Patent Publication Number: US-2007115066-A1

Title: Radio-frequency amplifier and radio-frequency wireless communication apparatus

Description:
FIELD OF THE INVENTION  
      The present invention relates to a radio-frequency amplifier and a radio-frequency wireless communication apparatus used in a millimeter-wave band or a microwave band.  
     BACKGROUND OF THE INVENTION  
      Using slot lines in this type of radio-frequency amplifier enables easy connection with a semiconductor device or the like and propagation of a well-balanced radio-frequency signal. For this reason, slot lines have been used as input and output lines for a radio-frequency signal or part of a module in recent years, as disclosed in Patent Documents 1 to 3 (see Patent Documents 1 to 3).  
       FIG. 21  is a schematic plan view showing an amplifier disclosed in Patent Document 1.  
      In this amplifier, conductors  101  and  102  on a substrate  100  define an input slot  200 , conductors  103  and  104  define an output slot  201 , and a conductor  105  is placed between the input and output slots  200  and  201 , which extend along an almost straight line. Gate electrodes G of FETs  301  and  302  in a chip  300  connect to the conductors  101  and  102 , respectively; drain electrodes D thereof connect to the conductors  103  and  104 , respectively; and source electrodes S thereof connect to the conductor  105 . That is, the orientation of arrangement of the gate electrode G and the drain electrode D of each FET  301  ( 302 ) is parallel with the input and output slots  200  and  201 . A microwave signal from the input slot  200  is amplified by the FETs  301  and  302  and is output to the output slot  201 .  
      Patent Document 1: PCT Japanese Translation Patent Publication No. 2001-501418  
      Patent Document 2: PCT Japanese Translation Patent Publication No. 2000-500310  
      Patent Document 3: Japanese Unexamined Patent Application Publication No. 01-095602  
      However, in the above-described amplifier, in which the orientation of arrangement of the gate electrode G and the drain electrode D of each FET  301  ( 302 ) is parallel with the input and output slots  200  and  201 , if a microwave signal is in a TE mode or the like, the direction of a magnetic field of the microwave signal propagating through the input and output slots  200  and  201  is different by almost 90 degrees from the direction of a magnetic field of a current flowing through the gate electrode G and the drain electrode D of each FET  301  ( 302 ). Thus, energy loss occurs when the magnetic field propagates from the input slot  200  to the gate electrode G side of each FET  301  ( 302 ) and when the magnetic field propagates from the drain electrode D side to the output slot  201 . Accordingly, significant insertion loss is caused by the FETs  301  and  302  disadvantageously.  
      In each FET  301  ( 302 ) applied in this amplifier, the gate electrode G, the drain electrode D, and the source electrode S are dispersed. Therefore, when each FET  301  ( 302 ) itself is to be measured, each electrode needs to be separately measured with a probe in order to evaluate the characteristic thereof. In that case, the measured characteristic may be different from a characteristic obtained when each FET  301  ( 302 ) is mounted. Furthermore, since the direction of a magnetic field of a microwave signal propagating through the input and output slots  200  and  201  is different from the direction of a magnetic field of each FET  301  ( 302 ) when the FETs are mounted, the electromagnetic field mode input to each FET  301  ( 302 ) when each FET  301  ( 302 ) itself is measured is different from the electromagnetic field mode input to each FET  301  ( 302 ) when the FETs are mounted, so that designing a circuit on the basis of a measurement result is difficult.  
      This problem arises also in the techniques according to the above-mentioned Patent Documents 2 and 3.  
     SUMMARY OF THE INVENTION  
      The present invention has been made to solve the above-described problem and an object thereof is to provide a radio-frequency amplifier and a radio-frequency wireless communication apparatus enabling reduction of insertion loss caused by a transistor and precise circuit design based on a measurement result.  
      In order to solve the above-described problem, the radio-frequency amplifier according to an embodiment of the invention includes an input-side line portion which is formed on a substrate, which has an input slot line whose one end is shorted, and which is used to input an electromagnetic-field-mode signal whose magnetic field is parallel to the input slot line into the input slot line; an output-side line portion including an output slot line which is substantially parallel to the input slot line and whose one end is shorted; and a transistor which includes a coplanar connecting portion in which source electrodes are arranged on both sides of a gate electrode and a drain electrode arranged along a straight line and which is mounted on the substrate such that the gate electrode is positioned on the input slot line side, that the drain electrode is positioned on the output slot line side, and that the orientation of arrangement of the gate electrode and the drain electrode is perpendicular to the input slot line and the output slot line.  
      With this configuration, an input signal propagates through the input slot line and enters the transistor. Then, the signal is processed in the transistor, reaches the output slot line, and propagates through the output slot line to be output therefrom. In the transistor, the gate electrode, the drain electrode, and the both source electrodes are arranged in a coplanar manner, so that all of the electrodes can be measured at one time with a probe. A coplanar electromagnetic field mode is induced in the mounted transistor, and thus the electromagnetic field mode input to the mounted transistor is the same as the electromagnetic field mode measured in the transistor itself. Furthermore, a signal input to the input slot line of the input-side line portion is a signal of an electromagnetic field mode in which the magnetic field is parallel to the input slot line, and the orientation of arrangement of the gate electrode and the drain electrode of the transistor is perpendicular to the input slot line and the output slot line. Thus, the directions of magnetic fields of a microwave signal propagating through the input slot line, the transistor, and the output slot line are the same.  
