Abstract:
A field-effect transistor with protection diodes includes: a field-effect transistor; and a two-terminal electrostatic protection circuit connected between a gate and a source of the field-effect transistor, wherein the two-terminal electrostatic protection circuit comprises: a first diode that is positioned on a reverse-biased side when a voltage lower than a potential of the source is applied to the gate and has a reverse withstand voltage lower than a reverse withstand voltage between the gate and the source of the field-effect transistor; a second diode that is positioned on a forward-biased side when a voltage lower than a potential of the source is applied to the gate and is connected in anti-series to the first diode; and a resistor that is connected in series to a diode pair comprising the first diode and the second diode and formed using a same channel layer as that of the field-effect transistor.

Description:
BACKGROUND OF THE INVENTION 
     Field 
       [0001]    The present invention relates to an improvement of electrostatic discharge (ESD) resistance of a field-effect transistor (FET) used in a microwave band/millimeter wave band. 
       Background 
       [0002]    In recent years, application of GaN-based FET amplifiers featuring high withstand voltage, high output and low thermal resistance to microwave/millimeter wave hands is progressing in the fields of mobile phone base stations, satellite communication amplifiers and radar amplifiers. However, although the GaN-based FETs exhibit sufficiently high two-terminal and three-terminal withstand voltage characteristics compared to conventional GaAs-based FETs, in terms of liability to failures caused by ESD (static electricity discharge), the resistance of the GaN-based FETs cannot be said to he sufficiently high. For example, ESD resistance of GaN-based FETs often has been proven to be on the order of 250 to 500 V according to human model (HBM) tests. In assembly of semiconductor devices and in actual use after mounting on printed circuit boards, minimizing ESD malfunction generally requires resistance of 1 kV or higher in HBM tests. In not only GaN-based FETs but also conventionally used GaAs-based FETs, insufficient ESD resistance (resistance to ESD-caused malfunction) often has been questioned in actual use. 
       CITATION LIST 
       [0000]    
       
         Patent Literature 1: JP 2001-332567 A 
         Patent Literature 2: JP 59-100579 A 
         Non Patent Literature 1: 2006 IEEE International Microwave Symp., pp. 647-650, “A Nonlinear Drain Resistance Model for a High Power Millimeter-wave PHEMT” 
       
     
       SUMMARY 
       [0006]    As described above, against the technical background, several solutions have been disclosed so far. Patent Literature 1 and Patent Literature 2 describe an ESD protective circuit using a Schottky junction diode between a gate terminal and a source terminal of a GaAs FET. 
         [0007]    For example, according to a configuration in  FIG. 11A  described in Patent Literature 1, when an excessive gate voltage is applied to a gate-source Schottky junction in a positive direction, it is possible to prevent malfunction caused by an overcurrent flowing through diodes Da 1  and Da 2 , and an overcurrent flowing through the gate-source Schottky junction of an FET F 1 . When a negative surge is applied to the gate, a surge current passes through diodes Db 1  and Db 2 , and it is thereby possible to protect the gate-source Schottky junction of the FET F 1 . However, such a configuration poses a problem that the gain is reduced when an FET amplifier is constructed of a capacitance of the junction of the diodes connected in anti-parallel. 
         [0008]    In the case of a GaAs-based or GaN-based FET, since a depression type (normally on type) is generally used, the depression type is used by applying a negative gate bias thereto. For example, in the case of a GaN FET, its threshold voltage is as deep as −2 V to −4 V and its gate bias voltage during amplification operation is also as deep as −1.5 V to −3 V. For this reason, to prevent current from flowing through the protective circuit: even during RF signal amplification operation, if a GaN gate-source Schottky barrier potential is assumed to be approximately 1 V, a longitudinal stack of at least five stages is required as shown by reverse diodes Db 1  to Db 5  in  FIG. 11B . This involves a problem that the occupancy area of the protective circuit increases and prevents the FET chip from being downsized. 
