Abstract:
A RF switch can be used in a wide frequency range and can be manufactured at a low cost. The RF switch changes a. signal passing through a waveguide with a variable device that is switchable between the first state in which the variable device has a high resistance and the second state in which the variable device has a low resistance, depending on the direction in which current flows through the variable device. The RF switch includes a high-frequency transmission circuit including the waveguide and at least one variable device, a driver circuit including at least one variable device, and a signal circuit for changing current supplied to the variable devices of the high-frequency transmission circuit and the driver circuit for switching between the first and second states of the variable devices. The variable devices are disposed such that the variable device of the high-frequency transmission circuit and the variable device of the driver circuit are in different states as viewed from the junction between the drive circuit and the high-frequency transmission circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a RF switch, and more particularly to a minute RF switch which can be used in a high frequency range from several MHz to several hundreds GHz.  
         [0003]     2. Description of the Related Art  
         [0004]     With the rapid progress of mobile telecommunication technology in recent years, the data rate that can be handled by mobile terminals has significantly been increased. Furthermore, to meet market demands for the higher telecommunication data rate, higher frequencies of signal career are being used so that mobile terminals can have wide bandwidth. At present, although mobile terminals use career frequencies ranging from several hundreds MHz to 2 GHz, they are expected, in the near future, to widely use higher career frequencies in the range of several GHz. In the field of wireless communications, high frequencies in a Ka bands from 20 to 30 GHz and a millimeter wave of about 60 GHz for vehicle communications have already been widely used.  
         [0005]     FETs fabricated on a GaAs substrate are widely known as switches for handling such high-frequency signals. However, the FETs have a problem that they are expensive since they have to use GaAs substrate. Then, they cannot be constructed as large-scale components because they are expensive, making it difficult to integrate FETs with other devices. Another problem is that the higher frequencies of several GHz or higher tend to produce an increased energy loss, which fails to satisfy requirements for mobile terminals with low power consumption.  
         [0006]     There are another known switches based on micro-electro-mechanical systems (MEMS). Since such switches can fabricate on any substrates, they can easily be integrated with other components. Furthermore, because they cause an extremely low energy loss, they are highly expected to be used in high-frequency applications. However, MEMS switches have a disadvantage that they are large in dimension, e.g., approximately size of 100 μm square, and need a high voltage of about 20 V to operate  
         [0007]     As described above, the existing RF switches have disadvantages of their own. There has been a need for a new RF switch different from those existing devices. Generally, a switch is used to pass or block a signal flowing in a circuit by bringing about a large change in resistance or capacitance. OUM (Ovonic Unified Memory) developed by Intel utilizing the calcogenide semiconductor and PMC (Programmable Metallization Cell) invented by Axon are known as devices for causing large resistance changes.  
         [0008]     The PMC disclosed in U.S. Pat. No. 5,761,115 will be described below. In U.S. Pat. No. 5,761,115, a device based on a phenomenon in which a metal dendrite is grown or retracted by a voltage applied thereto is referred to as a PMC, and the idea of using a PMC as a nonvolatile memory is described. Though it is not proposed to use a PMC as a RF switch in the description of U.S. Pat. No. 5,761,115, a PMC is interesting as a RF switch.  
         [0009]      FIG. 1 ( a ) of the accompanying drawings is a plan view of a PMC according to an embodiment disclosed in U.S. Pat. No. 5,761,115 and  FIG. 1 ( b ) of the accompanying drawings is a cross-sectional view taken along line A-A′ of  FIG. 1 ( a ). Lower electrode  93  is disposed over substrate  91  with insulating layer  98  interposed therebetween. Lower electrode  93  is patterned in a horizontal direction in  FIG. 1 ( a ). Second insulating layer  96  is disposed on lower electrode  93  and areas of insulating layer  98  where lower electrode  93  is not provided. Second insulating layer  96  has a via hole  99  defined therein which extends down to the surface of lower electrode  93 . Fast ion conductor layer  92  is deposited on the inner side wall of via hole  99 . Thereafter, the unfilled portion of via hole  99  is filled up with via filling layer  97 . Upper electrode  94  is disposed on via hole  99 . Upper electrode  94  is patterned in a vertical direction in  FIG. 1 ( a ).  
