# Patent

## Title
Switching circuit including nitride semiconductor devices

## Abstract

A switching circuit includes a switching device including the first and second main electrodes and a control electrode; and a driver including: a first rectifying device having an anode terminal connected to the first main electrode of the switching device; a first driving device having a first main electrode connected to a cathode terminal of the first rectifying device and a second main electrode connected to the control electrode of the switching device; a second driving device having a first main electrode connected to the control electrode of the switching device and a second main electrode connected to the second main electrode of the switching device; and input terminals receiving control signals inputted to a control electrode of the first driving device and a control electrode of the second driving device.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE
This application is based upon and claims the benefit of priority from prior Japanese Patent Application P2011-082650 filed on Apr. 4, 2011; the entire contents of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching circuit including a switching device having a nitride semiconductor layer.
2. Description of the Related Art
For example, high-frequency devices, high voltage power devices, and the like include electric field effect transistors each including a nitride semiconductor layer having a layered structure of gallium nitride (GaN) and aluminum gallium nitride (AlGaN) or the like (hereinafter, referred to as nitride FETs). Various types of integrated circuits have been proposed using nitride FETs each including a gate electrode which is provided on a nitride semiconductor layer and forms a Schottky barrier junction (hereinafter, referred to as a nitride-based Schottky gate FETs), nitride-based FETs having an MIS structure in which a gate electrode is provided on the nitride semiconductor layer with an insulating film interposed therebetween (hereinafter, referred to as a nitride-based MIS gate FETs), and the like.
In order to draw out the excellent operation characteristics of a nitride-based FET, it is necessary to sufficiently examine the configuration of a driver to drive the nitride-based FET. For example, nitride-based Schottky gate FETs have a problem of an increase in power consumption due to gate current flowing while the FETs are on. Nitride-based MIS gate nitride FETs have large gate capacities and require large gate current for high-speed operation. Accordingly, drivers capable of solving such problems are desired.
SUMMARY OF THE INVENTION
An aspect of the present invention is a switching circuit comprising a switching device and a driver. The switching circuit includes first and second main electrodes provided with a distance from each other on a principal surface of a nitride semiconductor layer; and a control electrode provided between the first and second main electrodes on the principal surface. The driver includes a first rectifying device having an anode terminal connected to the first main electrode of the switching device; a first driving device having a first main electrode connected to a cathode terminal of the first rectifying device and a second main electrode connected to the control electrode of the switching device; a second driving device having a first main electrode connected to the control electrode of the switching device and a second main electrode connected to the second main electrode of the switching device; and input terminals receiving control signals inputted to a control electrode of the first driving device and a control electrode of the second driving device.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic circuit diagram showing the configuration of a switching circuit according to a first embodiment of the present invention.
FIG. 2 is a schematic view showing an example of the structure of a switching device used in the switching circuit according to the first embodiment of the present invention.
FIG. 3 is a timing diagram for explaining the operation of the switching circuit according to the first embodiment of the present invention.
FIG. 4 is a schematic circuit diagram showing the configuration of a switching circuit according to a first modification of the first embodiment of the present invention.
FIG. 5 is a schematic circuit diagram showing the configuration of a switching circuit according to a second modification of the first embodiment of the present invention.
FIG. 6 is a schematic circuit diagram showing the configuration of a switching circuit according to a second embodiment of the present invention.
FIG. 7 is a schematic circuit diagram showing the configuration of a switching circuit according to a modification of the second embodiment of the present invention.
FIG. 8 is a schematic circuit diagram showing the configuration of a switching circuit according to a third embodiment of the present invention.
FIG. 9 is a schematic circuit diagram showing the configuration of a switching circuit according to a modification of the third embodiment of the present invention.
FIG. 10 is a schematic circuit diagram showing the configuration of a switching circuit according to a fourth embodiment of the present invention.
FIG. 11 is a schematic circuit diagram showing the configuration of a switching circuit according to a fifth embodiment of the present invention.