      In a further embodiment of the radio-frequency amplifier according to the present invention, the input-side line portion includes the input slot line, a first DC cut line which branches off at almost 90 degrees from the input slot line to an edge of the substrate and which has a short stub of a predetermined length at the middle thereof, and a second DC cut line which branches off at a point far from the one end of the input slot line relative to the first DC cut line to the edge of the substrate and which has a short stub of a predetermined length at the middle thereof. The output-side line portion includes the output slot line, a third DC cut line which branches off from the output slot line in the direction opposite to the first DC cut line to an edge of the substrate and which has a short stub of a predetermined length at the middle thereof, and a fourth DC cut line which branches off at a point far from the one end of the output slot line relative to the third DC cut line in the direction opposite to the second DC cut line to the edge of the substrate and which has a short stub of a predetermined length at the middle thereof. The transistor is mounted on the substrate such that the gate electrode of the connecting portion is connected to a first DC electrode separated by the first DC cut line and the second DC cut line of the input-side line portion and that the drain electrode is connected to a second DC electrode separated by the third DC cut line and the fourth DC cut line of the output-side line portion, so that the orientation of arrangement of the gate electrode and the drain electrode is perpendicular to the input slot line and the output slot line, and such that the both source electrodes are connected to a ground electrode separated by the input slot line, the first DC cut line, the output slot line, and the third DC cut line.  
      With this configuration, the gate electrode of the transistor is connected to the first DC electrode separated by the first DC cut line and the second DC cut line of the input-side line portion, whereas the drain electrode is connected to the second DC electrode separated by the third DC cut line and the fourth DC cut line of the output-side line portion. Accordingly, a biasing DC can be directly supplied from the gate electrode and the drain electrode serving as the hot-side of a coplanar line.  
      Preferably, the gate electrode and the drain electrode of the transistor are placed before the one ends of the input slot line and the output slot line, respectively, by a distance of ¼ wavelength.  
      With this configuration, the positions of the input slot line and the output slot line corresponding to the positions where the gate electrode and the drain electrode of the transistor are arranged are electrically open.  
      According to yet another embodiment of the invention, the short stubs of the first and second DC cut lines are placed at positions of ¼ wavelength from branch points of the input slot line, and the short stubs of the third and fourth DC cut lines are placed at positions of ¼ wavelength from branch points of the output slot line.  
      With this configuration, the branch points between the input slot line and the first and second DC cut lines and the branch points between the output slot line and the third and fourth DC cut lines are electrically shorted.  
      Preferably, an air bridge to electrically connect the both source electrodes is provided in the connecting portion of the transistor.  
      With this configuration, the both source electrodes of the transistor are at the same potential.  
      According to a further embodiment of the present invention, a part of the input slot line between the first DC cut line and the second DC cut line is curved toward the output slot line side and a part of the output slot line between the third DC cut line and the fourth DC cut line is curved toward the input slot line side so that pad portions are formed on the first and second DC electrodes. The gate electrode and the drain electrode are connected to the pad portions of the first and second DC electrodes, respectively.  
      With this configuration, the pad portions of the first and second DC electrodes are protruded while facing each other, so that the connecting point between the pad portion of the first DC electrode and the gate electrode can be placed close to the connecting point between the ground electrode and the source electrode, and that the connecting point between the pad portion of the second DC electrode and the drain electrode can be placed close to the connecting point between the ground electrode and the source electrode.  
      According to another embodiment of the present invention, the connecting portion of the transistor faces the substrate, and the gate electrode, the drain electrode, and the both source electrodes are connected to the first and second DC electrodes and the ground electrode, respectively, in a flip chip method using bumps.  
      With this configuration, the distances between the connecting points of the source electrodes on the substrate and the connecting points of the gate electrode and the drain electrode on the substrate can be shortened.  
      According to a further embodiment of the invention, the connecting portion of the transistor is oriented to the side opposite to the substrate. The gate electrode and the drain electrode are connected to the first and second DC electrodes, respectively, via wires, and the both source electrodes are connected to the ground electrode via through holes provided in the transistor.  
      With this configuration, the distance between the connecting point between one source electrode and the ground electrode and the connecting point between the other source electrode and the ground electrode becomes short, so that the both source electrodes on the transistor are at almost the same potential.  
      According to the another embodiment of the invention, the connecting portion of the transistor is oriented to the side opposite to the substrate. The gate electrode, the drain electrode, and the both source electrodes are connected to the pad portions of the first and second DC electrodes and the ground electrode, respectively, via through holes provided in the transistor.  
      With this connection via through holes, the connecting point between the pad portion of the first DC electrode and the gate electrode can be placed close to the connecting point between the ground electrode and the source electrode, whereas the connecting point between the pad portion of the second DC electrode and the drain electrode can be placed close to the connecting point between the ground electrode and the source electrode.  
      Preferably, one or more heat-dissipating through holes are provided in a portion of the substrate corresponding to a connecting position of the transistor.  
      With this configuration, heat generated in the transistor is dissipated through the heat-dissipating through holes.  
