This application is a continuation and claims the benefit of U.S. application Ser. No. 10/044,522, filed Jan. 11, 2002, now U.S. Pat. No. 6,737,930. U.S. application Ser. No. 10/044,522 claimed the benefit of U.S. Provisional Application 60/283,093, filed Apr. 11, 2001 and incorporated Provisional Application 60/283,093 by reference. U.S. Provisional Application 60/283,093 is hereby incorporated by reference in this application. In addition, this application relates to U.S. applications Ser. No. 09/904,631, “Tunable Ferro-Electric Filter,” filed on Jul. 13, 2001; 09/912,753, “Tunable Ferro-Electric Multiplexer,” filed on Jul. 24, 2001; 09/927,732, “Low Loss Tunable Ferro-Electric Device and Method of Characterization,” filed on Aug. 8, 2001; and 09/927,136, “Tunable Matching Circuit,” filed on Aug. 10, 2001, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Description of Related Art 
     Capacitors are commonly used in filters for wireless communication. Capacitors with capacitances in the range of 0.5 to 10.0 pF are typically employed in radio frequency signal paths to set resonant frequencies of filters to specific values. Additionally, capacitors are typically employed in matching circuits to match impedances between components in wireless communication devices. A capacitor, in fact, is a fundamental component in electrical circuit design. As is well known in the art, capacitors can be found in many circuits throughout electronic industries and wherever electronic circuits are required. 
     Referring specifically to filters for use in wireless communication devices, related application Ser. No. 09/904,631, discloses a tunable capacitor that has been developed for tuning the resonant frequency of a filter for use at different frequencies. Tunability can be achieved by applying a variable bias electric field to a ferro-electric (FE) material located in the field induced by the capacitor. FE materials have a dielectric constant that varies with the bias electric field. As the dielectric constant varies, the capacitance of the capacitor varies. This changes the resonant frequency of the filter. 
     As disclosed in patent application Ser. No. 09/904,631, there are three basic types of capacitors in common use: gap capacitors, overlay capacitors and interdigital capacitors. Gap capacitors and interdigital capacitors are both planar structures. That is, both electrodes of the capacitors are in the same plane. Overlay capacitors have electrodes that are in different planes, that is, planes that overlay each other. Typically, overlay capacitors can develop higher capacitances, but they are harder to fabricate than planar capacitors. Thus, this invention is focused on improving the biasing scheme for planar capacitors. The discussion below is directed to gap capacitors, but it will be understood that the methods and devices described herein apply equally to all planar capacitors. 
     It has proven difficult to apply the variable electric field to the FE material in RF applications without introducing (1) increased loss, (2) circuit complexity or (3) circuit size, or a combination of these three. The variable electric field is applied by applying a variable DC voltage to the FE material. Typically, in a planar capacitor, FE material is placed between the electrodes of the capacitor and the substrate. Thus, the FE layer is formed on the substrate. The capacitor electrodes are formed on the FE layer, with a gap between the electrodes, forming the capacitor. 
     One way of applying the DC voltage is to connect the DC voltage source to an electrode of the capacitor through a resistor. Often, a DC blocking capacitor must be used in the RF signal path so as to provide an RF ground for example, to the FE capacitor without shorting out the DC bias applied. The DC blocking capacitor invariably introduces added loss into the RF signal. This increased loss results in a lower signal to noise ratio for receive applications, which results in dropped communications, and increased power consumption in transmit applications, among other things. Additionally, the resistor and the DC blocking capacitor add to the cost, size and complexity of the device that the capacitor is used in. Thus, this method of applying the variable DC electric filed to the FE material is not an optimal solution. 
     While planar FE capacitors are relatively simple to fabricate, they require a larger DC bias voltage to tune, as the gap dimensions are necessarily large (typically greater than or equal to 2.0 microns) due to conventional patterning constraints. Overlay FE capacitors, alternatively, can be tuned with a minimum DC voltage, as the plate separation can be made quite small (about 0.1 micron FE film thickness is possible and greater than about 0.25 microns is typical). Thus, the required DC bias field strength can be a factor of 20 to 40 times smaller for an overlay capacitor than for a gap capacitor. Furthermore, most all of the DC bias field is constrained within the FE film in an overlay capacitor. This is not true in a gap or interdigital capacitor, where a significant portion of the DC bias field is located outside of the FE film. 
