a second resistance exists between the control electrode and the signal electrode, whereby the capacitor and the control electrode charge to a voltage level determined by such resistances and V_b -V_a.

43. The radiation detector of claim 35 wherein at least one of the cathode, the anode, and the control electrode is formed on the surface of the semiconductor.

44. The radiation detector of claim 43 wherein at least one of the cathode, the anode, and the control electrode comprises gold film.

45. The radiation detector of claim 35 wherein at least one of the cathode, the anode, and the control electrode is buffed under the surface of the semiconductor.

46. The radiation detector of claim 35 wherein the anode and the control electrode are separated on the surface of the semiconductor.

47. The radiation detector of claim 35 wherein the anode and the control electrode are separated by a bulk material on the surface of the semiconductor.

48. The radiation detector of claim 35 wherein the control electrode is a circular ring.

49. The radiation detector of claim 35 wherein the control electrode is a rectangular ring.

50. The radiation detector of claim 35 wherein V_c =V_b.

51. The radiation detector of claim 35 wherein the control electrode is connected to the bias electrode.

52. The radiation detector of claim 35 wherein the control electrode is part of the bias electrode.

53. A method for detecting an ionizing radiation emanating from a source using a radiation detector, the radiation detector comprising a semiconductor having a plurality of sides, the semiconductor including a control electrode, a cathode side and an anode side, the cathode side having a cathode and the anode side having an anode, the method comprising the steps of:

(a) placing the radiation detector in a path of the radiation emanating from the source;

(b) generating an electric field internal to the semiconductor via voltages applied on the cathode and anode;

(c) creating an electron charge cloud and an associated hole cloud within the semiconductor by the ionizing radiation;

(d) guiding the electron-charge cloud toward the anode by the electric field;

(c) sharing, between the anode and the control electrode, charge induced by the charge clouds when they are located a relatively significant distance from the anode, such that the charge induced on the anode is very small compared with the charge induced on the control electrode; and

(f) decreasing toward zero the induced charge on the control electrode by the electron-charge cloud and increasing the induced charge on the anode to the full value of the electron-charge cloud that arrives at the anode.

54. The method of claim 53 wherein the anode has an anode dimension;

and wherein the full value of the electron-charge cloud is induced on the anode when the electron-charge cloud is within a distance away from the anode approximately equal to the anode dimension.

55. The method of claim 53 wherein the electron-charge cloud comprises a plurality of moving electrons;

and wherein the step of guiding the electron-charge cloud further includes the steps of;

(a) forming a plurality of guide paths by the electric field, each guide path having a beginning and an end, the plurality of guide paths being substantially parallel to one another at the beginnings and turning toward the anode near the ends; and

(b) guiding each of the plurality of moving electrons along a corresponding guide path toward the anode.

56. The method of claim 55 wherein the control electrode is connected to the bias electrode.

57. The method of claim 55 wherein the control electrode is part of the bias electrode.

58. The method of claim 53, further comprising the steps of:

(a) biasing the control electrode to a control voltage, V_c ;

(b) biasing the anode to an anode voltage, V_a ;

(c) biasing the cathode to a cathode voltage, V_b ; and

(d) maintaining V_c at a voltage level less than V_a.

59. The method of claim 58 comprising the step of biasing the cathode and the control detector so that V_c =V_b.

60. The method of claim 53, further comprising the step of forming the control electrode such that it surrounds the anode.

61. The method of claim 53, further comprising the steps of:

(a) coupling the control electrode to a capacitor; and

(b) coupling the capacitor to ground.

62. The method of claim 61, further comprising the steps of:

(a) biasing the cathode to a cathode voltage, V_b ;

(b) biasing the anode to an anode voltage, V_a ; and

(c) charging the capacitor and the control electrode to a voltage determined in accordance with V_b -V_a.

63. The method of claim 53 wherein the semiconductor includes a plurality of movable electrons, the electron-charge cloud comprising at least some of the plurality of movable electrons.

64. The method of claim 53, further comprising the step of segmenting the anode into a plurality of segments.

65. The method of claim 53, further comprising the step of segmenting the control electrode into a plurality of segments.

66. A system for detecting an ionizing radiation, comprising:

(a) a radiation detector, including;

(1) a semiconductor having a plurality of sides,

(2) a cathode formed on a cathode side of the semiconductor,

(3) an anode formed on at least one side of the semiconductor different from the cathode side,

(4) a control electrode formed on at least one side of the semiconductor different from the cathode side; and

(b) means for forming an electric field internal to the semiconductor;

wherein, when the ionizing radiation is absorbed in the radiation detector, a movable charge cloud is generated within the semiconductor as a result of the ionizing radiation;

wherein the electric field guides the electron-charge cloud toward the anode;

wherein the anode and the control electrode share a charge induced by the charge cloud when the charge cloud is located a relatively significant distance away from the anode, such that the charge on the anode is small; and

wherein the charge induced by the electron-charge cloud on the anode increases as the electron-charge cloud becomes close to the anode.

67. The system of claim 66 wherein the anode has an anode dimension;

and wherein the means for forming the electric field, the control electrode, and the anode are configured so that the charge is completely induced on the anode when the electron-charge cloud arrives at the anode, most of which occurs within a distance away from the anode approximately equal to the anode dimension.

68. The system of claim 66 wherein the electron-charge cloud comprises a plurality of movable electrons;

and wherein the anode for forming the electric field is configured so that a plurality of guide paths are formed by the electric field, each guide path having a beginning and an end, the plurality of guide paths being substantially parallel to one another at the beginnings and turning toward the anode near the ends; and

wherein each of the plurality of movable electrons is guided along a corresponding guide path toward the anode.