      A radio-frequency wireless communication apparatus according to the present invention includes a mixer to receive an intermediate-frequency signal and a local oscillation signal from a local oscillator through a slot line, convert the intermediate-frequency signal to a radio-frequency signal, and output the radio-frequency signal through a slot line; the radio-frequency amplifier as described above to receive the radio-frequency signal from the mixer through an input slot line of an input-side line portion and amplify the signal; and a slot antenna to transmit the radio-frequency signal output from an output slot line of an output-side line portion of the radio-frequency amplifier.  
      According to a further embodiment of the radio-frequency wireless communication apparatus according to the present invention, the radio-frequency wireless communication apparatus includes the radio-frequency amplifier as described above to receive a radio-frequency signal received by the slot antenna through the input slot line of the input-side line portion and amplify the radio-frequency signal; and a mixer to receive the radio-frequency signal output from the output slot line of the output-side line portion of the radio-frequency amplifier and a local oscillation signal from the local oscillator through a slot line, convert the radio-frequency signal to an intermediate-frequency signal, and output the intermediate-frequency signal through a slot line.  
      As described above, in the radio-frequency amplifier according to the invention, all of the electrodes can be measured at one time with a probe, so that the transistor itself can be easily measured. Further, since the electromagnetic field mode input to the mounted transistor is the same as the electromagnetic field mode obtained when the transistor itself is measured, the characteristic of the mounted transistor matches the measurement characteristic of the transistor before being mounted. Accordingly, the radio-frequency amplifier can be precisely designed on the basis of the measurement result. In addition, the directions of magnetic fields of a microwave signal propagating through the input slot line, the transistor, and the output slot line are the same, so that loss occurring when the signal enters the transistor and loss occurring when the signal is output from the transistor, that is, insertion loss of the transistor is very low.  
      As described above, since the first and second DC electrodes are DC-separated from the ground electrode by the first to fourth DC cut lines, the input slot line, and the output slot line, DC can be directly supplied from the first and second DC electrodes to the gate electrode and the drain electrode serving as the hot-side of a coplanar line in the mounted transistor without necessity of mounting a special element such as a lead to supply bias or multi-layering the substrate. As a result, a special jig to supply DC is not necessary, which simplifies measurement and reduces the cost.  
      As described above, since the positions of the input slot line and the output slot line corresponding to the positions where the gate electrode and the drain electrode of the transistor are arranged are electrically open, a signal from the input slot line can be reliably input to the gate electrode side and a signal from the drain electrode side can be reliably output to the output slot line.  
      As described above, since the branch points between the respective DC cut lines and the slot lines are electrically shorted, a signal propagating through the slot line can be prevented from entering any DC cut line.  
      As described above, since both source electrodes of the transistor are at the same potential, the coplanar mode of a signal propagating in the connecting portion is reinforced. As a result, a signal of a mode other than the coplanar mode can be prevented from being generated at the connecting portion.  
      As described above, since the connecting point between the pad portion of the first DC electrode and the gate electrode can be placed close to the connecting point between the ground electrode and the source electrode, and the connecting point between the pad portion of the second DC electrode and the drain electrode can be placed close to the connecting point between the ground electrode and the source electrode, a coplanar mode can be easily induced in the mounted transistor, and as a result, transmission with low loss can be maintained in a higher frequency band.  
      In addition, because the distances between the connecting points of the source electrodes on the substrate and the connecting points of the gate electrode and the drain electrode on the substrate can be shortened, a coplanar mode can be easily induced in the mounted transistor, and low loss can be maintained even in a high frequency band.  
      Further, since the distance between the connecting point between one source electrode and the ground electrode and the connecting point between the other source electrode and the ground electrode is short so that the both source electrodes on the transistor are at the same potential, a signal of other mode excited by the transistor can be suppressed. As a result, a coplanar mode can be easily induced in the mounted transistor, and thus transmission with low loss can be maintained in a higher frequency band.  
      Because the connecting point between the pad portion of the first DC electrode and the gate electrode can be placed close to the connecting point between the ground electrode and the source electrode, and the connecting point between the pad portion of the second DC electrode and the drain electrode can be placed close to the connecting point between the ground electrode and the source electrode, a coplanar mode can be easily induced in the mounted transistor, and as a result, transmission with low loss can be maintained in a higher frequency band. Furthermore, this configuration has an advantage in that the transistor need not be die-bonded and connected with wires.  
      As described above, since heat generated in the transistor is efficiently dissipated through the heat-dissipating through holes, thermal isolation of the transistor can be improved.  
      In the radio-frequency wireless communication apparatus according to the invention  2 , the respective electronic elements are connected through slot lines parallel with each other. With this configuration, the respective elements can be smoothly connected and a signal can be transmitted with low loss by allowing the signal to propagate through the slot lines in a TE mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view showing a substrate and a transistor constituting a radio-frequency amplifier according to a first embodiment of the present invention.  
       FIG. 2  is a schematic plan view showing the substrate.  
       FIG. 3  is a schematic plan view showing a connecting portion of the transistor.  
       FIG. 4  is a cross-sectional view taken along the line A-A in  FIG. 2  in a state where the transistor is mounted.  
       FIG. 5  is a partial enlarged plan view showing a connection state of the transistor.  