         [0010]     When a voltage is applied between lower electrode  93  and upper electrode  94  with a negative voltage level on lower electrode  93 , metal dendrite  95  grows from lower electrode  93  toward upper electrode  94  and finally reaches upper electrode  94 . At this time, the electric resistance between upper electrode  94  and lower electrode  93  decreases. When the voltage polarity is reversed to apply a voltage between lower electrode  93  and upper electrode  94  with a positive voltage level on lower electrode  93 , metal dendrite  95  is retracted from upper electrode  94  toward lower electrode  93 . At this time, the electric resistance between upper electrode  94  and lower electrode  93  increases. U.S. Pat. No. 5,761,115 reveals an example in which the fast ion conductor layer is made of As 2 S 3 —Ag or a silver sulfide such as AgAsS 2 , the upper electrode (anode electrode) of silver or silver-aluminum alloy, and the lower electrode (cathode electrode) of aluminum. Interestingly, when the materials are combined as described above, the metal dendrite grows only when the voltage is applied between the lower electrode and the upper electrode with a negative voltage level on the lower electrode.  
         [0011]     It has been found that some problems arise if the PMC disclosed in U.S. Pat. No. 5,761,115 is used as a RF switch.  
         [0012]     The first problem is that the device is of a structure wherein two electric interconnects are connected to a switch, and a driver circuit for driving the switch is not isolate from a line for passing a data signal. To drive the switch, therefore, a signal has to be mixed with a data signal, posing a significant limitation on the design of the circuit.  
         [0013]     The second problem occurs if the driver circuit is connected parallel to the line for passing the data signal in order to solve the first problem. In a high-frequency waveguide circuit, great care must be taken about an impedance change in the path along which the signal passes. The signal passing through the switch may leak to the driver circuit, thus allowing the switch to cause an increased loss. Depending on the impedance change, the signal may be reflected in the input port, and may not be transmitted in the output port.  
         [0014]     The third problem develops if the driver circuit is connected to the signal line through an isolation circuit such as a transistor or the like in order to solve the second problem. In a low frequency range, it is possible to reduce the attenuation of the signal because the driver circuit is isolated from the signal line. At higher frequencies, however, a loss of the signal increases because the isolation characteristic of the transistor is degraded. The signal loss manifests itself at frequencies of several GHz or higher.  
         [0015]     The fourth problem is that the whole switch is complex due to the need for a complex driver circuit. With the above intervening transistor, it is necessary to position the transistor as closely to the signal line as possible for the purpose of reducing reflections from the branch at the junction. However, sophisticated packaging technology is required to position the transistor as closely to the signal line as possible. An additional problem is that since the isolation device such as a transistor or the like is incorporated in the switch, the switch as a whole has increased dimensions, and the cost of the switch is high because an additional GaAs substrate is required to integrate the driver circuit.  
         [0016]     As described above, even if conventional RF switches are improved using existing techniques, some problems remain unsolved.  
       SUMMARY OF THE INVENTION  
       [0017]     It is an object of the present invention to provide a RF switch which solves the conventional problems, has a low-loss high isolation characteristic, is small in size, can be used in a wide frequency range, and can be fabricated at a low cost.  
         [0018]     According to the present invention, there is provided a RF switch for changing a signal passing through a waveguide with a variable device switchable between the first state in which the variable device has a high resistance-and the second state in which the variable device has a low resistance, depending on the direction in which a current flows through the variable device, the RF switch comprising a high-frequency transmission circuit including the waveguide and at least one variable device, a driver circuit including at least one variable device, and a signal circuit for changing a current supplied to the variable devices of the high-frequency transmission circuit and the driver circuit to switch between the first and second states of the variable devices, the drive circuit and the high-frequency transmission circuit being electrically connected to each other at a junction, the variable devices being disposed such that the variable device of the high-frequency transmission circuit and the variable device of the driver circuit are in different states as viewed from the junction.  