FIG. 12 is a schematic circuit diagram showing the configuration of a switching circuit according to a modification of the fifth embodiment of the present invention.
FIG. 13 is a schematic circuit diagram showing the configuration of a switching circuit according to another modification of the fifth embodiment of the present invention.
FIG. 14 is a schematic circuit diagram showing the configuration of a switching circuit according to still another modification of the fifth embodiment of the present invention.
FIG. 15 is a schematic circuit diagram showing the configuration of a switching circuit according to a sixth embodiment of the present invention.
FIG. 16 is a schematic circuit diagram showing the configuration of a switching circuit according to another embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified.
In the following descriptions, numerous specific details are set forth such as specific signal values, etc., to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail.
(First Embodiment)
A switching circuit 1 according to a first embodiment of the present invention includes a switching device TSW and a driver 10 driving the switching device TSW. The switching device TSW includes a drain electrode DSW, a source electrode SSW, and a gate electrode GSW. The drain and source electrodes DSW and SSW are placed with a distance from each other on a principal surface of a nitride semiconductor layer. The gate electrode GSW is placed between the drain and source electrodes DSW and SSW on the principal surface of the nitride semiconductor layer. The drain and source electrodes DSW and SSW of the switching device TSW are connected to a drain terminal D and a source terminal S, respectively.
The switching device TSW is a nitride-based FET including the nitride semiconductor layer having a layered structure of a GaN layer and an AlGaN layer, for example. FIG. 2 shows a structure example of the switching device TSW. In the example of FIG. 2, the drain, source, and gate electrodes DSW, SSW, and GSW are arranged on a flat principal surface of a nitride semiconductor layer 22 provided on a substrate 21. The principal surface of the nitride semiconductor layer 22 may have a rough profile with recesses formed.
The driver 10 includes a first rectifying device D1 and first and second driving devices TD1 and TD2. As shown in FIG. 1, an anode terminal of the first rectifying device D1 is connected to the drain electrode DSW of the switching device TSW, and a cathode terminal of the first rectifying device D1 is connected to a drain electrode DD1, of the first driving device TD1. A source electrode SD1 of the first driving device TD1 is connected to the gate electrode GSW of the switching device TSW. A drain electrode DD2 of the second driving device TD2 is connected to the gate electrode GSW of the switching device TSW, and a source electrode SD2 thereof is connected to the source electrode SSW of the switching device TSW.
A gate electrode GD1 of the first driving device TD1 is connected to an input terminal IN_H, and a gate electrode GD2 of the second driving element TD2 is connected to an input terminal IN_L. The input terminals IN_H and IN_L receive control signals to control on/off of the switching device TSW. Between the input terminal IN_H and the gate electrode GSW of the switching device TSW, a gate resistor R1 is connected.
In the switching circuit 1, a control signal to control on/off of the first driving device TD1 is inputted to the gate electrode GD1 of the first driving device TD1 through the input terminal IN_H. A control signal to control on/off of the second driving device TD2 is inputted to the gate electrode GD2 of the second driving device TD2 through the input terminal IN_L. The conducting states of the first and second devices TD1 and TD2 are set by the control signals inputted to the driver 10 in order to control on/off of the switching device TSW. Moreover, the first rectifying device D1 prevents current from flowing from the gate electrode GD1 of the first driving element TD1 to the drain terminal D. Similarly, the first rectifying device D1 prevents current from flowing from the source electrode SD1 of the first driving device TD1 to the drain terminal D.
Preferably, the first and second driving devices TD1 and TD2 are nitride-based FETs similarly to the switching device TSW. Employing nitride-based FETs operable at high speed as the first and second driving devices TD1 and TD2 allows for high-speed operation of the entire switching circuit 1. For example, it is effective that the switching device TSW and driver 10 are formed in a same semiconductor chip. The first rectifying device D1 can be also formed in the semiconductor chip as a diode including a nitride semiconductor layer. For example, as the first rectifying device D1, a Schottky barrier diode can be formed by using a nickel (Ni) film, and a diode can be formed by short-circuiting the gate and the source of the FET structure shown in FIG. 2.
