Abstract:
A protective circuit connected between a terminal of a semiconductor integrated circuit and ground (GND), comprises: a first diode having an anode connected to the terminal of the semiconductor integrated circuit; a second diode having an anode connected to GND and a cathode connected to the cathode of the first diode; a transistor having a collector or drain connected to the terminal of the semiconductor integrated circuit, and an emitter or source connected to GND; and at least one third diode connected in series in a forward direction from the cathode of the first diode toward the base or gate of the transistor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a protective circuit for protecting a semiconductor integrated circuit from electrostatic discharge, and more specifically to a protective circuit that can reduce a mounting area by decreasing the number of elements. 
         [0003]    2. Background Art 
         [0004]    A protective circuit is used for protecting semiconductor integrated circuits from electrostatic discharge (ESD) (for example, refer to Patent Documents 1 to 6). The protective circuit is connected between a power source terminal or input-output terminal of the semiconductor integrated circuit and GND. The protective circuit is composed of, for example, a plurality of stages of diodes connected in series so as not to operate at a certain voltage or below. In the protective circuit for the power source terminal, the number of stages of diodes is determined so as not to operate at a power-source voltage or below. On the other hand, in the protective circuit for the input-output terminal, the number of stages of diodes is determined so as not operate at the voltage amplitude of normal input-output power. 
       [Patent Document 1] Japanese Patent Laid-Open No. 6-104712 
     [Patent Document 2] Japanese Patent Laid-Open No. 10-274663 
     [Patent Document 3] Japanese Patent Laid-Open No. 10-164748 
     [Patent Document 4] Japanese Patent Laid-Open No. 58-58827 
     [Patent Document 5] Japanese Patent Laid-Open No. 5-36979 
     [Patent Document 6] Japanese Patent Laid-Open No. 64-55017 
     SUMMARY OF THE INVENTION 
       [0005]      FIG. 6  is a circuit diagram showing a protective circuit, according to the first reference example. Five stages of diodes D 61  to D 65  are connected in series in a forward direction, and five stages of diodes D 66  to D 70  are connected in series in a backward direction from the terminal T of a semiconductor integrated circuit to GND. Thereby, the semiconductor integrated circuit can be protected from positive and negative surge signals. The number of elements composing the circuit of  FIG. 6  is ten. However, if the circuit is composed of n-stage diodes, (n×2) diodes are required. 
         [0006]      FIG. 7  is a circuit diagram showing a protective circuit, according to the second reference example. The anode of the diode D 71  is connected to the terminal T. The anode of the diode D 72  is connected to GND, and the cathode of the diode D 72  is connected to the cathode of the diode D 71 . The cathode of the diode D 73  is connected to the terminal T. The cathode of the diode D 74  is connected to GND, and the anode of the diode D 74  is connected to the anode of the diode D 73 . From the cathodes of diodes D 71  and D 72  toward the anodes of diodes D 73  and D 74 , diodes D 75  to D 77  are connected in series in a forward direction. As described, since the circuit shown in  FIG. 7  uses diodes D 75  to D 77  in both forward and backward directions, the number of elements can be reduced compared with the circuit shown in  FIG. 6 . 
         [0007]    It is required that the diodes in the protective circuit have a sufficiently large area for p-n junction so as not to be damaged by surge current. Therefore, if the number of stages of diodes is large, there is possibility that the mounting area of the protective circuit that occupies on a chip is widened. 
         [0008]    To solve the above-described problems, it is an object of the present invention to provide a protective circuit that can reduce a mounting area by decreasing the number of elements. 
         [0009]    According to one aspect of the present invention, a protective circuit connected between a terminal of a semiconductor integrated circuit and GND, comprises: a first diode whose anode is connected to the terminal of a semiconductor integrated circuit; a second diode whose anode is connected to GND and whose cathode is connected to the cathode of said first diode; a transistor whose collector or drain is connected to the terminal of said semiconductor integrated circuit, and whose emitter or source is connected to GND; and at least one third diode connected in series in a forward direction from the cathode of said first diode toward the base or gate of said transistor. 
         [0010]    According to the present invention, the number of elements that compose a protective circuit can be decreased, and the mounting area of the protective circuit that occupies the chip can be reduced. 
         [0011]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a circuit diagram showing a protective circuit according to the first embodiment. 
           [0013]      FIG. 2  is a circuit diagram showing a protective circuit according to the second embodiment. 
           [0014]      FIG. 3  is a circuit diagram showing a protective circuit according to the third embodiment. 
           [0015]      FIG. 4  is a circuit diagram showing a protective circuit according to the fourth embodiment. 
           [0016]      FIG. 5  is a circuit diagram showing a protective circuit according to the fifth embodiment. 
           [0017]      FIG. 6  is a circuit diagram showing a protective circuit, according to the first reference example. 