         [0009]    The configuration in  FIG. 11C  described in Patent Literature 2 provides ESD protection Schottky junction diodes connected in anti-series, thereby eliminates the need for the multi-stage connection as shown in  FIG. 11B , and the Db 1  is reversely biased when a negative bias voltage is applied thereto, and it is thereby possible to drastically reduce increases in capacitances by the Da 1  and Db 1 . However, the reverse current capacity of a Schottky junction diode is generally extremely small compared to the reverse current capacity of a pn junction diode, and such a configuration poses a problem that a reverse overcurrent, when a negative surge is applied, causes the protection diode itself to malfunction. 
         [0010]    It is an object: of the present invention to provide a small FET with protection diodes having a function of suppressing malfunction (burning) of the protection diodes connected in anti-series within a preset negative gate surge current range, and capable of suppressing a gain reduction during amplification operation. 
         [0011]    According to the present invention, a field-effect transistor with protection diodes includes: a field-effect transistor; and a two-terminal electrostatic protection circuit connected between a gate and a source of the field-effect transistor, wherein the two-terminal electrostatic protection circuit comprises: a first diode that is positioned on a reverse-biased side when a voltage lower than a potential of the source is applied to the gate and has a reverse withstand voltage lower than a reverse withstand voltage between the gate and the source of the field-effect transistor; a second diode that is positioned on a forward-biased side when a voltage lower than a potential of the source is applied to the gate and is connected in anti-series to the first diode; and a resistor that is connected in series to a diode pair comprising the first diode and the second diode and formed using a same channel layer as that of the field-effect transistor. 
         [0012]    The FET with protection diodes according to the present invention can prevent the diodes inside the protective circuit from malfunctioning due to an overcurrent by a current saturation characteristic of a resistor provided in the protective circuit and also allows surge resistance between the gate and the source of the FET to be used to pass a gate surge current. As a result, the present FET has an effect of being able to reduce the size (junction area) of the diodes in the protective circuit. Moreover, since a pair of diodes connected in anti-series is used for the protective circuit, the junction capacitance added between the gate and the source is small, and it is thereby possible to prevent a gain reduction when an FET amplifier is composed. 
         [0013]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is an example of a circuit diagram of an FET with protection diodes according to a first embodiment of the present invention. 
           [0015]      FIG. 2A  is a cross-sectional structure diagram of the FET, the diode and the channel resistor of the circuit in  FIG. 1 . 
           [0016]      FIG. 2B  is a layout of the circuit in  FIG. 1 . 
           [0017]      FIG. 3A  illustrates an example of a forward current-voltage characteristic (Fw), and an example of a reverse current-voltage characteristic (Rv) of the Schottky junction diode between the gate and source of the FET. 
           [0018]      FIG. 3B  illustrates an example of a current-voltage characteristic of the channel resistor. 
           [0019]      FIG. 3C  illustrates an example of a reverse current-voltage characteristic of the diodes formed of a Schottky junction. 
           [0020]      FIG. 3D  illustrates an example of a current-voltage characteristic when a negative surge is applied to the gate of the FET with a protective circuit in  FIG. 1 . 
           [0021]      FIG. 3E  illustrates a comparative example of a current-voltage characteristic when a negative surge is applied to the gate of the FET with a protective circuit in  FIG. 1  without the resistor. 
           [0022]      FIG. 4  is an example of a protective circuit according to a second embodiment of the present invention. 
           [0023]      FIG. 5  is an example of a protective circuit according to a third embodiment of the present invention, 
           [0024]      FIG. 6  is an example of a protective circuit according to a fourth embodiment of the present invention. 
           [0025]      FIG. 7A  is a layout diagram of a protection diode according to a fifth embodiment of the present invention. 
           [0026]      FIG. 7B  is a cross-sectional view of a protection diode according to a fifth embodiment of the present invention. 
           [0027]      FIG. 7C  is a layout diagram of a conventional diode. 
           [0028]      FIG. 7D  is a cross-sectional view of a conventional diode. 
           [0029]      FIG. 8A  is a top view of a protection diode according to a sixth embodiment of the present invention. 