         [0019]     The driver circuit may include a resistor having a substantially constant resistance, and the signal circuit may be connected to one end of the variable device of the high-frequency transmission circuit through the variable device of the driver circuit, and connected to another end of the variable, device of the high-frequency transmission circuit through the resistor.  
         [0020]     The substantially constant resistance of the resistor may have a value of at least 10 kΩ.  
         [0021]     The driver circuit may include first and second variable devices, and the signal circuit may be connected to one end of the variable device of the high-frequency transmission circuit through the first variable device of the driver circuit, and connected to another end of the variable device of the high-frequency transmission circuit through the second variable device of the driver circuit.  
         [0022]     The RF switch may further include a bias circuit connected to one end of the variable device of the high-frequency transmission circuit, and the signal circuit may be connected to one end of the variable device of the high-frequency transmission circuit through the variable device of the driver circuit, and also connected to a bias voltage source.  
         [0023]     The resistance of each of the variable devices may be variable in a range from 1 Ω to 1 kΩ.  
         [0024]     High-frequency waveguide circuits are usually designed to have an impedance of 50 Ω. When the resistance of a resistor inserted in series in such a high-frequency waveguide circuit is changed, a signal passing through the high-frequency waveguide circuit is attenuated to a different degree depending on the resistance of the resistor. For example, when the resistance of the resistor is 1 Ω or less, the signal is attenuated by 1%, and when the resistance of the resistor is 10 kΩ, the signal is attenuated by 99%. This is the principle of a RF switch having a variable resistor connected in series in a high-frequency waveguide circuit.  
         [0025]     If a circuit connected as a branch to a high-frequency waveguide circuit at a junction has a resistance of 10 kΩ near the junction, then the attenuation of a signal passing through the high-frequency waveguide circuit can be reduced to 1% or less. That is, any adverse effect that the branch has on the signal can be essentially ignored.  
         [0026]     As described above with respect to the related art, some devices have a resistance highly variable depending on the direction in which a voltage is applied thereto or the direction in which current flows therethrough. According to the present invention, at least two of such variable-resistance devices are combined, with one connected in series to a waveguide, thereby providing a high-frequency signal for switching a data signal. The other variable-resistance device is connected between the waveguide and a driver circuit for branching and actuating the waveguide. The latter variable-resistance device serves to transmit a control signal from the driver circuit to the waveguide, and also to prevent the data signal from leaking from the waveguide.  
         [0027]     While variable-resistance devices have been described above, the present invention is not limited to variable-resistance devices, but may be applied to devices having a variable electric capacitance or inductance. The RF switch is not limited to an arrangement in which a resistor is connected in series to a waveguide, but is also applicable to an arrangement in which a resistor is connected in parallel to a waveguide.  
         [0028]     According to the present invention, there is provided a RF switch having a waveguide and a driver circuit isolated therefrom. Since the driver circuit is isolated from the waveguide, the RF switch can easily be incorporated into circuits. The RF switch has a low-loss high isolation characteristic, is small in size, can be used in a wide frequency range, and can be fabricated at a low cost.  
         [0029]     The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     FIGS.  1 ( a ) and  1 ( b ) are plan and cross-sectional views, respectively, of a conventional switch;  
         [0031]      FIG. 2  is a plan view of a RF switch according to the first embodiment of the present invention;  
         [0032]     FIGS.  3 ( a ) and  3 ( b ) are schematic views showing the manner in which the RF switch according to the first embodiment of the present invention operates;  
         [0033]     FIGS.  4 ( a ) through  4 ( d ) are plan and cross-sectional views illustrative of a process of fabricating the RF switch according to the first embodiment of the present invention;  
         [0034]      FIG. 5  is a schematic view of a RF switch according to the second embodiment of the present invention;  
         [0035]     FIGS.  6 ( a ) and  6 ( b ) are views showing the manner in which the RF switch operates according to the second embodiment of the present invention;  
         [0036]      FIG. 7  is a schematic view of a RF switch according to the third embodiment of the present invention; and  
         [0037]      FIG. 8  is a schematic view of a RF switch according to the fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     RF switches according to preferred embodiments of the present invention will be described in detail below with reference to the drawings.  