     One significant problem with overlay capacitors is that they are more difficult to fabricate than gap capacitors, as they are multi-layer structures. They typically need a common bottom electrode on which the desired FE thin film is deposited. Ideally the desired metals for the bottom electrodes are typically the low loss noble metals like gold, silver or preferably copper. The deposition requirements for most FE films, however, would cause the unacceptable formation of metal oxides. To prevent unwanted oxidation, the deposition of a high refractory metal, such as platinum as a cap, or covering, layer is needed, which adds an extra mask or layer as well as increases cost. Additionally, the bottom layer metal thickness should be increased to greater than about 2.0 skin depths, to minimize the metal loss in the bottom electrode. 
     Rather than relying on overlay capacitors, a compromise solution is to introduce a pair of bias electrodes into the vicinity of the gap of a planar capacitor. One version would pattern one bias electrode in the gap itself and place the other electrode between the substrate and the FE layer. The variable DC electric field is applied to the FE material by putting bias electrodes in the form of doped silicon on both sides of the FE material. Thus, a first doped silicon layer is formed on the substrate. A FE layer is formed on the first doped silicon layer. The capacitor electrodes are formed on the FE layer. A second doped silicon layer is formed inside the gap region of the capacitor. The bias voltage is applied to the second doped silicon layer and the first doped silicon layer is grounded, or vice versa. This approach is not preferred, as it requires the presence of two bias electrodes, one above and one below the FE layer as well as the presence of a bias electrode between the RF electrodes in the gap capacitor. 
     Further, the gap typically must be widened to make room for the bias electrode between the two RF (capacitor) electrodes. Widening the gap reduces the capacitance of the structure. To bring the capacitance back to a useful level, the capacitor must be made wider. This increases the size and cost of the capacitor. Additionally, it is difficult and costly to manufacture a gap capacitor with a conducting layer of doped silicon in the gap, since one must provide added grounding as well as bias for a two layer bias scheme. 
     Accordingly, it would be beneficial to have a tunable FE capacitor with a less complex, cheaper and smaller bias scheme for applying the variable DC electric field to the FE material in a planar tunable capacitor. 
     SUMMARY 
     Variable capacitors using a variable DC voltage to tune the capacitance typically employ costly and overly large components to apply the variable DC voltage to the capacitor. Furthermore, at least one method of applying the variable DC voltage in the prior art introduces added signal loss into the RF signal path due to the need for a DC blocking capacitor. 
     Thus, it is an object of the present invention to provide methods and devices for applying a variable DC voltage to a tunable capacitor which introduce lower loss, lower cost and are smaller than those methods and devices previously available. 
     A bias electrode is positioned near a FE material. The capacitor electrodes are also positioned near the FE material, such that the capacitor electrodes and the bias electrode are not touching. There are only non-conductive materials, including possibly air, in the gap formed between the capacitor electrodes. The bias electrode is used to apply a variable DC voltage to the FE material. In a wide range of useful instances, one or both capacitor electrodes serve as a DC ground for producing a variable DC field between the bias electrode and the capacitor electrodes, thus eliminating the need for the extra DC blocking capacitor. Alternatively, one of the capacitor electrodes can be biased to, among other reasons, provide a modified capacitive response in that electrode. In other words, a single bias underlay electrode is added to a planar capacitor to achieve the biasing of the FE material. This allows for the elimination of biasing from either capacitor electrode. Alternatively, if bias is retained at either capacitor electrode, the underlay bias electrode allows for further biasing schemes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a tunable ferro-electric gap capacitor. 
         FIG. 2A  is a top view of a tunable ferro-electric gap capacitor. 
         FIG. 2B  is a circuit diagram equivalent of the tunable ferro-electric gap capacitor shown in  FIG. 2A . 
         FIG. 3  is a top view of a tunable ferro-electric gap capacitor, having a finger-like bias electrode. 
         FIG. 4  is a top view of a tunable ferro-electric gap capacitor, having a center portion of a bias electrode missing. 
         FIG. 5  is a diagram that shows the top view of an interdigital capacitor. 
         FIG. 6  is a diagram of a wireless communication device. 