           [0030]      FIG. 8B  is a cross-sectional view of a protection diode according to a sixth embodiment of the present invention. 
           [0031]      FIG. 8C  is a top view of an amplification FET according to a sixth embodiment of the present invention. 
           [0032]      FIG. 8D  is a cross-sectional view of an amplification FET according to a sixth embodiment of the present invention. 
           [0033]      FIG. 9A  is a layout example of the diode Db 1  in  FIG. 1 . 
           [0034]      FIG. 9B  is a layout example of the diode Da 1  in  FIG. 1 . 
           [0035]      FIG. 10  is an example of a circuit diagram of an FET with protection diodes according to an eighth embodiment of the present invention. 
           [0036]      FIGS. 11A, 11B, and 11C  are examples of circuit diagrams of conventional FETs with protection diodes connected between gate terminals and source terminals. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]    A FET with protection diodes according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted. 
       First Embodiment 
     (Description of Configuration) 
       [0038]      FIG. 1  is an example of a circuit diagram of an FET with protection diodes according to a first embodiment of the present invention. In  FIG. 1 , a forward diode Da 1  having a polarity the same direction as that of a Schottky junction diode between a gate and a source of an FET F 1  and a reverse diode Db 1  having a polarity in the reverse direction are connected in anti-series to each other. Furthermore, a resistor Ra is connected in series to the pair of diodes (Da 1  and Db 1 ) connected in anti-series to each other. An ESD protective circuit is constructed of the Da 1 , the Db 1  and the Ra, and is connected in parallel between the gate and the source of the FET F 1 . Here, it is a feature of the present invention that the resistor Ra is a channel resistor formed of the same layer as that of a channel of the FET F 1  or a resistor having a property equivalent thereto (current saturation characteristic). Note that the current saturation characteristic refers to a characteristic in which when a voltage exceeding a certain voltage value is applied to both ends of a resistor, the current exhibits not an ohmic characteristic but a substantially fixed current value as shown in  FIG. 3B . 
         [0039]      FIG. 2A  is a cross-sectional structure diagram of the FET, the diode Db 1  and the channel resistor Ra of the circuit in  FIG. 1 ,  FIG. 2B  is a layout of the circuit in  FIG. 1 . The cross-sectional structure diagram in  FIG. 2A  is a cross-sectional structure diagram along a plane X-X in  FIG. 2B . Since the cross-sectional structure diagram of the diode Da 1  is the same as that of the diode Db 1 , it is not shown in  FIG. 2A , shown in  FIG. 2A , a buffer layer  13  is provided on a semiconductor substrate  14  such as SiC or Si, and a GaN layer  12  and an AlGaN layer  11  are provided thereon. An FET region  51 , a diode (Db 1 ) region  53  and a resistor region  52  are separated from each other by a separation region  61  formed using ion implantation or the like. 
         [0040]    A drain electrode  21 , a gate electrode  22  and a source electrode  23  are on AlGaN in the FET region  51 . Resistor electrodes  24  and  25  are on AlGaN of the resistor region  52 . To improve a withstand voltage, a structure may also be provided in which both ends of the gate electrode  22  are “stranded on” an insulating film  41 . An insulating film  42  is a protective film that covers the electrode, a drain lead wire  31  which is a first layer wire is connected to the drain electrode  21  via a contact hole  44  on the insulating film  42 , a gate lead wire  32  which is the first layer wire is connected to the gate electrode  22  via contact hole  43  on the insulating film  42 , and a source lead wire  33  which is the first layer wire is connected to the source electrode  23  via a contact hole  44  on the insulating film  42 . 
         [0041]    Similarly, the resistor electrodes  24  and  25  are connected to first layer wires  34  and  35  via the contact hole  44  on the insulating film  42 . The resistor Ra which is the feature of the present invention is for med of a channel made up of the AlGaN layer  11  and the GaN layer  12  as in the case of the FET section to obtain the saturation current characteristic. Note that the saturation current characteristic which is the feature of the present invention can be obtained also by performing ion implantation with Si or the like on the channel section of the resistor appropriately and adjusting the sheet resistance value. 