The first and second driving devices TD1 and TD2 need to have driving capabilities at least high enough to control the operation of the switching device TSW. Accordingly, the sizes of the first and second driving devices TD1 and TD2 may be smaller than that of the switching device TSW.
With reference to the timing diagram of FIG. 3, the operation of the switching circuit 1 is described below. In FIG. 3, input voltage V(IN_H) refers to a voltage level of a signal inputted to the input terminal IN_H, and input voltage V(IN_L) refers to a voltage level of a signal inputted to the input terminal IN_L. Drain voltage V(D) refers to voltage of the drain terminal D, and drain current I(D) refers to current flowing through the drain terminal D. Gate current IG(TSW) refers to gate current of the switching device TSW. Rectifying current I(D1) and gate resistor current I(R1) refer to current flowing through the first rectifying device D1 and current flowing through the gate resistor R1, respectively.
If high and low signals are inputted into the input terminals IN_H and IN_L at time t1, respectively, the first driving device TD1 is turned on while the second driving device TD2 is turned off. The gate current IG(TSW) of the gate current IGSW large enough to turn on the switching device TSW therefore flows to the gate electrode GSW of the switching device TSW through the first rectifying device D1 and the first driving device TD1. The switching device TSW is then turned on. At this time, the gate resistor current I(R1) also flows to the gate electrode GSW of the switching element TSW.
After the switching device TSW is turned on, the drain voltage V(D) gradually drops. The first driving device TD1 is therefore turned off at time t2, thus stopping the current flowing through the first rectifying device D1. The gate current IG(TSW) is therefore rapidly reduced. Accordingly, the gate current IG(TSW) having a gate current value IGON which is large enough to keep the switching device TSW on flows to the gate electrode GSW of the switching device TSW through the gate resistor R1.
If low and high signals are inputted to the input terminals IN_H and IN_L at time t3, respectively, the second driving device TD2 is turned on while the first driving device TD1 remains off. The switching device TSW is therefore turned off.
As described above, in the switching circuit 1 shown in FIG. 1, large current is allowed to flow from the first driving device TD1 to the gate electrode GSW of the switching device TSW only at the start of switching operation requiring gate charge for turning on the switching device TSW. After the first driving device TD1 becomes the non-conducting state, the minimum gate current needed to keep the switching device TSW on is supplied to the gate electrode GSW of the switching device TSW through the gate resistor R1. According to the switching circuit 1, it is possible to efficiently drive the switching device TSW at high speed operation while preventing an increase in power consumption.
The resistance value of the gate resistor R1 is set in the light of the size of the switching device TSW, the voltage level of the high signal inputted to the input terminal IN_H, and the like so that the gate current IG(TSW) has a current value needed to keep the on state of the switching device TSW.
When the switching device TSW is a nitride-based Schottky gate FET, the switching device TSW has a device structure similar to a junction field effect transistor (JFET) in which gate current flows to a gate electrode. In order to cause the switching device TSW to operate at high speed, therefore, it is necessary to apply high voltage to the gate electrode GSW. Implementation of high speed operation increases the gate current. On the other hand, if the gate current to be supplied is reduced, the switching operation slows down.
Accordingly, the switching circuit 1, which requires large current only during the switching operation of the switching device TSW and is supplied with minimum current needed to keep the on state of the switching device TSW while the switching device TSW is on, is an ideal switching circuit.
Moreover, the nitride-based FETs are capable of operating at higher speed than devices by silicon (Si) process technology such as bipolar transistors, MOSFETs, and insulated gate bipolar transistors (IGBTs). For maximizing the performance thereof, it is preferable that the gate resistance inserted in the driver for preventing oscillation be set to 0 or minimized. It is therefore necessary to reduce parasitic inductance of the gate electrode which causes the oscillation. Furthermore, threshold voltage Vth of nitride-based FETs is lower than that of silicon-based MOSFETs. It is therefore preferable that the parasitic inductance be reduced also for the purpose of preventing malfunction.