           [0018]      FIG. 7  is a circuit diagram showing a protective circuit, according to the second reference example. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0019]      FIG. 1  is a circuit diagram showing a protective circuit according to the first embodiment. The protective circuit is connected between a terminal T, which is a power source terminal or an input-output terminal of a semiconductor integrated circuit (not shown), and GND, to protect the semiconductor integrated circuit from electrostatic discharge. The semiconductor integrated circuit is, for example, a GaAs-HBT, and the base-emitter voltage or the base-collector voltage of this GaAs-HBT is about 1.2 V. Whereas, when a DC voltage of about 6 V or above is applied to the terminal T, the protective circuit is operated. 
         [0020]    The anode of a diode D 11  (first diode) is connected to the terminal T. The anode of a diode D 12  (second diode) is connected to GND, and the cathode of the diode D 12  is connected to the cathode of the diode D 11 . The collector of a transistor Q 11  is connected to the terminal T, and the emitter of the transistor Q 11  is connected to GND. Diodes D 13  to D 15  (third diodes) are connected in series in a forward direction from the cathodes of the diodes D 11  and D 12  toward the base of the transistor Q 11 . 
         [0021]    Here, the portions between the base and the emitter and between the base and the collector of the transistor Q 11  are equivalent to a diode, respectively. Therefore, in the portions in the forward direction and the backward direction from the terminal T toward GND, the above-described protective circuit is equivalent to five stages of diodes. 
         [0022]    The operation of the protective circuit in the forward direction when a positive voltage is applied to the terminal T will be described. When a voltage of the clamp voltage determined by the five stages of diodes in the forward direction (6 V) or lower is applied to the terminal T, the transistor Q 11  operates as a diode between the base and the emitter. On the other hand, when a voltage of the clamp voltage (6 V) or higher is applied to the terminal T, the transistor Q 11  performs three-terminal operation. Therefore, the current flows mainly between the collector and the emitter of the transistor Q 11  as a collector current. 
         [0023]    The operation of the protective circuit in the backward direction when a negative voltage is applied to the terminal T will be described. Different from the operation in the forward direction, even if a voltage of the clamp voltage determined by the five stages of diodes in the backward direction (−6 V) or lower is applied to the terminal T, the transistor Q 11  does not perform three-terminal operation. Therefore, the current flows through the diode D 12 , diodes D 13  to D 15 , and between the base and the collector of the transistor Q 11 . 
         [0024]    The circuit according to the first embodiment is equivalent to the circuit shown in  FIG. 7  wherein two diodes D 73  and D 74  are replaced by the transistor Q 11 . Therefore, compared with the circuit shown in  FIG. 7 , the mounting area can be reduced by the size of one element. The mounting area for the transistor is only larger than the area for a diode by the size of the electrodes. Since only the base current of the transistor Q 11  flows in the diode D 11 , the area for p-n junction of the diode D 11  can be made smaller than the area of the transistor Q 11 . Therefore, the number of elements composing the protective circuit can be reduced to make the mounting area of the protective circuit on the chip smaller. 
         [0025]    In the first embodiment, although there are three diodes between the cathodes of the diodes D 11  and D 12 , and the base of the transistor Q 11 , the present invention is not limited thereto, but at least one is sufficient. By increasing or decreasing the number of diodes, the number of stages in the protective circuit can be adjusted. For example, when the number of the diode is one, the number of the stages in the protective circuit is three. 
       Second Embodiment 
       [0026]      FIG. 2  is a circuit diagram showing a protective circuit according to the second embodiment. This protective circuit is a circuit wherein the terminal T and GND are replaced by each other in the circuit according to the first embodiment. Therefore, the operation of the protective circuit is opposite to the operation of the circuit according to the first embodiment in terms of the forward direction and the backward direction. 
         [0027]    The anode of a diode D 21  (first diode) is connected to the terminal T. The anode of a diode D 22  (second diode) is connected to GND, and the cathode of the diode D 22  is connected to the cathode of the diode D 21 . The emitter of a transistor Q 21  is connected to the terminal T, and the collector of the transistor Q 21  is connected to GND. Diodes D 23  to D 25  (third diodes) are connected in series in a forward direction from the cathodes of the diodes D 21  and D 22  toward the base of the transistor Q 21 . 
       Third Embodiment 
       [0028]      FIG. 3  is a circuit diagram showing a protective circuit according to the third embodiment. This protective circuit is connected between the terminal T, which is the power source terminal or the input-output terminal and GND, and protects the semiconductor integrated circuit (not shown) from electrostatic discharge. The semiconductor integrated circuit is, for example, a GaAs-HBT, and the base-emitter voltage or the base-collector voltage of the GaAs-HBT is about 1.2 V. Whereas, when a DC voltage of about 6 V or higher is applied to the terminal T, the protective circuit operates. 