         [0042]    (Description of Characteristics of Diode and Resistor) 
         [0043]      FIG. 3A  illustrates an example of a forward current-voltage characteristic (Fw), and an example of a reverse current-voltage characteristic (Rv) of the Schottky junction diode between the gate and source of the FET F 1 . In  FIG. 3A , the vertical axis shows an anode-cathode (gate and source) current and the horizontal axis shows an anode-cathode voltage.  FIG. 3B  illustrates an example of a current-voltage characteristic of the channel resistor Ra, the vertical axis showing a current flowing through the resistor and the horizontal axis showing a voltage applied to both ends of the resistor.  FIG. 3C  illustrates an example of a reverse current-voltage characteristic of the diodes Da 1  and Db 1  formed of a Schottky junction, the vertical axis showing a current and the horizontal axis showing a reverse voltage. A difference in the reverse withstand voltage between  FIGS. 3A and 3C  is that reverse withstand voltages of the Da 1  and the Db 1  in  FIG. 3C  are lower than the gate-source withstand voltage of the FET F 1  in  FIG. 3A . In other words, the circuit is manufactured so that the reverse withstand voltages of the Da 1  and the Db 1  are lower than the gate-source withstand voltage of the FET F 1 .  FIG. 3D  illustrates an example of a current-voltage characteristic when a negative surge is applied to the gate of the FET with a protective circuit in  FIG. 1 , the vertical axis showing a current and the horizontal axis showing a voltage.  FIG. 3E  illustrates a comparative example of a current-voltage characteristic when a negative surge is applied to the gate of the FET with a protective circuit in  FIG. 1  without the resistor Ra, the vertical axis showing a current and the horizontal axis showing a voltage, 
         [0044]    Note that in  FIG. 3 , point A 1  is a point at which breakdown starts in a gate-source reverse direction of the FET F 1 , point C is a point at which the breakdown reverse current reaches Io 2 , point E is a point at which the breakdown reverse current reaches Io 3  and point B is a point at which a current reaches saturation with respect to the voltage between both ends of the resistor. Point A is a point at which the diodes Da 1  and Db 1  start breakdown in the reverse direction, Io 1  is a reverse current value at that time, point B is a point at which a current that has been snapped back at point A when a negative surge is applied reaches lot in the protective circuit made up of the Da 1 , the Db 1  and the Ra 1  in  FIG. 1 . 
         [0045]    Using this difference in the withstand voltage, the diode Db 1  is subjected to breakdown ahead of the FET F 1  when a negative gate surge is applied. Note that when practicality is taken into consideration, the difference in the withstand voltage is preferably lower by on the order of 5 to 10 V. A feature of the characteristic of the channel resistor formed of the same layer as that of the FET F 1  shown in  FIG. 3B  is that the current reaches saturation when a voltage equal to or higher than a certain voltage is applied thereto. This is similar to a static characteristic of a drain-source voltage with respect to the drain current of the FET. 
       (Description of Operation) 
       [0046]    Next, operation when a negative gate surge (a voltage lower than the source potential is applied to the gate) is applied between the gate and source will be described using the example of the current-voltage characteristic shown in  FIG. 3D . For the surge, a human body model (HBM) which is most widely used as a standard for ESD resistance is assumed. When the surge is applied, the Db 1  is subjected to breakdown at a voltage at point A as shown by a solid line (wR) in  FIG. 3D  and a snapback occurs. After the snap back, the current increases and reaches point B. When the current reaches point B, the current is limited to Io 2  due to the current saturation characteristic of the resistor Ra, and the voltage then increases up to point C. When the voltage increases up to point C, the gate-source reverse junction of the FET F 1  is subjected to breakdown and the current reaches Io 3 . As long as the difference between the currents Io 3  and Io 2  is lower than the current resistance between the gate and the source of the FET F 1 , the circuit in  FIG. 1  can keep the desired surge current Io 3  flowing without destroying the gate-source junction of the FET F 1 . 