For the reduction of the parasitic inductance, the switching circuit 1, which allows the switching device TSW and the driver 10 to be easily integrated into a single chip, is effective. The driver 10 of the switching circuit 1 can be composed of the first and second driving devices TD1 and TD2 of nitride-based FETs and the first rectifying device D1 formed using the nitride semiconductor layer, and the gate resistor R1. Accordingly, the driver 10 can be formed by the process to manufacture the nitride semiconductor device forming the switching device TSW without another additional process. Furthermore, a general nitride-based FET has a horizontal FET structure with a chip's back side set to the source potential. The switching device TSW and the driver 10 can be therefore easily formed in a same semiconductor chip.
By integrating the switching device TSW and driver 10 into a single chip, wire can be eliminated from the gate wiring, and the circuit can be composed of only metal wiring. This can reduce the parasitic inductance. Accordingly, the gate resistance R1 can be reduced without causing oscillation, thus allowing for high speed operation of the switching device TSW. Moreover, since the driver 10 is composed of a nitride-based FET, the high speed operation performance of the switching device TSW can be sufficiently exerted.
Furthermore, since the switching circuit 1 is formed in a single chip, the switching device TSW and the first driving device TD1 can share the drain terminal. Accordingly, compared with the case where the switching device TSW and the first driving device TD1 are provided with different drain terminals, the number of pins of a package of the switching circuit 1 can be reduced. Moreover, sharing the pad of the drain terminal by the switching device TSW and the first driving device TD1 has effects on reduction of the chip area, the number of wires, and the package size.
Moreover, short-circuiting the source terminals of the switching device TSW and the second driving device TD2 stabilizes the gate voltage of the switching device TSW, thus stabilizing the switching operation of the switching circuit 1.
Furthermore, since the gate terminal of the switching device TSW and the drain terminal of the second driving device TD2 are shorted, the switching device TSW allows current to flow from the source terminal to the drain terminal when the second driving device TD2 is on. This eliminates the need for a regenerative diode.
As described above, according to the switching circuit 1 according to the first embodiment of the present invention, the switching device TSW is supplied with the amount of gate current needed by the switching operation only at the start of the switching operation and is supplied with the minimum gate current needed to keep the on state after the switching operation. According to the switching circuit 1, it is therefore possible to provide a switching circuit capable of operating at high speed with reduced gate current.
<First Modification>
FIG. 4 shows a switching circuit 1 according to a first modification of the first embodiment of the present invention. The switching circuit 1 shown in FIG. 4 further includes a second rectifying device D2 with the cathode terminal connected to the anode terminal of the first rectifying device D1 and the anode terminal connected to the cathode terminal of the first rectifying device D1.
While the switching device TSW is on, voltage across the input terminal IN_H and the drain terminal D is equal to the sum of forward voltage across the gate electrode GD1 and the drain electrode DD1 of the first driving device TD1 and the forward voltage of the second rectifying device D2, that is, double the forward voltage Vf. On the other hand, voltage across the input terminal IN_H and the source terminal S is equal to the sum of forward voltage across the gate electrode GD1 and the source electrode SD1 of the first driving device TD1 and the forward voltage across the gate electrode GSW and the source electrode SSW of the switching device TSW, that is, double the forward voltage Vf.
Because of a voltage drop between the drain terminal ID and source terminal S, the voltage across the input terminal IN_H and the source terminal S is slightly higher than the voltage across the input terminal IN_H and the drain terminal D. Accordingly, the amount of gate current needed to keep the on state of the switching device TSW is supplied to the gate electrode GSW. On the other hand, current unnecessary to keep the on state of the switching device TSW flows from the input terminal IN_H to the drain terminal D through the second rectifying device D2. According to the switching circuit 1 shown in FIG. 4, the minimum gate current needed to keep the on state of the switching device TSW is supplied to the switching device TSW, and the voltage across the gate electrode GSW and the source electrode SSW of the switching device TSW is optimized.