         [0029]    The anode of a diode D 31  (first diode) is connected to the terminal T. The anode of a diode D 32  (second diode) is connected to GND. The collector of a transistor Q 31  (first transistor) is connected to the terminal T, and the emitter thereof is connected to the cathode of the diode D 32 . The collector of a transistor Q 32  (second transistor) is connected to GND, and the emitter thereof is connected to the cathode of the diode D 31 . The anode of a diode D 33  (third diode) is connected to the cathode of the diode D 31 , and the cathode of the diode D 33  is connected to the base of the transistor Q 31 . The anode of a diode D 34  (fourth diode) is connected to the cathode of the diode D 32 , and the cathode of the diode D 34  is connected to the base of the transistor Q 32 . 
         [0030]    Here, the portions between the base and the emitter of the transistor Q 31  and between the base and the collector of the transistor Q 32  are equivalent to a diode, respectively. Therefore, in the portion in the forward direction from the terminal T toward GND, the above-described protective circuit is equivalent to five stages of diodes. The portion in the backward direction is also the same. 
         [0031]    The operation of the protective circuit in the forward direction when a positive voltage is applied to the terminal T will be described. When a voltage of the clamp voltage determined by the five stages of diodes in the forward direction (6 V) or lower is applied to the terminal T, the transistor Q 31  operates as a diode between the base and the emitter. On the other hand, when a voltage of the clamp voltage (6 V) or higher is applied to the terminal T, the transistor Q 31  performs three-terminal operation. Therefore, the current flows mainly between the collector and the emitter of the transistor Q 31  as a collector current, and further flows through the diode D 34  and between the base and the collector of the transistor Q 32  to GND. 
         [0032]    Since the protective circuit according to the third embodiment is symmetric between the terminal T and GND, the protective circuit operates in the same way when a negative voltage is applied to the terminal T as when a positive voltage is applied to the terminal T. 
         [0033]    Since the circuit according to the third embodiment has six elements, the number of elements can be reduced compared with the circuit shown in  FIG. 7 . The mounting area for the transistor is only larger than the area for a diode by the size of the electrodes. Since only the base current of the transistor Q 31  flows in the diode D 31 , the area for p-n junction of the diode D 31  can be made smaller than the area of the transistor Q 31 . Therefore, the number of elements composing the protective circuit can be reduced to make the mounting area of the protective circuit on the chip smaller. 
       Fourth Embodiment 
       [0034]      FIG. 4  is a circuit diagram showing a protective circuit according to the fourth embodiment. This protective circuit is connected between the terminal T, which is the power source terminal or the input-output terminal and GND, and protects the semiconductor integrated circuit (not shown) from electrostatic discharge. The semiconductor integrated circuit is, for example, a GaAs-HBT, and the base-emitter voltage or the base-collector voltage of the GaAs-HBT is about 1.2 V. Whereas, when a DC voltage of about 6 V or higher is applied to the terminal T, the protective circuit operates. 
         [0035]    The collector of a transistor Q 41  (first transistor) is connected to the terminal T. The base of a transistor Q 42  (second transistor) is connected to the emitter of the transistor Q 41 , the collector thereof is connected to GND, and the emitter thereof is connected to the base of the transistor Q 41 . Diodes D 41  to D 43  (first diodes) are connected in series in a forward direction from the terminal T toward the base of the transistor Q 41 . Diodes D 44  to D 46  (second diodes) are connected in series in a forward direction from GND toward the base of the transistor Q 42 . 
         [0036]    Here, the portions between the base and the emitter of the transistor Q 41  and between the base and the collector of the transistor Q 42  are equivalent to a diode, respectively. Therefore, in the portion in the forward direction from the terminal T toward GND, the above-described protective circuit is equivalent to five stages of diodes. The portion in the backward direction is also the same. 
         [0037]    The operation of the protective circuit in the forward direction when a positive voltage is applied to the terminal T will be described. When a voltage of the clamp voltage determined by the five stages of diodes in the forward direction (6 V) or lower is applied to the terminal T, the transistor Q 41  operates as a diode between the base and the emitter. On the other hand, when a voltage of the clamp voltage (6 V) or higher is applied to the terminal T, the transistor Q 41  performs three-terminal operation. Therefore, the current flows mainly between the collector and the emitter of the transistor Q 41  as a collector current, and further flows between the base and the collector of the transistor Q 42  to GND. 
         [0038]    Since the protective circuit according to the fourth embodiment is symmetric between the terminal T and GND, the protective circuit operates in the same way when a negative voltage is applied to the terminal T as when a positive voltage is applied to the terminal T. 