     
    
    
     DETAILED DESCRIPTION 
     A tunable gap capacitor is formed on a substrate. A bias electrode is positioned between the substrate and the capacitor electrodes. Only non-conductive material is in the gap between the capacitor electrodes. Between the bias electrode and the capacitor electrodes is a FE material for tuning the capacitance of the capacitor. 
     In other words, only one bias electrode is introduced, as an underlay, beneath the FE film layer deposited on the base substrate. In this configuration the RF electrodes provide the DC return paths for the DC bias signal. In this realization there is no need for an external DC blocking capacitor as the DC bias introduced in this manner is inherently isolated from the rest of the circuit. A further advantage of this arrangement is that one need not increase the gap in the gap capacitor to accommodate the presence of a two layer bias electrode structure. Thus the most compact gap capacitor realization can be obtained in this manner. 
     The gap capacitor will now be described with reference to  FIG. 1 .  FIG. 1  is a side view of a tunable FE capacitor  10 . A substrate  12  is shown. The substrate  12  is typically a low loss ceramic material such as magnesium oxide, sapphire, or some other such similar material on which the desired FE film can be deposited, preferably without the need for an adhesion or buffer layer. The substrate can also be a more lossy material like silicon dioxide, alumina or a printed circuit board material such as the well known material, FR4 as long as one can tolerate the added loss arising from its use, along with the added cost and complexity of using one or more buffer layers or an adhesion layer that may be necessary with these substrates. 
     Formed on the substrate  12  is a bias electrode  14 . The bias electrode  14  is preferably doped silicon, as it can have a much lower conductivity than any metal, and its conductivity can be controlled by doping. Alternatively, the bias electrode  14  can be a metal such as gold, silver, platinum or copper. Over the bias electrode  14  is a FE layer  16 . The FE layer  16  provides the tunability to the capacitor. Over the FE layer  16  are the capacitor electrodes  21  and  24 . The capacitor is part of a RF signal path. The capacitor electrodes  21  and  24  define a space between the electrodes called a gap  47 . The gap  47  is defined by the electrodes. The gap  47  is shown as a dotted line. The dotted line is separated somewhat from the solid line defining the capacitor electrodes  21  and  24 . This is for the sake of distinguishing between the lines defining the gap  47  and the electrodes  21  and  24 , not to indicate that there is any space between the gap  47  and the electrodes  21  and  24 . The gap  47  and the electrodes  21  and  24  are coterminous. 
     The gap capacitor will now be described with reference to  FIG. 2A .  FIG. 2A  is a top view of the gap capacitor. A first capacitor electrode  43  and a second capacitor electrode  45  form a capacitor gap  47 . In one implementation, the second electrode  45  is positioned within 3.0 microns of the first electrode  43 . A ferro-electric material  53  lies preferably underneath the first and second capacitor electrodes  43  and  45 . The ferro-electric material  53  could alternatively lie over the top of the first and second capacitor electrodes  43  and  45  assuming the proper precautions are taken to prevent the oxidation or melting of the metal traces  43  and  45  during the deposition of the FE film on top of the electrodes. Due to these limitations, the FE film will almost always be under the RF metal contacts,  43  and  45 . In one implementation, the FE material  53  comprises barium strontium titanate and is formed in a layer having a thickness equal to about one micron. 
     A bias electrode  55  lies preferably underneath the ferro-electric material  53 . The bias electrode  55  is preferably more narrow than the ferro-electric material  53 , so that the bias electrode  55  does not make electrical contact with the first or second capacitor electrodes  43  and  45 . 
     In some cases, it may be desirable to have a bias electrode of sufficient size and electrical thickness relative to the gap region that some noticeable capacitance exists between the capacitor electrodes and the bias electrode. An example of this is in the case where the bias electrodes extends underneath the capacitor electrodes as shown in  FIG. 1 . In this case, the electrical equivalent circuit is shown in  FIG. 2B . 
     In  FIG. 2B , a capacitor  44  is shown coupled between two terminals  46  and  48 . The capacitor  44  represents the capacitance developed between the capacitor terminals  43  and  45  of  FIG. 2A . The terminals  46  and  48  represent the capacitor electrodes  43  and  45  shown in  FIG. 2A . A third terminal  50  represents the bias electrode  55  shown in  FIG. 2A . Two other capacitors  52  and  54  are shown coupled between the terminals  46  and  48  and the third terminal  50 . The other capacitors  52  and  54  represent capacitances developed between the capacitor electrodes  43  and  45  shown in  FIG. 2A  and the bias electrode  55  shown in  FIG. 2A . 