<Second Modification>
FIG. 5 shows a switching circuit 1 according to a second modification of the first embodiment of the present invention. The switching circuit 1 shown in FIG. 5 further includes a protection rectifying device DSD with the anode terminal connected to the source electrode SD1 of the first driving device TD1 and the cathode terminal connected to the gate electrode GSW of the switching device TSW.
The protection rectifying device DGD prevents the first rectifying device D1 and the first driving device TD1 from being destroyed by excessive current. According to the switching circuit 1 shown in FIG. 5, the first rectifying device D1 and the first driving device TD1 are protected from breakdown by current.
(Second Embodiment)
As shown in FIG. 6, a switching circuit 1 according to a second embodiment of the present invention further includes an inverter circuit 11 outputting to the gate electrode GD1 of the first driving device TD1 a signal having a phase opposite to a signal inputted to the gate electrode GD2 of the second driving device TD2.
The switching circuit 1 according to the second embodiment differs from that shown in FIG. 1 in that the gate electrode GD1 of the first driving device TD1 is connected to the output of the inverter circuit 11 instead of the input terminal IN_H, and the input of the inverter circuit 11 and the gate electrode GD2 of the second driving device TD2 are connected to the input terminal IN. The other configuration of the second embodiment is the same as that of the first embodiment.
As shown in FIG. 6, the inverter circuit 11 includes a resistor RINV and a transistor TINV. The transistor TINV can be composed of a nitride-based FET. One terminal of the resistor RINV is connected to a power supply terminal POS, and the other terminal thereof is connected to the drain electrode of the transistor TINV. The gate electrode of the transistor TINV is connected to the input terminal IN, and the drain electrode of the transistor TINV is connected to the gate electrode GD1 of the first driving device TD1. The source electrode of the transistor TINV is connected to the source terminal S. The power supply terminal POS is supplied with power supply voltage high enough to operate the inverter circuit 11. Accordingly, the input terminal of the inverter circuit 11 is the gate electrode of the transistor TINV, and the output terminal of the inverter circuit 11 is the drain electrode of the transistor TINV.
If a low signal is inputted to the input terminal IN, the second driving device TD2 is turned off. On the other hand, the low signal inputted into the input terminal IN is inverted to a high signal, which is then inputted to the gate electrode GD1 of the first driving device TD1. The switching device TSW is therefore turned on. Similar to the operation described with reference to FIG. 3, when a high signal is inputted to the input terminal IN after the first driving device TD1 is turned off to reduce the gate current IG(TSW), the second driving device TD2 is turned on, and the switching device TSW is turned off.
According to the switching circuit 1 shown in FIG. 6, controlling on/off of the switching device TSW requires only one terminal to input the control signal. Accordingly, the control of the switching device TSW is easier than the switching circuit 1 shown in FIG. 1.
In the switching circuit 1, flow-through current flowing through the first and second driving devices TD1 and TD2 does not flow when the switching device TSW changes from on to off or from off to on. Accordingly, even if the inverter circuit 11 is employed, it is not necessary to set dead time for preventing flow-through current.
According to the switching circuit 1 according to the second embodiment, it is possible to provide a switching circuit which allows for easy input of the control signal and is capable of operating at high speed with the gate current reduced. The other is substantially the same as that of the first embodiment, and the overlapping description is omitted.