         [0047]    As a specific example, a case will be considered where an HBM surge resistance of 1 kV is protected. When the reverse direction surge resistance possessed by the FET F 1  is assumed to be 250 V, the current that is allowed to flow between the gate and the source of the F 1  is 250 V/1.5 kΩ=0.17 A. Since 1 kV/1.5 kΩ=0.67 A, in order for the circuit in  FIG. 1  to possess the resistance of 1 kV, a current of 0.5 A may be made to flow through the ESD protective circuit (Da 1 , Db 1 , Ra). Here, 1.5 kΩ is a resistance value of a test system used to measure the ESD resistance in HBM. 
         [0048]    If a reverse current capacity per unit area of the Schottky junction acquired by an experiment in advance is assumed to be, for example, 0.2 A/1 mm 2 , a junction area necessary to pass 0.5 A is calculated to be 2.5 mm 2 . On the other hand, regarding the resistor, in the case where the saturation current per unit width is 2.5 A/min, a saturation current characteristic of 0.5 A can be achieved using a channel resistor having a width of 0.2 mm. When the circuit is designed in this way, a current Ida 1  that flows through the protective circuit in  FIG. 1  is 0.5 A (=Io 2 ), and a current Isgf 1  that flows from the source toward the gate of the FET F 1  in  FIG. 1  becomes 0.17 A, and a total surge current of Io 3 −0.67 A can flow. As a result, the ESD resistance of 1 kV in HBM can be achieved. 
         [0049]    As a comparative example of this, a current-voltage characteristic when there is no resistor Ra (corresponding to  FIG. 11C ) is shown by a single-dot dashed line (woR) in  FIG. 3E . When an HBM surge of 1 kV is applied to the gate of the FET F 1 , since there is no current limitation by the resistor Ra, the current Io 2  at point B exceeds an allowable reverse current of 0.5 A of the diode Db 1  as the voltage increases as shown by a single-dot dashed line (woR) in  FIG. 3E , and reaches 0.67 A of a current Io 3 . As a result, the diode Db 1  malfunctions. 
       (Effects of First Embodiment) 
       [0050]    As described above, regarding the ESD resistance of the FET with protection diodes according to the first embodiment, the overall allowable reverse current can be increased by an amount corresponding to the allowable gate-source current of the F 1  compared to the conventional case in  FIG. 11C . In other words, when realizing the same ESD allowable surge current as that in  FIG. 11C , it is possible to reduce the junction area of the protection diodes used for ESD by an amount corresponding to the allowable current of the F 1 . Furthermore, when implementing the protection diodes Da 1  and Db 1  using a Schottky junction, the diode and the channel resistor can be manufactured in the same step as the FET producing step, and no additional steps are involved, and it is therefore possible to achieve a cost reduction. Moreover, since an anti-series diode configuration is used, it is possible to reduce influences of parasitic capacitance and suppress a gain reduction of the FET amplifier by protection diode loading compared to the reverse parallel configuration in  FIGS. 11A and 11B . 
         [0051]    An example of a case has been described above where the Da 1  and Db 1  are manufactured using the same Schottky junction as that of the FET F 1 , but it is apparent that even when the Da 1  and Db 1  are formed of pn junction, the junction area of the protection diode can be reduced by an amount corresponding to the allowable current of the F 1 . The present embodiment has been described by taking a GaN FET as an example, but it is apparent that similar effects can also be expected for conventional GaAs FETs. 
       Second Embodiment 
       [0052]      FIG. 4  is an example of a protective circuit according to a second embodiment of the present invention. A difference from  FIG. 1  of the first embodiment is that a diode Db 2  is connected in series to the Db 1  in the same direction. Although the area of the diodes increases by an amount corresponding to the Db 2 , the number of diodes reversely biased when a negative gate bias is applied is incremented by one compared to  FIG. 1  in the first embodiment, and influences of a parasitic capacitance by the protective circuit can be reduced by that amount. Other effects are the same as those of the first embodiment. 