       FIG. 6  is a cross-sectional view showing an electromagnetic field distribution propagating in electrodes of the transistor.  
       FIG. 7  is a cross-sectional view taken along the line B-B in  FIG. 2  in a state where the transistor is mounted.  
       FIG. 8  includes schematic plan views showing a radio-frequency amplifier used in a simulation.  
       FIG. 9  is a characteristic diagram showing a relationship between frequency and insertion loss.  
       FIG. 10  is a schematic plan view showing a radio-frequency amplifier according to a second embodiment of the present invention.  
       FIG. 11  is a cross-sectional view taken along the line C-C in  FIG. 10 .  
       FIG. 12  is a schematic plan view showing a radio-frequency amplifier according to a third embodiment of the present invention.  
       FIG. 13  is a cross-sectional view taken along the line D-D in  FIG. 12 .  
       FIG. 14  is a schematic plan view showing a radio-frequency amplifier according to a fourth embodiment of the present invention.  
       FIG. 15  is a cross-sectional view taken along the line E-E in  FIG. 14 .  
       FIG. 16  is a schematic plan view showing a radio-frequency amplifier according to a fifth embodiment of the present invention.  
       FIG. 17  is a cross-sectional view taken along the line F-F in  FIG. 16 .  
       FIG. 18  is a block diagram showing a radio-frequency wireless communication apparatus according to a sixth embodiment of the present invention.  
       FIG. 19  is a schematic partial plan view showing a modification of the present invention.  
       FIG. 20  is a schematic partial plan view showing another modification of the present invention.  
       FIG. 21  is a schematic plan view showing a known amplifier. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Hereinafter, a best mode for carrying out the invention is described with reference to the drawings.  
     First Embodiment  
       FIG. 1  is a perspective view showing a substrate and a transistor constituting a radio-frequency amplifier according to a first embodiment of the present invention;  FIG. 2  is a schematic plan view showing the substrate; FIG.  3  is a schematic plan view showing a connecting portion of the transistor;  FIG. 4  is a cross-sectional view taken along the line A-A in  FIG. 2  in a state where the transistor is mounted; and  FIG. 5  is a partial enlarged plan view showing a connection state of the transistor.  
      As shown in  FIG. 1 , the radio-frequency amplifier according to this embodiment includes the substrate  1  and the transistor  2  in a chip shape.  
      As shown in  FIG. 1 , the substrate  1  includes a dielectric plate  1   a  and a conductor  1   b  provided on both surfaces thereof. Predetermined portions of the conductor  1   b  are removed so that a plurality of slot lines are formed. These slot lines define an input-side line portion  3  and an output-side line portion  4 .  
      The input-side line portion  3  includes an input slot line  30  and first and second DC (direct current) cut lines  31  and  32  which are parallel with each other. These lines  30  to  32  define a separated first DC electrode  10 .  
      The input slot line  30  is a line to receive and transmit a microwave signal and extends straight from an open end  30   a  as a signal input end to a center of the substrate  1 . An end  30   b  thereof is short-circuited.  
      As shown in  FIG. 2 , the DC cut line  31  branches off at almost 90 degrees from the input slot line  30  and opens at an edge (a lower edge in the figure) of the substrate  1 . At the middle thereof, a short stub  33  whose one end is shorted is provided. The short stub  33  is placed at a position of λ/4 (λ is a wavelength of a microwave signal) from a branch point S 1  of the input slot lint  30 , and the length thereof is also set to λ/4.  
      The DC cut line  32  also branches off at almost 90 degrees from the input slot line  30  at a point before the DC cut line  31  and opens at the edge of the substrate  1 . The DC cut line  32  also has a short stub  34  having a length of λ/4 at the middle. The short stub  34  is placed at a position of λ/4 from a branch point S 2  of the input slot line  30 .  
      On the other hand, as shown in  FIG. 1 , the output-side line portion  4  is substantially point-symmetrically placed with the input-side line portion  3  and includes an output slot line  40  and third and fourth DC cut lines  41  and  42  which are parallel with each other. These lines  40  to  42  define a separated second DC electrode  11 .  
      The output slot line  40  is a line to output and transmit a microwave signal and is substantially parallel with the input slot line  30 . Specifically, the output slot line  40  extends straight from a shorted one end  40   a  positioned at a center of the substrate  1  to an open end  40   b  as a signal output end.  
      As shown in  FIG. 2 , the DC cut line  41  branches off at almost 90 degrees from the output slot line  40  in a direction opposite to the DC cut line  31  and opens at an edge (an upper edge in the figure) of the substrate  1 . At the middle thereof, a short stub  43  is provided. The short stub  43  is placed at a position of λ/4 from a branch point S 3 , and the length thereof is set to λ/4.  
      The DC cut line  42  branches off at almost 90 degrees from the output slot line  40  in a direction opposite to the DC cut line  32  behind the DC cut line  41  and opens at the edge of the substrate  1 . The DC cut line  42  also has a short stub  44  having a length of λ/4 at the middle. The short stub  44  is placed at a position of λ/4 from a branch point S 4 .  
      As described above, in the substrate  1 , the separated DC electrode  10  is defined by the lines  30  to  32  of the input-side line portion  3 . Also, the separated DC electrode  11  is defined by the lines  40  to  42  of the output-side line portion  4 . Furthermore, a separated ground electrode  12  is defined by the input slot line  30 , the DC cut lien  31 , the output slot line  40 , and the DC cut line  41 .  