<Modification>
FIG. 7 shows a switching circuit 1 according to a modification of the second embodiment of the present invention. In the switching circuit 1 shown in FIG. 7, the inverter circuit 11 composed of the resistor RINV and transistor TINV outputs to the gate electrode GD2 of the second driving device TD2, a signal having a phase opposite to the signal inputted to the gate electrode GD1 of the first driving device TD1. Therefore, the input terminal IN is connected to the gate electrode GD1 of the first driving device TD1 and the input of the inverter 11, and the output of the inverter 11 is connected to the gate electrode GD2 of the second driving device TD2. In other words, the gate electrode of the transistor TINV is connected to the input terminal IN, and the drain electrode of the transistor TINV is connected to the gate electrode GD2 of the second driving device TD2.
If a high signal is inputted to the input terminal IN, the first driving device TD1 is turned on. On the other hand, the high signal inputted to the input terminal IN is inverted to a low signal, which is then inputted to the gate electrode GD2 of the second driving device TD2. The switching device TSW is therefore turned on. Similar to the operation described with reference to FIG. 3, when a low signal is inputted to the input terminal IN after the first driving device TD1 is turned off to reduce the gate current IG(TSW), the second driving device TD2 is turned on, and the switching device TSW is turned off.
In the switching circuit 1 shown in FIG. 7, at the change of the switching device TSW from off to on, the first driving device TD1 can be turned on earlier than the second driving device TD2 is turned off, thus causing flow-through current to flow through the first and second driving devices TD1 and TD2. However, the switching device TSW is turned on immediately after the first driving device TD1 is turned on. The drain voltage V(D) therefore drops to turn off the first driving device TD1. Accordingly, the flow-through current cannot become large enough to destroy the first and second driving devices TD1 and TD2.
(Third Embodiment)
As shown in FIG. 8, a switching circuit 1 according to the third embodiment of the present invention differs from the second embodiment shown in FIG. 6 in that the drain terminal ID serves as the power supply terminal POS. In the switching circuit 1 shown in FIG. 8, a transistor TPOS is provided between the drain terminal D and the resistor RINV of the inverter circuit 11. The drain electrode of the transistor TPOS is connected to the drain terminal D, and the source electrode thereof is connected to the resistor RINV. The gate electrode of the transistor TPOS is connected to the source terminal S. The other configuration is the same as that of the switching circuit 1 shown in FIG. 6.
The transistor TPOS is composed of a normally-on nitride-based FET. By connecting the gate electrode of the transistor TPOS to the source terminal S, the voltage of the source electrode of the transistor TPOS is set to threshold voltage Vth, thus supplying power to drive the inverter 11. The transistor TPOS may be replaced with a diode, a resistor, or the like.
FIG. 9 shows a switching circuit 1 in which the drain terminal D serves as the power supply terminal POS in the modification of the second embodiment shown in FIG. 7 similar to the switching circuit 1 shown in FIG. 8.
According to the switching circuit 1 of the third embodiment, compared with the switching circuits 1 shown in FIGS. 6 and 7, the power supply terminal POS can be omitted, thus reducing the number of terminals. The other configuration is substantially the same as those of the first to second embodiments, and the overlapping description is omitted.
(Fourth Embodiment)
As shown in FIG. 10, a switching circuit 1 according to a fourth embodiment of the present invention is obtained by adding a controller 12 to the switching circuit 1 shown in FIG. 9. The controller 12 is a circuit to turn off the switching device TSW even in the case where the second driving device TD2 is not turned on when a low signal is inputted to the input terminal IN in the switching circuit 1 shown in FIG. 9.
The controller 12 includes a diode D12, a capacitor C12, resistors R121 and R122, and transistors T121 and T122. The transistors T121 and 122 can be composed of nitride-based FETs.
The drain electrode of the transistor T121 is connected to the gate electrode GSW of the switching device TSW; the source electrode thereof is connected to the source terminal S; and the gate electrode thereof is connected to the drain electrode of the transistor T122. The drain electrode of the transistor 122 is connected to one terminal of the resistor R121; the source electrode thereof is connected to the source terminal S; the gate electrode thereof is connected to the input terminal IN through the resistor R122. The other terminal of the resistor R121 is connected to the cathode terminal of the diode D12, and the anode terminal of the diode D12 is connected to the input terminal IN. The capacitor C12 is connected to between the cathode terminal of the diode D12 and the source terminal S.
