Patent Publication Number: US-2013249044-A1

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-066890, filed Mar. 23, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate a semiconductor device. 
     BACKGROUND 
     Traditionally, to protect internal circuits from a surge, a protection circuit is placed between the power supply terminal and the input-output terminal as well as between the input-output terminal and a ground terminal. This protection circuit controls the flow of an electric current if a surge is applied to the power supply terminal, input-output terminal, or the ground terminal, and functions so that high voltage will not be impressed on the internal circuit. If a surge is applied, the diode used in the protection circuit must not break when the electric current flows in the forward or reverse direction. To withstand a current flow in the reverse direction, which is lower compared with the forward direction, it is necessary to make the size of the element larger and to secure protection by lowering the current density. For this reason, there is a tendency for the size of the integrated semiconductor circuit to increase. However, it is desirable to design this kind of protection circuit to have a small circuit area. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram that shows a semiconductor device according to a first embodiment. 
         FIG. 2  is a circuit diagram that shows a semiconductor device according to a second embodiment. 
         FIG. 3  is a circuit diagram that shows a semiconductor device according to a third embodiment. 
         FIG. 4  is a circuit diagram that shows a semiconductor device according to a fourth embodiment. 
         FIG. 5  is a circuit diagram that shows a semiconductor device according to a fifth embodiment. 
         FIG. 6  is a circuit diagram that shows a semiconductor device according to a sixth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments provide a semiconductor device that includes a protection circuit configuration that can have a scaled-down circuit area. 
     A semiconductor device according to an embodiment includes a first diode, a second diode, and a third diode. The first diode has an anode connected to a first power supply terminal that is supplied with a first power-supply voltage and a cathode connected to an input-output terminal at which input-output signals are input and output. The second diode has an anode connected to the input-output terminal and a cathode connected to a second power supply terminal to which a second power-supply voltage that is higher than the first power-supply voltage is applied. The third diode has an anode connected to the first power supply terminal and a cathode connected to the second power supply terminal. The breakdown voltage of at least one of the first diode and the second diode is higher than the breakdown voltage of the third diode. 
     First Embodiment 
     The composition of the semiconductor device according to the first embodiment will be described with reference to  FIG. 1 . The semiconductor device according to the first embodiment, as shown in  FIG. 1 , includes a protection circuit  10  and an internal circuit  20 . The protection circuit  10  provides protection so that a surge is not applied to the internal circuit  20  if a surge is applied to the power supply terminal T 1 , the input-output terminal T 2 , or the ground terminal T 3 . The internal circuit  20  is supplied with a power-supply voltage Vdd from the power supply terminal T 1 , and is supplied with a ground voltage Vss (Vss&lt;Vdd) from the ground terminal T 3 . Also, the internal circuit  20  is input with various signals from the input-output terminal T 2 , and outputs various signals to the input-output terminal T 2 . 
     The protection circuit  10 , as shown in  FIG. 1 , includes diodes  11 ,  12 , and  13 . Diode  11  has an anode connected to the ground terminal T 3  and a cathode connected to the input-output terminal T 2 . The diode  12  has an anode connected to the input-output terminal T 2  and a cathode connected to the power supply terminal T 1 . The diode  13  has an anode connected to the ground terminal T 3  and a cathode connected to the power supply terminal T 1 . The breakdown voltage of the diode  11  is higher than the breakdown voltage of the diodes  12  and  13 . In contrast, the breakdown voltage of the diode  12  is nearly equal to the breakdown voltage of the diode  13 . Due to the relationships among these breakdown voltages (which will be discussed later), a reverse current does not flow through the diode  11 , and the element size (e.g., element area size) of the diode  11  can be made smaller than the element size of the diodes  12  and  13 . 
     Next, the flow of the electric current in the first embodiment when a negative surge is applied to the input-output terminal T 2  with the ground terminal T 3  as the reference is described. In this case, as shown in path P 1 , a current in the forward direction flows through the diode  11 . With this, the negative surge is discharged, and the protection circuit  10  protects the internal circuit  20 . 
     Next, the flow of the electric current in the first embodiment when a positive surge is applied to the input-output terminal T 2  with the ground terminal T 3  as the reference is described. In this case, because the breakdown voltage of the diode  11  is higher than the breakdown voltage of the diode  13 , a reverse current does not flow through the diode  11 . Therefore, as shown in path P 2 , an electric current in the forward direction flows through the diode  12 , and an electric current in the reverse direction flows through the diode  13 . With this, the positive surge is discharged, and the protection circuit  10  protects the internal circuit  20 . 
     Next, the flow of the electric current in the first embodiment when a negative charge is applied to the input-output terminal T 2  with the power supply terminal T 1  as the reference is described. In this case, as shown in path P 3 , an electrical current in the reverse direction flows through the diode  12 . With this, the negative surge is discharged, and the protection circuit  10  protects the internal circuit  20 . 