         [0039]    Since the circuit according to the fourth embodiment has eight elements, the number of elements can be reduced compared with the circuit shown in  FIG. 6 . The mounting area for the transistor is only larger than the area for a diode by the size of the electrodes. Since only the base current of the transistors Q 41  and Q 42  flows in the diodes D 41  to D 46 , the area for p-n junction of the diodes D 41  to D 46  can be made smaller than the area of the transistors Q 41  and Q 42 . Therefore, the number of elements composing the protective circuit can be reduced to make the mounting area of the protective circuit on the chip smaller. 
         [0040]    In the fourth embodiment, although three diodes, D 41  to D 43 , are used as the first diodes, the present invention is not limited thereto, but at least one diode is sufficient. Although three diodes, D 44  to D 46 , are used as the second diodes, the present invention is not limited thereto, but at least one diode is sufficient. By increasing or decreasing the number of these diodes, the number of stages of the protective circuit can be adjusted. 
       Fifth Embodiment 
       [0041]      FIG. 5  is a circuit diagram showing a protective circuit according to the fifth embodiment. This protective circuit is connected between the terminal T, which is the power source terminal or the input-output terminal and GND, and protects the semiconductor integrated circuit (not shown) from electrostatic discharge. The semiconductor integrated circuit is, for example, a GaAs-HBT, and the base-emitter voltage or the base-collector voltage of the GaAs-HBT is about 1.2 V. Whereas, when a DC voltage of about 6 V or higher is applied to the terminal T, the protective circuit operates. 
         [0042]    The collector of a transistor Q 51  is connected to the terminal T, and the emitter thereof is connected to GND. Diodes D 51  to D 54  (first diodes) are connected in series in a forward direction from the terminal T toward the base of the transistor Q 51 . A diode D 55  (second diode) is connected in a forward direction from GND toward the base of the transistor Q 51 . 
         [0043]    Here, the portions between the base and the emitter and between the base and the collector of the transistor Q 51  are equivalent to a diode, respectively. Therefore, in the portions in the forward direction from the terminal T toward GND, the above-described protective circuit is equivalent to five stages of diodes, and in the backward direction, the protective circuit is equivalent to two stages of diodes. 
         [0044]    The operation of the protective circuit in the forward direction when a positive voltage is applied to the terminal T will be described. When a voltage of the clamp voltage determined by the five stages of diodes in the forward direction (6 V) or lower is applied to the terminal T, the transistor Q 51  operates as a diode between the base and the emitter. On the other hand, when a voltage of the clamp voltage (6 V) or higher is applied to the terminal T, the transistor Q 51  performs three-terminal operation. Therefore, the current flows mainly through between the collector and the emitter of the transistor Q 51  as a collector current. 
         [0045]    The operation of the protective circuit in the backward direction when a negative voltage is applied to the terminal T will be described. Different from the operation in the forward direction, even if a voltage of the clamp voltage determined by the two stages of diodes in the backward direction (−2.4 V) or lower is applied to the terminal T, the transistor Q 51  does not perform three-terminal operation. Therefore, the current flows through the diode D 55 , and between the base and the collector of the transistor Q 51 . 
         [0046]    Since the circuit according to the fifth embodiment has six elements, the number of elements can be reduced compared with the circuit shown in  FIG. 7 . The mounting area for the transistor is only larger than the area for a diode by the size of the electrodes. Since only the base current of the transistor Q 51  flows in the diodes D 51  to D 54 , the area for p-n junction of the diodes D 51  to D 54  can be made smaller than the area of the transistors Q 51 . Therefore, the number of elements composing the protective circuit can be reduced to make the mounting area of the protective circuit on the chip smaller. 
         [0047]    In the fifth embodiment, although four diodes, D 51  to D 54 , are used as the first diodes, the present invention is not limited thereto, but at least one diode is sufficient. Although one diode D 55  is used as the second diode, the present invention is not limited thereto, but at least one diode is sufficient. By increasing or decreasing the number of these diodes, the number of stages of the protective circuit can be adjusted. Although the minimum number of stages of the protective circuits according to the first to fourth embodiments is three, the minimum number of stages of the protective circuits according to the fifth embodiment is two in both forward and backward directions. 
         [0048]    In the above-described first to fifth embodiments, although bipolar transistors are used as transistors Q 11 , Q 21 , Q 31 , Q 32 , Q 41 , Q 42  and Q 51 , the present invention is not limited thereto, but n-channel FETs of an enhancement mode may also be used. In this case, the bases, collectors and emitters of the transistors Q 11 , Q 21 , Q 31 , Q 32 , Q 41 , Q 42  and Q 51  are gates, drains and sources, respectively. 
         [0049]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0050]    The entire disclosure of a Japanese Patent Application No. 2008-023884, filed on Feb. 4, 2008 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.