      As shown in  FIG. 1 , the transistor  2  is connected to the DC electrodes  10  and  11  and to the ground electrode  12 .  
      The transistor  2  is an active element to amplify a microwave signal input from the input slot line  30  of the input-side line portion  3  and outputs the signal to the output slot line  40  of the output-side line portion  4 . In this embodiment, a FET (field-effect transistor) is applied.  
      As shown in  FIG. 3 , the transistor  2  includes a connecting portion  20  on one surface.  
      The connecting portion  20  includes a gate electrode G, a drain electrode D, and source electrodes S that are arranged in a coplanar manner. More specifically, the gate electrode G and the drain electrode D are placed along a straight line at the center of the connecting portion  20 , and a pair of source electrodes S are placed in parallel on both sides of the gate electrode G and the drain electrode D. An air bridge  21  is provided on the pair of source electrodes S, so that the pair of source electrodes S electrically connect to each other.  
      In this embodiment, as indicated in  FIG. 4  and with broken lines shown in  FIG. 5 , the transistor  2  is mounted on the substrate  1  such that the connecting portion  20  faces the substrate  1  and that the gate electrode G, the drain electrode D, and the both source electrodes S connect to the DC electrodes  10  and  11  and the ground electrode  12 , respectively, via bumps  22  in a flip chip method.  
      Accordingly, the orientation of arrangement of the gate electrode G and the drain electrode D of the transistor  2  is perpendicular to the input slot line  30  and the output slot line  40 . Further, the gate electrode G and the drain electrode D are placed at positions before the one ends  30   b  and  40   a  of the input slot line  30  and the output slot line  40  by a distance of λ/4.  
      Next, an effect and advantage of the radio-frequency amplifier according to this embodiment are described.  
       FIG. 6  is a cross-sectional view showing an electromagnetic-field distribution propagating in the electrodes of the transistor, and  FIG. 7  is a cross-sectional view taken along the line B-B in  FIG. 2  in a state where the transistor is mounted.  
      In the above-described connection state, a biasing DC current is applied to the DC electrodes  10  and  11  shown in  FIG. 1  and a microwave signal M 1  whose electromagnetic field mode is a TE mode is input to the input slot line  30  of the input-side line portion  3 , the microwave signal M 1  propagates in the input slot line  30  in a state where an electric field E is vertical to a propagation direction and a magnetic field H is parallel with the propagation direction. In other words, the magnetic field H of the microwave signal M 1  propagates in a direction parallel with the input slot line  30 .  
      This microwave signal M 1  propagates in the input slot line  30  toward the transistor  2  side and reaches the branch point S 2  of the DC cut line  32  (see  FIG. 2 ). Since the length of the short stub  34  of the DC cut line  32  is λ/4, the one end of the short stub  34  is shorted, and the short stub  34  is at a position of λ/4 from the branch point S 2 , a branch point S 5  of the short stub  34  is electrically open and the branch point S 2  is electrically shorted when viewed from the input slot line  30  side. With this configuration, the microwave signal M 1  cannot propagate to the DC cut line  32  side but propagates in the input slot line  30  toward the transistor  2  side, while not causing any loss.  
      When the microwave signal M 1  reaches a connecting position P 10  of the bump  22  of the gate electrode G, the input slot line  30  becomes electrically open at a position corresponding to the connecting position P 10  because the one end  30   b  of the input slot line  30  is shorted and because the connecting position P 10  of the gate electrode G is at a position of λ/4 from the one end  30   b . Therefore, the microwave signal M 1  does not propagate in the input slot line  30  any more but propagates to the gate electrode G and the both source electrodes S through the bumps  22 .  
      Then, the microwave signal M 1  input from the gate electrode G side is amplified in the transistor  2  and an amplified microwave signal M 2  is output from the drain electrode D side. At this time, the gate electrode G and the drain electrode D function as a so-called hot-line, because the gate electrode G, the drain electrode D, and the both source electrodes S are arranged in a coplanar manner. Thus, the electromagnetic field distribution of the microwave signal M 1  (M 2 ) propagating through these electrodes is that shown in  FIG. 6 , in which both the electric field E and the magnetic field H of the microwave signal M 1  (M 2 ) are vertical to the propagation direction. In other words, the microwave signal propagates to the output slot line  40  side of the output-side line portion  4  in a coplanar mode (TEM) mode in the connecting portion  20 .  
      The output microwave signal M 2  that has been amplified in the transistor  2  reaches the output slot line  40  shown in  FIGS. 1 and 2  through the bump  22  of the drain electrode D. Since a connecting position P 11  of this bump  22  is at a position of λ/4 from the one end  40   a  of the output slot line  40 , the output slot line  40  becomes electrically open at a position corresponding to the connecting position P 11  when viewed from the open end  40   b  side. Thus, the microwave signal M 2  does not propagate to the one end  40   a  side but propagates toward the open end  40   b  in the output slot line  40 . Then, the microwave signal M 2  propagates toward the open end  40   b  in the output slot line  40  and reaches the branch point S 4  of the DC cut line  42  (see  FIG. 2 ). In this case, as in the case of the DC cut line  31 , the branch point S 4  is electrically shorted when viewed from the output slot line  40  side. Thus, the microwave signal M 2  cannot propagate to the DC cut line  42  side but propagates in the output slot line  40  toward the open end  40   b , while not causing any loss.  