         [0039]      FIG. 2  shows a plan of a RF switch according to the first embodiment of the present invention. The RF switch according to the first embodiment comprises high-frequency transmission circuit  10  for passing a high-frequency signal therethrough and driver circuit  19  for controlling the transmission of the signal. High-frequency transmission circuit  10  comprises high-frequency waveguides  13   a,    13   b  and first variable-resistance device  11  having a resistance variable depending on the direction of the voltage or current.  
         [0040]     High-frequency waveguides  13   a,    13   b  are constructed as microstrip waveguide circuits, coplanar waveguide circuits, or the like, and are suitable for the transmission of high-frequency signals without any loss. For example, high-frequency waveguides  13   a,    13   b,  each comprising a gold interconnect layer having a thickness of 2 μm and a width of 40 μm, are mounted on insulating substrate  18  made of glass or the like, and a thin metal film on the reverse side of substrate  18  is kept as a ground potential.  
         [0041]     High-frequency waveguide  13   a  is connected to an output port of an external waveguide circuit (not shown) by a gold wire or the like, and high-frequency waveguide  13   b  is connected to an input port of the external waveguide circuit by a gold wire or the like. High-frequency waveguides  13   a,    13   b  are connected to each other by first variable-resistance device  11 . First variable-resistance device  11  comprises variable-resistance layer  113 , insulating film  115  in the form of a silicon nitride film or the like, and upper electrode  111  which are successively deposited.  
         [0042]     Variable-resistance layer  113  is formed by successively depositing a layer of copper having a thickness of 200 nm and a layer of copper sulfide having a thickness of 20 nm on high-frequency waveguide  13   a.    
         [0043]     Upper electrode  111  comprises a layer of metal such as gold or the like having a thickness of 2 μm and a width of 30 microns, and is connected to variable-resistance layer  113  through contact hole  114  that is defined in insulating film  115 . Upper electrode  111  is also connected to high-frequency waveguide  13   b  through contact hole  112  that is defined in insulating film  115 .  
         [0044]     First variable-resistance device  11  has a low resistance when a voltage is applied thereto that causes a current to flow in a direction from high-frequency waveguide  13   a  to high-frequency waveguide  13   b,  and has a high resistance of 10 k Ω or higher when a voltage is applied thereto to cause a current to flow in the reverse direction. A device which was actually fabricated as first variable-resistance device  11  was measured for its resistance. When a voltage of 0.2 V was applied to the device to cause a current to flow in a direction from high-frequency waveguide  13   a  to high-frequency waveguide  13   b,  the device had a resistance of 2 Ω or less (at this time, a current of about 100 mA flowed through the device). When a voltage of 0.06 V was applied to the device to cause a current to flow in the reverse direction-, the device had a resistance of 100 kΩ (at this time, a current of about 1 μA or less flowed through the device).  
         [0045]     Driver circuit  19  comprises second variable-resistance device  12  and fixed resistor  14  having a resistance of about 10 k Ω and is connected to external signal circuit  15 . Second variable-resistance device  12  comprises variable-resistance layer  123 , insulating film  125  in the form of a silicon nitride film or the like, and upper electrode  121  which are successively deposited in the order named.  
         [0046]     Variable-resistance layer  123  is formed by successively depositing a layer of copper having a thickness of 200 nm and a layer of copper sulfide having a thickness of 20 nm on metal interconnect  17 .  