     Next, the flow of the electric current in the first embodiment when a positive surge is applied to the input-output terminal T 2  with the power supply terminal T 1  as the reference is described. In this case, as shown in path P 4 , an electric current in the forward direction flows through the diode  12 . With this, the positive surge is discharged, and the protection circuit  10  protects the internal circuit  20 . 
     Next, the flow of the electric current in the first embodiment when a negative surge is applied to the power supply terminal T 1  with the ground terminal T 3  as the reference is described. In this case, as shown in path P 5 , an electric current in the forward direction flows through the diode  13 . With this, the negative surge is discharged, and the protection circuit  10  protects the internal circuit  20 . 
     Next, the flow of the electric current in the first embodiment when a positive surge is applied to the power supply terminal T 1  with the ground terminal T 3  as the reference is described. In this case, as shown in path P 6 , an electric current in the reverse direction flows the diode  13 . With this, the positive surge is discharged, and the protection circuit  10  protects the internal circuit  20 . 
     Thus, the first embodiment can discharge surges that have patterns based on the relationships among the breakdown voltages. Furthermore, because a reverse current does not flow through the diode  11 , the element size of the diode  11  can be made smaller than the element size of the diodes  12  and  13 . That is, the first embodiment can protect the internal circuit  20  with a reduced circuit area. 
     Second Embodiment 
     Next, the semiconductor device according to the second embodiment is described with reference to  FIG. 2 .  FIG. 2  is a circuit diagram of the semiconductor device according to the second embodiment. The protection circuit  10   a  according to the second embodiment, as shown in  FIG. 2 , includes the diode  11   a  instead of the diode  11  and includes the diode  12   a  instead of the diode  12 . 
     The diode  11   a  has an anode connected to the ground terminal T 3  and a cathode connected to the input-output terminal T 2 . The diode  12   a  has an anode connected to the input-output terminal T 2  and a cathode connected to the power supply terminal T 1 . In this point, the second embodiment is the same as the first embodiment. However, the breakdown voltage of the diode  11   a  is nearly equal to the breakdown voltage of the diode  13 , and the breakdown voltage of the diode  12   a  is higher than the breakdown voltage of the diode  13 . Due to the relationships among these breakdown voltages and because a reverse current does not flow through the diode  12   a , the element size of the diode  12   a  can be made smaller than the element size of the diodes  11   a  and  13 . 
     Next, the flow of the electric current in the second embodiment when a negative surge is applied to the input-output terminal T 2  with the ground terminal T 3  as reference is described. In this case, in the same way as the first embodiment and as shown in path P 1 , an electric current in the forward direction flows through the diode  11   a . With this, the negative surge is discharged, and the protection circuit  10   a  protects the internal circuit  20 . 
     Next, the flow of the electric current in the second embodiment when a positive surge is applied to the input-output terminal T 2  with the ground terminal T 3  as reference is described. In this case, as shown in path P 2   a , an electric current in the reverse direction flows through the diode  11   a . With this, the positive surge is discharged, and the protection circuit  10   a  protects the internal circuit  20 . 
     Next, the flow of the electric current in the second embodiment when a negative surge is applied to the input-output terminal T 2  with the power supply terminal T 1  as the reference is described. In this case, because the breakdown voltage of the diode  12   a  is higher than the breakdown voltage of the diode  13 , a reverse current does not flow through the diode  12   a . Therefore, as shown in path P 3   a , an electric current in the reverse direction flows through the diode  13 , and a current in the forward direction flows through the diode  11   a . With this, the negative surge is discharged, and the protection circuit  10   a  protects the internal circuit  20 . 
     Next, the flow of the electric current in the second embodiment when a positive surge is applied to the input-output terminal T 2  with the power supply terminal T 1  as the reference is described. In this case, in the same way as the first embodiment and as shown in path P 4 , an electric current in the forward direction flows through the diode  12   a . With this, the positive surge is discharged, and the protection circuit  10   a  protects the internal circuit  20 . 
     Next, the flow of the electric current in the second embodiment when a negative surge is applied to the power supply terminal T 1  with the ground terminal T 3  as the reference is described. In this case, in the same way as the first embodiment and as shown in path P 5 , an electric current in the forward direction flows through the diode  13 . With this, the negative surge is discharged, and the protection circuit  10   a  protects the internal circuit  20 . 
     Next, the flow of the electric current in the second embodiment when a positive surge is applied to the power supply terminal T 1  with the ground terminal T 3  as the reference is described. In this case, in the same way as the first embodiment and as shown in path P 6 , an electric current in the reverse direction flows through the diode  13 . With this, the positive surge is discharged, and the protection circuit  10   a  protects the internal circuit  20 . 
     Thus, the second embodiment can discharge surges that have patterns based on the relationships among the breakdown voltages. Furthermore, the element size of the diode  12   a  can be made smaller than the element size of the diodes  11  and  13 . That is, the second embodiment can protect the internal circuit  20  with a reduced circuit area. 