       Third Embodiment 
       [0053]      FIG. 5  is an example of a protective circuit according to a third embodiment of the present invention. A different from the drawing in the first embodiment is that the resistor Ra is replaced by a constant current source Fa in which a gate and a source of an FET are connected together. Since the constant current source Fa has the constant current characteristic shown in  FIG. 3B  as in the case of the channel resistor Ra, effects similar to those of the first embodiment can be expected. 
       Fourth Embodiment 
       [0054]      FIG. 6  is an example of a protective circuit according to a fourth embodiment of the present invention. This is a configuration in which the resistor Ra shown in  FIG. 1  is added to  FIG. 5  of the third embodiment. The addition of the channel resistor Ra can increase an impedance of the protective circuit compared to  FIG. 5 . Here, the current saturation characteristic deriving from the resistor Ra is set to be equivalent to the current saturation characteristic of the constant current source Fa. The impedance of the protective circuit is Sri increased by loading of the resistor Ra, and a gain reduction of the amplifier during amplification operation can be suppressed by an amount corresponding thereto compared to the third embodiment. Other effects are similar to those of the third embodiment. 
       Fifth Embodiment 
     (Description of Configuration) 
       [0055]    The first to fourth embodiments have described aspects of the circuit configuration according to the present invention, and fifth to seventh embodiments will describe embodiments relating to a layout of the diode section which is a feature of the present invention.  FIGS. 7A and 7B  are a layout diagram and a cross-sectional view of a protection diode according to the fifth embodiment of the present invention.  FIGS. 7C and 7D  are a layout diagram and a cross-sectional view of a conventional diode. The circuit configuration of the protective circuit is assumed to correspond to any one of the first, second and fourth embodiments. Schottky junction diodes are normally often manufactured as shown in  FIGS. 7C and 7D  according to a layout of the FET.  FIG. 7  is an example of a GaN FET. As shown in  FIGS. 7C and 7D , the buffer layer  13  is provided on the semiconductor substrate  14  made of SiC or Si, and the GaN layer  12  and the AlGaN layer  11  are provided thereon. 
         [0056]    The drain electrode  21 , the gate electrode  22  and the source electrode  23  are provided on AlGaN inside an active region  10 . To improve a withstand voltage, both ends of the gate electrode  22  are often stranded on the insulating film  41 . The insulating film  42  covers the electrodes, the drain lead wire  31 , which is the first layer wire, is connected to the drain electrode  21  via the contact hole  44  on the insulating film  42 , the gate lead wire  32 , which is the first layer wire, is connected to the gate electrode  22  via the contact hole  43  on the insulating film  42 , and the source lead wire  33 , which is the first layer wire, is connected to the source electrode  23  via the contact hole  44  on the insulating film  42 . The first layer wire  30  is a wire to cause the drain and source lead wires  31  and  33  to have the same potential. 
         [0057]    As is apparent from a comparison of  FIG. 7A  with  FIG. 7C , the gate electrode  22  which is a Schottky junction diode of the present invention (may also be called an “anode electrode” because  FIG. 7  illustrates a diode) is covered with the first layer wire  32  in substantially the same size. The contact hole  43  also has a large opening below the first layer wire. On the other hand, since the conventional Schottky junction diode may be configured into the same shape as the FET as shown in  FIG. 7C , a contact hole is conventionally manufactured only at an end of the gate electrode  22  with an opening that satisfies its minimum opening size, and the length of a thin portion as shown by a width Lgm 2  of the gate electrode  22  that plays an important role as the FET is shorter than a width Lgm 1  in  FIG. 7A . In the case of a microwave band FET, a gate length Lg as shown in  FIGS. 7C and 7D  is smaller than the minimum opening size of the contact hole  43  that normally connects the gate electrode  22  and the first layer wire. For this reason, the gate electrode  22  other than the end portion of the gate electrode  22  is never covered with the gate lead wire  32 . 