         [0047]     Upper electrode  121  comprises a layer of metal such as gold or the like having a thickness of 0.2 μm and a width of 30 microns, and is connected to variable-resistance layer  123  through contact hole  124  that is defined in insulating film  125 . Upper electrode  121  is also connected to high-frequency waveguide  13   b  through contact hole  122  that is defined in insulating film  125 .  
         [0048]     Second variable-resistance device  12  has a low resistance of 2 Ω or less when a voltage is applied thereto that causes current to flow in a direction from metal interconnect  17  to high-frequency waveguide  13   b,  and has a high resistance of 10 kΩ or higher when a voltage is applied thereto that causes current to flow in the reverse direction.  
         [0049]     Fixed resistor  14  has an actual constant resistance regardless of the direction of the current flowing therethrough and the magnitude of a voltage applied thereto, and is connected between high-frequency waveguide  13   a  and metal interconnect  16 . Fixed resistor  14  is made of high-resistance metal such as tantalum nitride or the like, and has a width of 5 μm, a length of 3 mm, and a thickness of 0.1 μm. Fixed resistor  14  may occupy a reduced area if it is folded into multiple layers. Each of two metal interconnects  16 ,  17  is made of metal such as aluminum, gold, or the like, and has a width of 20 μm and a thickness of 0.2 μm.  
         [0050]     Signal circuit  15  is connected to two metal interconnects  16 ,  17  for producing a signal to operate the RF switch, i.e., a signal to control a voltage applied to driver circuit  19  or a current flowing through driver circuit  19 . In the present embodiment, the directions from variable-resistance devices  11 ,  12  to high-frequency waveguide  13   b  are referred to as forward directions in which the resistance of variable-resistance devices  11 ,  12  is lower when current flows therethrough in those directions.  
         [0051]     Operation of the RF switch according to the present embodiment will be described below with reference to FIGS.  3 ( a ) and  3 ( b ). Those parts in FIGS.  3 ( a ) and  3 ( b ) which are identical to those shown in  FIG. 2  are denoted by identical reference characters.  
         [0052]      FIG. 3 ( a ) shows the manner in which a control signal is applied to signal circuit  15  to cause a current to flow clockwise in driver circuit  19 . At this time, since first variable-resistance circuit  11  is biased in the forward direction, first variable-resistance circuit  11  has a low resistance (r). Second variable-resistance circuit  12  has a high resistance (R) of 10 k Ω or higher because second variable-resistance circuit  12  is reverse-biased. A high-frequency signal applied to high-frequency waveguide  13   a  passes through first variable-resistance circuit  11  with a low loss, and is output to high-frequency waveguide  13   b.  As the high-frequency signal does not leak into the branch line connected to second variable-resistance device  12 , the high-frequency signal passes through high-frequency waveguide  13   b  with a low loss. This state continues until a control signal is applied in the reverse direction to signal circuit  15 , and there is no need to keep applying the forward control signal in the meantime.  
         [0053]      FIG. 3 ( b ) shows the manner in which a control signal is applied to signal circuit  15  to cause a current to flow counterclockwise in driver circuit  19 . A voltage which is expressed by about R/(R+R′) of the voltage that was first applied to signal circuit  15  is applied to second variable-resistance circuit  12 , where R′ represents the resistance of fixed resistor  14 . If the resistances R, R′ are about 10 kΩ, then a voltage which is about one-half of the voltage that was first applied to signal circuit  15  is applied to second variable-resistance circuit  12 . Since the voltage is applied to second variable-resistance circuit  12  in the forward direction, the resistance of second variable-resistance circuit  12  changes to a small value (r). Because the resistance of second variable-resistance circuit  12  changes quickly, a large reverse voltage is applied across first variable-resistance circuit  11 , whose resistance changes to a large value (R). At this time, a high-frequency signal applied to high-frequency wave guide  13   a  is reflected by first variable-resistance circuit  11  and hence is not outputted to high-frequency waveguide  13   b.  The high-frequency signal does not leak into the branch line connected to fixed resistor  14 . Therefore, the high-frequency signal is unable to travel through high-frequency waveguide  13   b.  This state continues until a control signal is applied in the forward direction to signal circuit  15 , and there is no need to keep applying the reverse control signal in the meantime.  