     Third Embodiment 
     Next, the semiconductor device according to the third embodiment is described with reference to  FIG. 3 .  FIG. 3  is a circuit diagram of the semiconductor device according to the third embodiment. The protection circuit  10   b  according to the third embodiment, as shown in  FIG. 3 , includes the diodes  11 ,  12   a , and  13 . The breakdown voltages of the diodes  11  and  12   a  are higher than the breakdown voltage of the diode  13 . Therefore, the element sizes of the diodes  11  and  12   a  can be made smaller than the element size of the diode  13 . 
     Next, the flow of the electric current in the third embodiment when a negative surge is applied to the input-output terminal T 2  with the ground terminal T 3  as the reference is described. In this case, in the same way as the first embodiment and as shown in path P 1 , an electric current in the forward direction flows through the diode  11 . With this, the negative surge is discharged, and the protection circuit  10   b  protects the internal circuit  20 . 
     Next, the flow of the electric current in the third embodiment when a positive surge is applied to the input-output terminal T 2  with the ground terminal T 3  as reference is described. In this case, because the breakdown voltage of the diode  11  is higher than the breakdown voltage of the diode  13 , the diode  11  does not apply a reverse current. Therefore, in the same way as the first embodiment and as shown in path P 2 , an electric current in the forward direction flows through the diode  12   a , and an electric current in the reverse direction flows through the diode  13 . With this, the positive surge is discharged, and the protection circuit  10   b  protects the internal circuit  20 . 
     Next, the flow of the electric current in the third embodiment when a negative surge is applied to the input-output terminal T 2  with the power supply terminal T 1  as the reference is described. In this case, because the breakdown voltage of the diode  12   a  is higher than the breakdown voltage of the diode  13 , a reverse current does not flow through the diode  12   a . Therefore, in the same way as the second embodiment and as shown in path P 3   a , an electric current in the reverse direction flows through the diode  13 , and an electric current in the forward direction flows through the diode  11 . With this, the negative surge is discharged, and the protection circuit  10   b  protects the internal circuit  20 . 
     Next, the flow of the electric current in the third embodiment when a positive surge is applied to the input-output terminal T 2  with the power supply terminal T 1  as the reference is described. In this case, in the same way as the first embodiment and as shown in path P 4 , an electric current in the forward direction flows through the diode  12   a . With this, the positive surge is discharged, and the protection circuit  10   b  protects the internal circuit  20 . 
     Next, the flow of the electric current in the third embodiment when a negative surge is applied to the power supply terminal T 1  with the ground terminal T 3  as the reference is described. In this case, in the same way as the first embodiment and as shown in path P 5 , an electric current in the forward direction flows through the diode  13 . With this, the negative surge is discharged, and the protection circuit  10   b  protects the internal circuit  20 . 
     Next, the flow of the electric current in the third embodiment when a positive surge is applied to the power supply terminal T 1  with the ground terminal T 3  as the reference is described. In this case, in the same way as the first embodiment and as shown in path P 6 , an electric current in the reverse direction flows through the diode  13 . With this, the positive surge is discharged, and the protection circuit  10   b  protects the internal circuit  20 . 
     Thus, the third embodiment can discharge surges that have patterns based on the relationships among the breakdown voltages. Furthermore, the element size of the diodes  11  and  12   a  can be made smaller than the element size of the diode  13 . That is, the third embodiment can protect the internal circuit  20  with a reduced circuit area. 
     Fourth Embodiment 
     Next, the semiconductor device according to the fourth embodiment is described with reference to  FIG. 4 .  FIG. 4  is a circuit diagram of the semiconductor device according to the fourth embodiment. The protection circuit  10   c  according to the fourth embodiment, as shown in  FIG. 4 , includes four diodes  11  that are series-connected and this is the only difference between the fourth embodiment and the first embodiment. 
     With the composition mentioned above, even if the breakdown voltage of one diode  11  is low, the breakdown voltage of the whole series can be made higher by series-connecting multiple diodes  11 . For this reason, setting the breakdown voltage becomes easier. 
     Fifth Embodiment 
     Next, the semiconductor device according to the fifth embodiment is described with reference to  FIG. 5 .  FIG. 5  is a circuit diagram of the semiconductor device according to the fifth embodiment. The protection circuit  10 d according to the fifth embodiment, as shown in  FIG. 5 , includes multiple diodes  12   a  that are series-connected and this is the only difference between the fifth embodiment and the second embodiment. 
     With the composition mentioned above, even if the breakdown voltage of one diode  12   a  is low, the breakdown voltage of the whole series can be made higher by series-connecting multiple diodes  12   a.    
     Sixth Embodiment 
     Next, the semiconductor device according to the sixth embodiment is described with reference to  FIG. 6 .  FIG. 6  is a circuit diagram of the semiconductor device according to the sixth embodiment. The protection circuit  10 e according to the sixth embodiment, as shown in  FIG. 6 , includes the multiple diodes  11  and  12   a  that are series-connected and this is the only difference between the sixth embodiment and the third embodiment. 
     Because the composition of the sixth embodiment is a combination of the fourth embodiment and the fifth embodiment, a detailed description is omitted. 
     While certain embodiments have been described, these embodiments have been presented by way of example only and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.