      Likewise, the branch points S 1  and S 3  of the DC cut lines  31  and  41  are electrically shorted when viewed from the slot line side, so that a leaked microwave signal M 1  or M 2  does not enter the DC cut lines  31  and  41 .  
      As described above, according to the radio-frequency amplifier of this embodiment, the microwave signal M 1  input into the input slot line  30  reliably propagates to reach the connecting portion  20  of the transistor  2  without causing any loss in the DC cut lines  31  and  32 . Then, the microwave signal M 2  amplified in the transistor  2  propagates in the output slot line  40  to be output from the open end  40   b  without causing any loss in the DC cut lines  41  and  42 .  
      As shown in  FIG. 6 , in such a propagation state, the microwave signal propagates from the input-side line portion  3  side to the output slot line  40  side in a coplanar mode in the connecting portion  20 . Furthermore, since the orientation of arrangement of the gate electrode G and the drain electrode D is perpendicular to the input slot line  30  and the output slot line  40 , the directions of magnetic fields H of the microwave signal propagating through the input slot line  30 , the transistor  2 , and the output slot line  40  are the same, as shown in  FIG. 7 . As a result, loss caused when the microwave signal M 1  enters the transistor  2  and loss caused when the microwave signal is output from the transistor  2 , that is, insertion loss of the transistor  2  is significantly reduced.  
      The inventors carried out the following electromagnetic field simulation in order to verify the above-described concept.  
       FIG. 8  includes schematic plan views of a radio-frequency amplifier used in the simulation.  FIG. 8 ( a ) shows a state where the transistor is connected such that the orientation of arrangement of the gate electrode and the drain electrode is parallel to the input slot line and the output slot line.  FIG. 8 ( b ) shows a state where the transistor is connected such that the orientation of arrangement of the gate electrode and the drain electrode is perpendicular to the input slot line and the output slot line.  FIG. 9  is a characteristic diagram showing the relationship between frequency and insertion loss.  
      First, as shown in  FIG. 8 ( a ), the transistor  2  was connected to the substrate such that the orientation of arrangement of the gate electrode G and the drain electrode D is parallel to the input slot line  30  and the output slot line  40 . In this state, a microwave signal of a 50 to 80 GHz band was input to simulate the insertion loss of the transistor  2 , and as a result, a characteristic curve B 1  shown in  FIG. 9  was obtained. On the other hand, as shown in  FIG. 8 ( b ), the transistor  2  was connected to the substrate such that the orientation of arrangement of the gate electrode G and the drain electrode D is perpendicular to the input slot line  30  and the output slot line  40 . In this state, a microwave signal of the above-described band was input to simulate the insertion loss, and as a result, a characteristic curve B 2  shown in  FIG. 9  was obtained. In this simulation, a FET made by forming the above-described coplanar line on GaAs (gallium arsenide) is used as the transistor  2 .  
      As indicated by these characteristic curves B 1  and B 2 , the insertion loss is much lower when the orientation of arrangement of the gate electrode and the drain electrode are perpendicular to the slot lines than when the orientation of arrangement of the gate electrode and the drain electrode are parallel to the slot lines.  
      In the transistor  2  applied to this embodiment, the gate electrode G, the drain electrode D, and the both source electrodes S are arranged in a coplanar manner, so that all of the electrodes can be measured at one time with a probe. Thus, the transistor  2  itself can be easily measured. Further, a biasing DC can be directly supplied from the gate electrode G and the drain electrode D serving as the hot-side of a coplanar line. Therefore, a special jig for supplying DC need not be prepared, which contributes to simplify the measurement and reduce the cost.  
      As described above, a coplanar electromagnetic field mode is induced in the mounted transistor  2 . Therefore, the electromagnetic field mode input to the mounted transistor  2  is the same as the electromagnetic field mode obtained when the transistor  2  is separately measured. Thus, the characteristic of the mounted transistor  2  matches the characteristic of the transistor  2  that is not mounted. As a result, the circuit of the radio-frequency amplifier can be precisely designed on the basis of the measurement result.  
      Since the DC electrodes  10  and  11  are DC-separated from the ground electrode  12  by the DC cut lines  31 ,  32 ,  41 , and  42 , a special element such as a lead to supply bias need not be mounted and the substrate need not be multi-layered. In the mounted transistor  2 , DC can be supplied from the gate electrode G and the drain electrode D serving as the hot-side of a coplanar line.  
      Since the both source electrodes S of the transistor  2  are electrically connected through the air bridge  21 , the source electrodes S are at the same potential, so that the coplanar mode of the microwave signal propagating in the connecting portion  20  is reinforced. As a result, a signal of a mode other than the coplanar mode can be prevented from occurring in the connecting portion  20 .  
      The transistor  2  is connected to the substrate  1  in a flip chip method by using the bumps  22 . With this configuration, the distance between connecting points of the source electrodes S on the substrate and connecting points of the gate electrode G and the drain electrode D on the substrate can be shorter than the distance between respective connecting points on the substrate in a case where the source electrodes S, the gate electrode G, and the drain electrode D are connected to the substrate  1  by using wires as in die bonding. As a result, a coplanar mode is easily induced in the mounted transistor  2 , so that low loss can be maintained even in a high-frequency band.  