         [0054]     A process for manufacturing the RF switch will be described below. FIGS.  4 ( a ) through  4 ( d ) are views showing successive steps of the process for manufacturing the RF switch. In each of FIGS.  4 ( a ) through  4 ( d ), the left figure is a plan view, and the right figure is a cross-sectional view taken along line A-A′ of the plan view.  
         [0055]     First, as shown in  FIG. 4 ( a ), the reverse side of glass substrate  30  is coated with a thin film of gold, providing ground layer  301 . A pattern of fixed resistor  34  is formed of chromium nitride to a thickness of 0.1 μm on the face side of glass substrate  30 . Then, the face side of glass substrate  30  is covered with a film of gold having a thickness of 0.3 μm, and a film of gold having a thickness of 1.7 μm is deposited only in those areas of the face side of glass substrate  30  which correspond to waveguides  33   a,    33   b.  Thereafter, patterns of waveguides  33   a,    33   b  and metal interconnects  36 ,  37  are formed as a resist on the surface formed. Using the resist as a mask, the film of gold having a thickness of 0.3 μm is etched away. Finally, the resist is removed.  
         [0056]     Then, as shown in  FIG. 4 ( b ), a thin film of copper having a thickness of 0.2 μm is deposited on glass substrate  30 . Thereafter, the surface of the thin film of copper is partly sulfurized into copper sulfide as follows: Substrate  30  is placed in a solution of sodium sulfide, and a power supply is connected to substrate  30  such that the thin film of copper on substrate  30  is positively biased with respect to the solution of sodium sulfide. The power supply is set to cause a current of about 100 μA to flow, thus forming a film of copper sulfide to a thickness of 20 nm. Thereafter, the film of copper sulfide and the film of copper are etched away to form patterns  313 ,  323  respectively on waveguide  33   a  and metal interconnect  37 .  
         [0057]     Then, as shown in  FIG. 4 ( c ), a film of silicon nitride having a thickness of about 0.3 μm is deposited on substrate  30 , forming patterns of insulating films  315 ,  325 . At the same time, contact holes  314 ,  312  are formed in the pattern of insulating film  315  so as to expose the surface of pattern  313  of copper sulfide and the surface of waveguide  33   b  of gold through contact holes  314 ,  31 . 2 , and contact holes  324 ,  322  are formed in the pattern of insulating film  325  so as to expose the surface of pattern  323  and the surface of waveguide  33   b  through contact holes  324 ,  322 .  
         [0058]     Then, as shown in  FIG. 4 ( d ), a film of gold having a thickness of 0.3 μm is deposited on substrate  30 , and then a film of gold having a thickness of about 1.7 μm is deposited only in the area which corresponds to upper electrode  311  by electric plating. Thereafter, using a resist as a mask, the deposited film of gold is etched away to form patterns of upper electrodes  311 ,  321 . Finally, the resist is removed. Upper electrode  311  is electrically connected to pattern  313  and waveguide  33   b  through contact holes  314 ,  312  in insulating film  315 . Upper electrode  321  is electrically connected to pattern  323  and waveguide  33   b  through contact holes  324 ,  322  in insulating film  325 .  
         [0059]     In the embodiment shown in  FIG. 2 , it is possible to switch around the positions of first variable-resistance device  11  and contact hole  112 . According to such a modification, variable-resistance layer  113  is formed by successively depositing a layer of copper sulfide and a layer of copper on high-frequency waveguide  13   b.  First variable-resistance device  11  changes its resistance in the same manner as described above depending on the direction of current flowing therethrough. Similarly, it is possible to switch around the positions of second variable-resistance device  12  and contact hole  122  while allowing second variable-resistance device  12  to change its resistance in the same manner as described above.  