In the switching circuit 1 shown in FIG. 9, when the switching device TSW is on, the drain voltage is 0 V, and the potential of the gate electrode GD2 of the second driving device TD2 is 0 V. When a low signal is inputted to the input terminal IN in order to turn off the switching device TSW, therefore, there is a possibility that no gate current flows to the gate electrode GD2 of the second driving device TD2. Accordingly, the second driving device TD2 is not turned on in some cases.
The controller 12 turns off the switching device TSW even in the case where the second driving device TD2 is not turned on. Hereinafter, the operation of the controller 12 is described.
While a high signal is inputted to the input terminal IN, the transistor T122 is on, and the gate electrode of the transistor t121 is low. If a low signal is inputted to the input terminal IN, the transistor T122 is turned off, and the gate electrode of the transistor T121 becomes high level by electric charges charged in the capacitor C12. The switching device TSW starts to be turned off. When the switching device TSW starts to be turned off, the second driving device TD2 is turned on, and then the switching device TSW is completely turned off.
According to the switching circuit 1 of the fourth embodiment, which includes the inverter circuit 11 outputting to the second driving device TD2 the signal having a phase opposite to the input of the first driving device TD1, the switching circuit 1 in which the drain terminal D serves as the power supply terminal POS can reliably turn off the switching device TSW. The others are substantially the same as those of the first to third embodiments, and overlapping descriptions are omitted.
(Fifth Embodiment)
As shown in FIG. 11, a switching circuit 1 according to a fifth embodiment of the present invention differs from that shown in FIG. 1 in including a diode DSGV as a protection circuit against surge voltage inputted to the drain terminal D. The anode terminal of the diode DSGV for surge protection is connected to the gate electrode GD1 of the first driving device TD1, and the cathode terminal thereof is connected to the drain terminal D.
The diode DSGV can be a diode composed of a nitride semiconductor formed together with the driver 10 in a same semiconductor chip. Alternatively, the diode DSGV may be an external device. For example, the diode DSGV may be a single discrete device or may be provided within a drive IC outputting control signals to the input terminals IN_H and IN_L.
As previously described, the size of the switching device TSW is larger than that of the first driving device TD1. The impedance of the first driving device TD1 is therefore larger than that of the switching device TSW. Accordingly, as shown in FIG. 11, the anode terminal of the diode DSGV is connected to the gate electrode GD1 of the first driving device TD1. The clamping operation is performed to turn on the first driving device TD1 and turn on the second driving device TD2 at breakdown of the diode DSGV even if the current flowing through the diode DSGV is small. The switching device TSW can be thus protected.
FIG. 12 shows a switching circuit 1 in which the diode DSGV for surge protection is connected to the switching circuit 1 shown in FIG. 9. In the switching circuit 1 shown in FIG. 12, similar to the switching circuit 1 shown in FIG. 11, the switching device TSW is protected from surge voltage.
FIG. 13 shows an example in which the diode DSGV shown in FIG. 11 is replaced with a surge protection circuit 13 including a plurality of diodes D131, D132, . . . , and D13n connected in series in the forward direction (n: an integer not less than 2). Nitride semiconductor devices have low avalanche withstand capabilities. Accordingly, if the diode DSGV is composed of a nitride semiconductor device, the diode DSGV could be destroyed at breakdown. In the case where the diodes D131, D132, . . . , D13n are composed of nitride semiconductor devices, the diodes D131, D132, . . . , D13n are connected in series to be prevented from being destroyed at breakdown. The number of diodes connected in series is about 200, for example.
FIG. 14 shows an example in which the diode DSGV in the switching circuit 1 shown in FIG. 12 is replaced with the surge protection circuit 13. In the switching circuit 1 shown in FIG. 14, it is possible to prevent the diodes from being destroyed at breakdown similar to the switching circuit 1 shown in FIG. 13.