         [0058]    In the present invention, since a diode is used as the protection diode, it is necessary to pass a surge current several tens of times to several hundreds of times as large as that in normal operation through the gate electrode in a short time. For this reason, resistance of any parts other than the junction section is preferably minimized so as to pass the current through the Schottky junction as evenly as possible. For that purpose, it is effective to reduce the wiring resistance. This is because the surge current can more easily flow through the entire junction section and the allowable current increases as the evenness improves. From this viewpoint, it is appreciated that the wiring resistance in areas other than the junction section decreases in the layout in  FIG. 7A  according to the present invention rather than in  FIG. 7C . A guideline for the sizes of the gate lead wire  32  and the contact hole  43  is that it is experimentally preferable that the gate electrode  22  should occupy ⅔ or more of the Schottky junction area. It should be additionally noted that, the effect of reducing wiring resistance is experimentally not much available when the gate electrode  22  occupies ⅓ or less of the Schottky junction area. 
       (Description of Effects) 
       [0059]    As described above, the layout example of the protection diode according to the fifth embodiment has an effect of increasing the allowable reverse current of the diode itself when the first, second and fourth embodiments are actually implemented on a GaN chip. Increasing the allowable current has an effect of being able to prevent the junction area for unnecessary protection diodes from increasing and contributing to a reduction of the chip layout of the protective circuit. 
       Sixth Embodiment 
     (Description of Configuration) 
       [0060]      FIG. 8  is a layout example of a diode and an FET for describing a sixth embodiment. As described in the first embodiment, according to the present invention, in order to ensure that the protection diode Db 1  is subjected to reverse breakdown, the reverse withstand voltage of the protection diode is preferably lower by on the order of 5 to 10 V than the gate-source reverse withstand voltage of the amplification FET F 1 . The sixth embodiment is an example of the layout technique of the diode and the FET that realizes the difference in the withstand voltage. As the circuit configuration of the protective circuit, the cases in  FIGS. 1, 4, 5 and 6  according to the first fourth embodiments are assumed. 
         [0061]      FIGS. 8A and 8B , and  FIGS. 8C and 8D  are examples of top views and cross-sectional views of the protection diode and the amplification FET respectively. As shown in  FIGS. 8C and 8D , the layout of the amplification FET intended for high output often has a gate-drain interval Lgd 2  greater than a gate-source interval Lgs 2 . This is because the layout is intended to increase the gate-drain withstand voltage to enable a high voltage operation. On the other hand, in the ease of the diode, as shown in  FIGS. 8A and 8B , a target structure with Lgs 1 =Lgd 1  is preferable where the source resistance and the drain resistance are equalized so that a gate current flows, through the gate electrode, into the source side and the drain side equally. 
         [0062]    An example has been shown in  FIG. 8  where in order to implement a low withstand voltage protection diode, the gate-source electrode interval Lgs 1  is made shorter than the gate-source electrode interval Lgs 2  of the FET F 1  and the withstand voltage of the protection diode Db 1  is thereby made lower than the gate-source reverse withstand voltage of the FET F 1 . For example, when Lgs 1  is experimentally made to be narrower by 0.2 to 0.5 μm than Lgs 2 , a reduction of withstand voltage by 5 to 10 V can be achieved. When a difference between Lgs 1  and Lgs 2  (Lgs 1 −Lgs 2 ) is less than 0.2 μm, the difference from the gate-source reverse withstand voltage is less than 5 V, which means a shortage of margin when seen from the previous standpoint of causing the protection diode Db 1  to undergo breakdown, which cannot be said to be desirable in terms of practicality. On the other hand, when the difference between Lgs 1  and Lgs 2  is 0.5 μm or greater, the difference in the withstand voltage becomes 10 V or greater, which is an excessive margin, but this does not mean that the effects of the present embodiment cannot be obtained. 
       (Description of Effects) 
       [0063]    Since control on the withstand voltages by changing intervals can be implemented using only a mask pattern, such control provides an effect of being able to prevent a cost increase. 