         [0060]     Variable-resistance devices  11 ,  12  do not need to be provided in each of the junctions of a circuit. Variable-resistance devices of the above structure may be provided on both ends of a junction of a circuit while being allowed to change their resistance in the same manner as described above depending on the direction of current flowing therethrough.  
         [0061]     Second variable-resistance device  12  of driver circuit  19  may be connected to upper electrode  111  rather than high-frequency waveguide  13   b.  According to this modification, upper electrode  111  and upper electrode  121  may be connected directly with each other without the need for a contact hole.  
         [0062]     In the above embodiment, the directions from variable-resistance devices  11 ,  12  to high-frequency waveguide  13   b  are referred to as forward directions in which the resistances of variable-resistance devices  11 ,  12  are low when a current flows therethrough in those directions. However, variable-resistance devices  11 ,  12  may be oriented such that the resistances of variable-resistance devices  11 ,  12  are high when current flows therethrough in both directions from variable-resistance devices  11 ,  12  to high-frequency waveguide  13   b.  Such an alternative arrangement offers the same advantages as described above.  
         [0063]      FIG. 5  is a schematic view showing the equivalent circuit of a RF switch according to the second embodiment of the present invention. Those parts in  FIG. 5  which are identical to those shown in FIGS.  3 ( a ) and  3 ( b ) are denoted by identical reference characters. According to the second embodiment, fixed resistor  44  and second variable-resistance device  42  are disposed in the respective positions of variable-resistance device  12  and fixed resistor  14  of the RF switch according to the first embodiment. The forward direction of second variable-resistance device  42  is opposite to the forward direction of second variable-resistance device  12  according to the first embodiment.  
         [0064]     Operation of the RF switch according to the present embodiment will be described below with reference to FIGS.  6 ( a ) and  6 ( b ). Those parts in FIGS.  6 ( a ) and  6 ( b ) which are identical to those shown in  FIG. 5  are denoted by identical reference characters.  
         [0065]      FIG. 6 ( a ) shows the manner in which a control signal is applied to signal circuit  15  to cause current to flow clockwise in driver circuit  19 . At this time, since first variable-resistance circuit  11  is biased in the forward direction, it has a low resistance (r). Second variable-resistance circuit  42  has a high resistance (R) of 10 kΩ or higher because second variable-resistance circuit  42  is reverse-biased. A high-frequency signal applied to high-frequency waveguide  13   a  passes through first variable-resistance circuit  11  with a low loss, and is outputted to high-frequency waveguide  13   b.  As the high-frequency signal does not leak into the branch lines connected to second variable-resistance device  42  and fixed resistor  44 , the high-frequency signal passes through high-frequency waveguide  13   b  with a low loss. This state continues until a control signal is applied in the reverse direction to signal circuit  15 , and there is no need to keep applying the forward control signal in the meantime.  
         [0066]      FIG. 6 ( b ) shows the manner in which a control signal is applied to signal circuit  15  to cause a current to flow counterclockwise in driver circuit  19 . A voltage which is expressed by about R/(R+R′) of the voltage that was first applied to signal circuit  15  is applied to second variable-resistance circuit  42 , where R′ represents the resistance of fixed resistor  44 . If the resistances R, R′ are about 10 kΩ, then a voltage which is about one-half of the voltage that was first applied to signal circuit  15  is applied to second variable-resistance circuit  42 . Since the voltage is applied to second variable-resistance circuit  42  in the forward direction, the resistance of second variable-resistance circuit  42  changes to a small value (r). As the resistance of second variable-resistance circuit  42  changes quickly, a large reverse voltage is applied across first variable-resistance circuit  11  whose resistance changes to a large value (R). At this time, a high-frequency signal applied to high frequency waveguide  13   a  is reflected by first variable-resistance circuit  11  and hence is not outputted to high-frequency waveguide  13   b.  The high-frequency signal is unable to enter high-frequency waveguide  13   b  through the branch line connected to fixed resistor  44  because of fixed resistor  44 . This state continues until a control signal is applied in the forward direction to signal circuit  15 , and there is no need to keep applying the reverse control signal in the meantime.  