According to the switching circuit 1 of the fifth embodiment, the switching device TSW can be prevented from being destroyed by surge voltage. The others are substantially the same as those of the first to fourth embodiments, and the overlapping description is omitted.
(Sixth Embodiment)
As shown in FIG. 15, a switching circuit 1 according to a sixth embodiment of the present invention differs from the switching circuit 1 shown in FIG. 1 in that a driving device turning on the switching device TSW is composed of a multistage transistor circuit. The other configurations are the same as those of the first embodiment shown in FIG. 1. FIG. 15 shows an example in which the multistage transistor circuit includes the first driving device TD1 and the driving device TD11 driving the first driving device TD1. The driving device TD11 can be composed of a nitride-based FET.
The drain electrode of the driving device TD11 is connected to the cathode terminal of the diode D11; the gate electrode thereof is connected to the input terminal IN; and the source electrode thereof is connected to the gate electrode GD1 of the first driving device TD1. The anode terminal of the diode D11 is connected to the drain terminal D. Furthermore, a resistor R11 is connected between the gate electrode GD1 of the first driving device TD1 and the input terminal IN_H.
As previously described, the size of the first driving device TD1 driving the switching device TSW can be made smaller than that of the switching device TSW. Furthermore, driving the first driving device TD1 with the driving device TD11 smaller than the first driving device TD1 can reduce the driving current. FIG. 15 shows an example where the multistage transistor circuit driving the switching device TSW is composed of two transistors, but the multistage transistor circuit may be composed of three or more transistors.
According to the switching circuit 1 of the sixth embodiment, it is possible to provide a switching circuit capable of operating at high speed with the driving current reduced and the gate current reduced. The others are substantially the same as those of the first embodiment, and the overlapping description is omitted.
(Other Embodiments)
In the description of the first to sixth embodiments described above, the gate electrode GSW of the switching device TSW is connected to the gate resistor R1. However, if the nitride-based FET employed in the switching circuit 1 is a Schottky gate FET, for example, current flows from the gate electrode GD1 to the source electrode SD1 in the first driving device TD1 even when the first driving device TD1 is off. Accordingly, when the first driving device TD1 is turned off after the switching device TSW is turned on, the switching device TSW is supplied with gate current through the first driving device TD1 even if the gate electrode GSW of the switching device TSW is not connected to the gate resistor R1. Accordingly, as shown in FIG. 16, the switching circuit 1 not including the gate resistor R1 connected to the gate electrode GSW of the switching device TSW can be implemented.
Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

## Claims

1. A switching circuit comprising:
a switching device including: first and second main electrodes provided with a distance from each other on a principal surface of a nitride semiconductor layer; and a control electrode provided between the first and second main electrodes on the principal surface; and
a driver including:
a first rectifying device having an anode terminal connected to the first main electrode of the switching device;
a first driving device having a first main electrode connected to a cathode terminal of the first rectifying device and a second main electrode connected to the control electrode of the switching device;
a second driving device having a first main electrode connected to the control electrode of the switching device and a second main electrode connected to the second main electrode of the switching device; and
input terminals receiving control signals inputted to a control electrode of the first driving device and a control electrode of the second driving device.

2. The switching circuit according to claim 1, further comprising a second rectifying device having a cathode terminal connected to the anode terminal of the first rectifying device and an anode terminal connected to the cathode terminal of the first rectifying terminal.

3. The switching circuit according to claim 1, further comprising a protection rectifying device including an anode terminal connected to the second main electrode of the first driving device and a cathode terminal connected to the control electrode of the switching device.

4. The switching circuit according to claim 1, further comprising an inverter circuit outputting to the control electrode of the first driving device, a signal having a phase opposite to the signal inputted to the control electrode of the second driving device.

5. The switching circuit according to claim 1, wherein the switching device and the driver are formed in a same semiconductor chip.