69. The system of claim 66 wherein the control electrode is formed such that it surrounds the anode.

70. The system of claim 66 wherein the means for forming the electric field includes a capacitor coupled to the control electrode and to ground.

71. The system of claim 70 wherein the anode has a voltage V_a, wherein the cathode is biased to a cathode voltage, V_b ;

and wherein the capacitor and the control electrode are charged to a voltage determined in accordance with V_b -V_a.

72. The system of claim 71 wherein the control electrode has a voltage V_c and wherein V_c =V_b.

73. The system of claim 66 wherein the control electrode is connected to the cathode.

74. The system of claim 66 wherein the control electrode is part of the cathode.

75. An apparatus for detecting ionizing radiation, comprising:

(a) a semiconductor having at least three electrodes, including a cathode, an anode, and a control electrode, and a means for biasing each electrode to a voltage potential, wherein an electric field is created within the semiconductor;

wherein, as the ionizing radiation is absorbed in the semiconductor, a charge cloud is generated within the semiconductor;

wherein the charge cloud induces a charge in the semiconductor, the charge being shared by the anode and the control electrode until the charge has a close proximity to the anode.

76. The apparatus of claim 75 wherein the voltage potential of the control electrode is less than the voltage potential of the anode.

77. The apparatus of claim 75 wherein the semiconductor is formed from a CdZnTe material.

78. The apparatus of claim 75 wherein the control electrode and anode are formed on a same side of the semiconductor, the control electrode forming a ring around the anode.

79. The apparatus of claim 75, further comprising a capacitor coupled to the control electrode and to ground.

80. The apparatus of claim 75 wherein the anode is formed on a side of the semiconductor;

and wherein the electric field has a plurality of electric field lines, the lines being substantially parallel to one another and substantially orthogonal to the side on which the anode is formed and converging toward the anode as the lines approach the side on which the anode is formed.

81. The apparatus of claim 80 wherein the electric field lines are configured to guide the charge cloud toward the anode.

82. The apparatus of claim 75 wherein the anode has an anode dimension;

and wherein the three electrodes are configured so that the control electrode has only a negligible share of the charge when the charge cloud is within a distance away from the anode substantially equivalent to the anode dimension.

83. The apparatus of claim 82 wherein the three electrodes are configured so that the anode has only a small share of the charge when the charge cloud is beyond the distance away from the anode.

84. The apparatus of claim 75 wherein the semiconductor includes a plurality of electrons movable within the semiconductor;

and wherein the three electrodes are configured so that the charge cloud comprises at least some of the movable electrons.

85. The system of claim 75 wherein the control electrode is biased to the same voltage potential as the cathode.

86. The system of claim 75 wherein the control electrode is connected to the cathode electrode.

87. The system of claim 75 wherein the control electrode is part of the bias electrode.

88. An apparatus for detecting ionizing radiation, comprising a semiconductor having a thickness of at least about 0.5 mm and having at least three electrodes, including a cathode, an anode, and a control electrode, the ionizing radiation creating a charge cloud within the semiconductor, the electrodes being formed on the semiconductor and means for biasing the electrodes to generate an electric field pattern within the semiconductor such that the electric field pattern focuses the charge cloud resulting from the ionizing radiation towards the anode, thereby substantially reducing the effect of hole trapping in the semiconductor on the anode, wherein the radiation detector is capable of detecting energies greater than about 20 KeV.

89. The apparatus of claim 88 wherein the control electrode is biased to a voltage level less than the voltage level to which the anode is biased.

90. The apparatus of claim 88 wherein the semiconductor has a plurality of sides, the control electrode and the anode being formed on a side of the semiconductor opposing a side on which the cathode is formed, the control electrode surrounding the anode.

91. The apparatus of claim 88 wherein the anode comprises a contact point.

92. The apparatus of claim 88 wherein the control electrode is coupled to a capacitor, the capacitor being coupled to ground, the control electrode and the capacitor being charged to a voltage level determined by voltage levels of the anode and the cathode.

93. The apparatus of claim 88 wherein the control electrode is biased to a voltage equal to the voltage to which the cathode is biased.

94. The apparatus of claim 88 wherein the control electrode is connected to the cathode.

95. The apparatus of claim 88 wherein the control electrode is part of the cathode.

96. A radiation detector array, comprising:

(a) a semiconductor having a plurality of sides and a thickness of at least about 0.5 mm;

(b) a cathode formed on at least one side of the semiconductor;

(c) an array of anodes formed on at least one side of the semiconductor; and

(d) a control electrode pattern, formed on at least one side of the semiconductor, for directing charge clouds resulting from ionizing events in the semiconductor toward a respective anode and for substantially reducing the effect on the array of anodes of hole trapping in the semiconductor, the control electrode pattern being formed such that it affects each of the anodes in the array of anodes,

wherein the radiation detector is capable of detecting energies greater than about 20 KeV.

97. The radiation detector of claim 96 wherein the control electrode pattern comprises a grid pattern.

98. The radiation detector of claim 96 wherein each of the anodes is surrounded by a corresponding section of the control electrode pattern;

and wherein each of the anodes and the corresponding section comprise a pixel.

99. The radiation detector of claim 96 wherein at least some of the anodes in the array of anodes are electrically coupled together.

100. The radiation detector of claim 96 wherein the control electrode has a voltage V_c, and the cathode electrode has a voltage V_b, and wherein V_c =V_b.

101. The radiation detector of claim 96 wherein the control electrode is connected to the cathode.

102. The radiation detector of claim 96 wherein the control electrode is part of the cathode.