         [0067]     In the present embodiment, the directions from variable-resistance devices  11 ,  42  to high-frequency waveguide  13   a  are referred to as reverse directions in which the resistances of variable-resistance devices  11 ,  12  are large when current flows therethrough in those directions. However, variable-resistance devices  11 ,  42  may be oriented such that the resistances of variable-resistance devices  11 ,  42  are low when current flows therethrough in both directions from variable-resistance devices  11 ,  42  to high-frequency waveguide  13   a.  Such an alternative arrangement offers the same advantages as described above with respect to the previous embodiment.  
         [0068]      FIG. 7  is a schematic view showing the equivalent circuit of a RF switch according to the third embodiment of the present invention. Those parts in  FIG. 7  which are identical to those shown in FIGS.  3 ( a ) and  3 ( b ) are denoted by identical reference characters. According to the third embodiment, variable-resistance device  62  is disposed in the position of resistor  14  of the RF switch according to the first embodiment. Therefore the driver circuit has two variable-resistance devices  12 ,  62 . The directions from variable-resistance devices  11 ,  12  to high-frequency waveguide  13   b  are referred to as forward directions in which the resistances of variable-resistance devices  11 ,  12  are low when current flows therethrough in those directions. The directions from variable-resistance devices  11 ,  62  to high-frequency waveguide  13   a  are referred to as reverse directions in which the resistances of variable-resistance devices  11 ,  62  are high when current flows therethrough in those directions. This arrangement offers the same advantages as described above with respect to the previous embodiments.  
         [0069]     Alternatively, variable-resistance devices  11 ,  12  may be oriented such that the resistances of variable-resistance devices  11 ,  12  are high when current flows therethrough in the directions from variable-resistance devices  11 ,  12  to high-frequency waveguide  13   b,  and variable-resistance devices  11 ,  62  may be oriented such that the resistances of variable-resistance devices  11 ,  62  are low when current flows therethrough in the directions from variable-resistance devices  11 ,  62  to high-frequency waveguide  13   a.  Such an alternative arrangement offers the same advantages as described above with respect to the previous embodiments.  
         [0070]      FIG. 8  is a schematic view showing the equivalent circuit of a RF switch according to the fourth embodiment of the present invention. Those parts in  FIG. 8  which are identical to those shown in FIGS.  3 ( a ) and  3 ( b ) are denoted by identical reference characters. According to the fourth embodiment, the RF switch has no fixed resistor and has bias circuit  70  connected to high-frequency waveguide  13   a.  Bias circuit  70  comprises bypass capacitor  701  and bias coil  702  having an end connected to one terminal of bypass capacitor  701 . The other end of bias coil  702  is connected to a certain bias voltage source or a ground potential. Bias circuit  70  may be included in the RF switch or may be external and connected to the RF switch. Signal circuit  15  has a terminal connected to second variable-resistance device  12  and another terminal connected to a ground or a bias potential source. In the present embodiment, the directions from variable-resistance devices  11 ,  12  to high-frequency waveguide  13   b  are referred to as forward directions in which the resistances of variable-resistance devices  11 ,  12  are low when current flows therethrough in those directions. This arrangement offers the same advantages as described above with respect to the previous embodiments.  
         [0071]     Alternatively, variable-resistance devices  11 ,  12  may be oriented such that the resistances of variable-resistance devices  11 ,  12  are high when current flows therethrough in the directions from variable-resistance devices  11 ,  12  to high-frequency waveguide  13   b.    
         [0072]     In the above embodiments, the variable-resistance devices have a variable-resistance layer including a layer of copper sulfide. However, the variable-resistance layer is not limited to copper sulfide, but may be made of a compound of calcogenide (arsenic, germanium, selenium, tellurium, bismuth, nickel, sulfur, polonium, zinc, etc.) and a metal belonging to groups I, II of the periodic table.  
         [0073]     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.