     Second Embodiment  
      Next, a second embodiment of the present invention is described.  
       FIG. 10  is a schematic plan view showing a radio-frequency amplifier according to the second embodiment of the present invention, and  FIG. 11  is a cross-sectional view taken along the line C-C in  FIG. 10 .  
      The radio-frequency amplifier according to this embodiment is different from that of the above-described first embodiment in the shape of lines on the substrate and in an attachment state of the transistor.  
      As shown in  FIG. 10 , in this embodiment, the transistor  2  is mounted in the reverse orientation between the input slot line  30  and the output slot line  40 .  
      Specifically, as shown in  FIGS. 10 and 11 , the transistor  2  is mounted on the ground electrode  12  and is die-bonded with a conductive paste such that the connecting portion  20  of the transistor  2  is oriented to the side opposite to the substrate  1 . The gate electrode G and the drain electrode D are set to positions before the one ends  30   b  and  40   a  of the input slot line  30  and the output slot line  40  by λ/4, respectively, and the gate electrode G and the drain electrode D connect to the DC electrodes  10  and  11 , respectively, with wires  5 . Further, the both source electrodes S connect to the ground electrode  12  via through holes  23  provided in the transistor  2 .  
      The other configuration, effect, and advantage are substantially the same as those in the above-described first embodiment, and thus the corresponding description is omitted.  
     Third Embodiment  
      Next, a third embodiment of the present invention is described.  
       FIG. 12  is a schematic plan view showing a radio-frequency amplifier according to the third embodiment of the present invention, and  FIG. 13  is a cross-sectional view taken along the line D-D in  FIG. 12 .  
      The radio-frequency amplifier according to this embedment is different from that of the above-described second embodiment in that pad portions  10   a  and  11   a  for connection with the transistor  2  are provided in the DC electrodes  10  and  11 .  
      That is, as shown in  FIGS. 12 and 13 , the input slot line  30  has a curved portion  30   c . Specifically, the line between the DC cut line  31  and the DC cut line  32  is curved toward the output-side line potion  4  so that the pad portion  10   a  is formed at an edge of the DC electrode  10 . The center of the pad portion  10   a  is positioned before the one end  30   b  of the input slot line  30  by a distance of λ/4.  
      Also, the output slot line  40  has a curved portion  40   c . That is, the line between the DC cut line  41  and the DC cut line  42  is curved toward the input slot line  30  so that the pad portion  11   a  is formed at an edge of the DC electrode  11 . The center of the pad portion  11   a  is positioned away from the one end  40   a  by a distance of λ/4.  
      The transistor  2  is connected in a flip chip method such that the bump  22  of the gate electrode G is positioned at the center of the pad portion  10   a  of the DC electrode  10 , that the bump  22  of the drain electrode D is positioned at the center of the pad portion  11   a  of the DC electrode  11 , and that the bumps  22  of the both source electrodes S are positioned on the ground electrode  12 .  
      The other configuration, effect, and advantage are substantially the same as those in the above-described first and second embodiment, and thus the corresponding description is omitted.  
     Fourth Embodiment  
      Next, a fourth embodiment of the present invention is described.  
       FIG. 14  is a schematic plan view showing a radio-frequency amplifier according to the fourth embodiment of the present invention, and  FIG. 15  is a cross-sectional view taken along the line E-E in  FIG. 14 .  
      The radio-frequency amplifier according to this embodiment is different from those of the above-described embodiments in that the transistor  2  is mounted on the substrate  1  by die-bonding and that all electrodes are connected via through holes.  
      Specifically, the transistor  2  is mounted over the DC electrodes  10  and  11  and the ground electrode  12  and is die-bonded with a conductive paste such that the connecting portion  20  of the transistor  2  is oriented to the side opposite to the substrate  1 . Through holes  23  are provided at positions corresponding to the gate electrode G, the drain electrode D, and the both source electrodes S. The gate electrode G, the drain electrode D, and the both source electrodes S electrically connect to the DC electrodes  10  and  11  and the ground electrode  12 , respectively, via these through holes  23 .  
      The other configuration, effect, and advantage are substantially the same as those of the above-described first to third embodiments, and thus the corresponding description is omitted.  
     Fifth Embodiment  
      Next, a fifth embodiment of the present invention is described.  
       FIG. 16  is a schematic plan view showing a radio-frequency amplifier according to the fifth embodiment of the present invention, and  FIG. 17  is a cross-sectional view taken along the line F-F in  FIG. 16 .  
      The radio-frequency amplifier according to this embodiment is different from those of the above-described first to fourth embodiments in that a through hole to dissipate heat of the transistor is provided in the substrate.  
      That is, as shown in  FIGS. 16 and 17 , a heat-dissipating through hole  13  is provided at a portion of the substrate  1  corresponding to the connecting portion of the transistor  2  in the radio-frequency amplifier according to the first embodiment. Specifically, the heat-dissipating through hole  13 , which is rectangular in a plan view, is provided at a portion of the ground electrode  12  between the input slot line  30  and the output slot line  40  and just under the position where the transistor  2  is mounted. This heat-dissipating through hole  13  is a thermal via hole filled with a member having high thermal conductivity and has a function of transferring heat generated by the transistor  2  to the lower side of the substrate  1  so as to cool the transistor  2 .  