         [0064]    Note that an example has been shown in the present embodiment where a difference in the withstand voltages has been achieved by correcting a mask pattern, but the withstand voltage may be reduced using a method of changing the material of the gate electrode  22  or slightly embedding the gate electrode  22  in the AlGaN layer  11 . It should be additionally noted that in the case of changing the gate electrode material or using an embedded structure, a mask step and a process step need to be added. 
       Seventh Embodiment 
     (Description of Configuration and Effects) 
       [0065]    A seventh embodiment relates to the Schottky junction area of the protection diodes Da 1  and Db 1  in  FIG. 1  according to the first to sixth embodiments, and  FIG. 9  shows a layout example of the Da 1  and Db 1 .  FIG. 9A  is a layout example of the diode Db 1  in  FIG. 1  and  FIG. 9B  is a layout example of the diode Da 1  in  FIG. 1 , Wg 1  denotes a gate width of the diode Db 1  and Wg 2  denotes a gate width of the diode Da 1 . The Schottky junction area is represented by an overlapping area between the gate electrode  22  and the active region  10  in  FIGS. 7A and 7B  as described above. 
         [0066]    When an allowable forward current of a diode is compared with an allowable reverse current, the allowable forward current is generally several times as great as the allowable reverse current. Therefore, when the first embodiment is actually expressed in the form of a chip layout, instead of implementing the diodes Da 1  and Db 1  with the same junction area, implementing the Da 1  with a smaller area than the Db 1  can improve the ESD resistance described in the first embodiment. The desired ESD resistance can be implemented even when the junction area of the Da 1  is set to ½ to ¼ of that of the diode Db 1 , for example. In the examples in  FIGS. 9A and 9B , the gate width Wg 2  of the diode Da 1  can be set to approximately ½ or less of the gate width Wg 1  of the diode Db 1 , the layout area occupied by the circuit in  FIG. 1  can be reduced by the amount corresponding to the reduced gate width of the Da 1 , thus allowing downsizing. 
         [0067]    Thus, reducing the area of the Da 1  to ½ to ¼ of the area of the diode Db 1  has an effect of being able to reduce the occupancy area by the protective circuit compared to the ease where both are implemented with the same area. 
       Eighth Embodiment 
     (Description of Configuration and Effects) 
       [0068]    An eighth embodiment is an example where the protective circuit section described in the first to fourth embodiments is implemented not on the same chip as the GaN FET but on another III-V group semiconductor substrate made of, for example, GaAs.  FIG. 10  illustrates an example of a circuit diagram thereof. In the example in  FIG. 10 , the FET F 1  is manufactured on a GaN substrate C 1  and the protective circuit section made up of the diodes Da 1  and Db 1 , and the resistor Ra is manufactured on a GaAs substrate C 2 . Both are connected together via a bonding wire Bw and constitute a circuit equivalent to that in  FIG. 1 . 
         [0069]    SiC substrates widely used as epitaxial substrates capable of manufacturing a GaN BET have excellent low thermal resistance, but they are normally more expensive than GaAs substrates. A high output, high gain, high efficiency characteristic and low thermal resistance in a GHz band are not strongly required for a protective circuit section intended for ESD protection. Therefore, the present embodiment featuring the protective circuit section manufactured on a GaAs substrate, the FET manufactured on a GaN substrate, both of which are connected together via a wire or the like, is effective for cost reduction. 
         [0070]    The effects described in the first to fourth embodiments can also be obtained even by manufacturing the FET F 1  and the protective circuit on different semiconductor substrates as described above and connecting them via the bonding wire Bw. Furthermore, creating the chip on a GaAs substrate has an effect of being able to reduce the size of the GaN FET chip by art amount corresponding to the area occupied by the protective circuit and achieving a cost reduction. It goes without saying that in addition to the GaAs substrate, an InP substrate, a Si substrate, a sapphire substrate or the like are also applicable. 
         [0071]    Obviously many modifications and variations of the present invention are possible light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may he practiced otherwise than as specifically described, 
         [0072]    The entire disclosure of Japanese Patent Application No. 2016-022499, filed on Feb. 9, 2016 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.