      With this configuration, heat generated by the transistor  2  can be efficiently dissipated, so that thermal isolation of the transistor  2  can be improved.  
      The other configuration, effect, and advantage are substantially the same as those in the above-described first to forth embodiments, and thus the corresponding description is omitted.  
     Sixth Embodiment  
      Next, a sixth embodiment of the present invention is described.  
       FIG. 18  is a block diagram showing a radio-frequency wireless communication apparatus according to the sixth embodiment of the present invention.  
      This radio-frequency wireless communication apparatus includes a transmitting unit  6 , a receiving unit  7 , a slot antenna  8 , and a separator  9  to separate transmission and reception signals.  
      The transmitting unit  6  includes a mixer  60 , a band-pass filter  61 , and a radio-frequency amplifier  62  according to any of the above-described embodiments, which are connected through slot lines  90 , and has a function of converting an intermediate-frequency signal IF to a radio-frequency signal RF and transmitting the RF signal from the slot antenna  8 .  
      Specifically, the mixer  60  receives an intermediate-frequency signal IF and a local oscillation signal Lo from a local oscillator  50  through the slot line  90 , converts the intermediate-frequency signal IF to a radio-frequency signal RF, and outputs the radio-frequency signal RF to the band-pass filter  61 . Then, the band-pass filter  61  filters the radio-frequency signal RF and outputs the signal to the radio-frequency amplifier  62 . The radio-frequency amplifier  62  receives the radio-frequency signal RF through the input slot line  30  of the input-side line portion  3  and amplifies the signal. Then, the radio-frequency amplifier  62  transmits the amplified radio-frequency signal RF to the slot antenna  8  through the separator  9 , so that the radio-frequency signal RF is transmitted from the slot antenna  8 .  
      On the other hand, the receiving unit  7  includes a radio-frequency amplifier  70  according to any of the above-described embodiments, a band-pass filter  71 , and a mixer  72 , which are connected through slot lines  90 , and has a function of converting a radio-frequency signal RF received from the slot antenna  8  to an intermediate-frequency signal IF.  
      Specifically, the slot antenna  8  receives a radio-frequency signal RF, and the radio-frequency signal RF is input to the input slot line  30  of the radio-frequency amplifier  70  through the separator  9 . Then, the radio-frequency amplifier  70  amplifies the radio-frequency signal RF and outputs it from the output slot line  40 . The band-pass filter  71  filters the radio-frequency signal RF and outputs it to the mixer  72 . The mixer  72  receives this radio-frequency signal RF and a local oscillation signal Lo from the local oscillator  50  through the slot line  90 , converts the radio-frequency signal RF into an intermediate-frequency signal IF, and outputs the intermediate-frequency signal IF.  
      As described above, according to the radio-frequency wireless communication apparatus according to this embodiment, the respective electronic elements are connected through slot lines parallel with each other. With this configuration, the respective elements can be smoothly connected and a signal can be transmitted with low loss by transmitting the signal through the slot lines in a TE mode.  
      The other configuration, effect, and advantage are substantially the same as those in the above-described first to fifth embodiments, and thus the corresponding description is omitted.  
      The present invention is not limited to the above-described embodiments, but various modifications or alterations can be accepted within the scope of the present invention.  
      For example, as shown in  FIG. 19 , in the radio-frequency amplifier according to the third embodiment, the transistor  2  can be connected to the substrate  1  via through holes  23 . That is, the transistor  2  may be placed on the substrate  1  such that the connecting portion  20  of the transistor  2  is oriented to the side opposite to the substrate  1 , and the gate electrode G, the drain electrode D, and the both source electrodes S may be electrically connected to the pad portion  10   a  of the DC electrode  10 , the pad portion  11   a  of the DC electrode  11 , and the ground electrode  12 , respectively, via the through holes  23  provided at positions corresponding to the gate electrode G, the drain electrode D, and the both source electrodes S.  
      In the radio-frequency amplifier according to the above-described second embodiment, the transistor  2  is connected to the substrate  1  via wires and through holes, as shown in  FIG. 10 . Alternatively, as shown in  FIG. 20 , in the radio-frequency amplifier according to the second embodiment, the transistor  2  may be connected to the substrate  1  such that the gate electrode G and the drain electrode D of the transistor  2  are connected to the DC electrodes  10  and  11  via the wires  5  and that the both source electrodes S are connected to the ground electrode  12  via the through holes  23 .  
      In the fifth embodiment, one heat-dissipating through hole  13  is provided. Alternatively, a plurality of heat-dissipating through holes  13  of a small diameter may be dispersed.  
      In the above-described embodiments, only a case where slot lines are formed on one side of the substrate is described. However, substantially the same effect and advantage as those of the above-described embodiments can be obtained when a PDTL (planar dielectric transmission line) is used.  
      In the above-described embodiments, a FET is used as the transistor. However, the present invention is not limited to the FET but any type of transistor may be used as long as the transistor is a MOS transistor having a gate electrode, a drain electrode, and a source electrode.