     Depending on geometry and the materials used, the capacitances of capacitors  52  and  54  may be negligible, or not, when zero volts is applied to the bias electrode  55 . Also, capacitors  52  and  54  may have some non-negligible tuning characteristics, as the bias voltage applied to bias electrode  55  is varied. 
     Additionally, a voltage may be applied to either terminal  46  or  48 , in addition to the voltage applied to terminal  50 . This further modifies the tuning characteristics of the complete device shown in FIG.  2 B. In other words, there are two voltage differences that can be manipulated. The two differences are (1) between terminal  46  and terminal  50  and (2) between terminal  48  and terminal  50 . By varying the geometries and electrode materials different tuning characteristics can be achieved without changing FE materials and thicknesses. One drawback of the embodiment employing a bias voltage at either terminal  46  or  48  is, as already stated, that a DC blocking capacitor is then required. A DC blocking capacitor increases RF loss. 
     The bias electrodes need not be rectangular, as shown in  FIG. 2 . Preferably, the bias electrode has more than one finger as shown in  FIG. 3 . Alternatively, the bias electrode may have a portion removed from its center, a shown in  FIG. 4 . These shapes further reduce the loss introduced by the bias electrode by reducing any RF coupling to the bias electrode. 
     A preferred bias electrode shape will now be described with reference to  FIG. 3 . There are two capacitor electrodes  63  and  65  defining a gap  67 . The bias electrode  80  is split into two fingers  72  and  74 . A finger is defined herein as a strip thinner than the whole object. Here it is used to mean a strip of bias electrode material thinner than the whole bias electrode. This limits the RF current that can flow in the bias electrode  70 , thereby reducing the loss in the bias electrode  70 . Alternatively, the bias electrode  70  may have more than two fingers (only two fingers  72  and  74  shown). Preferably, the finger width  76  is about 1 to 2 microns. 
     A joining member  70  connects the fingers. In another embodiment, not shown, the joining member  70  is not inside the gap  67 . The figners  72  and  74  are longer and the joining member  70  is outside the gap  67  on the side where the voltage is applied. It will be understood that many variations of this shape are possible. 
     The bias electrode  70  is adapted to be coupled to a voltage source  78  which is coupled to a control signal generator  83 . Note that the ferro-electric layer is not shown, to more clearly show the other items described. 
     Another bias electrode shape will now be described with reference to  FIG. 4 . Again, there are two capacitor electrodes  86  and  89  defining a gap  92 . The bias electrode  95  is similar in shape to the bias electrode  70  described with reference to  FIG. 3 . The bias electrode  95 , however, has its fingers connected at the ends. In other words, the bias electrode  95  is like a rectangular bias electrode, but with its center missing. Note that the shapes of bias electrodes described with reference to  FIGS. 2A ,  3  and  4  are simply by way of example. Other shapes, such as those having rounded corners, and asymmetrical shapes, would be within the spirit of the invention. 
     A variable DC voltage source  57  is coupled to the bias electrode  53  for applying a variable DC voltage to the bias electrode. Note that DC is intended to mean slowly varying with respect to a RF signal. The voltage on the capacitor electrodes will have some DC component. The DC component may be zero. The difference between the variable DC voltage applied to the bias electrode  53  and the DC component of the voltage in the capacitor electrodes  43  and  45  creates a DC electric field in the FE material  53 . The variable DC voltage applied to the bias electrode  55  can be varied to change the dielectric constant of the FE material  53 . This changes the capacitance of the capacitor. This changes the operating parameters of the device incorporating the capacitor, such as, for example, a filter or a matching circuit. 
     A control signal generator  59  is coupled to the voltage source  57  for controlling the voltage source  57 . The capacitor electrodes  43  and  45 , the bias electrode  55  and the ferro-electric material  53  are all located on a substrate  61 . The control signal generator  59  and the voltage source  57  may be located on the substrate  61  (as shown) or off the substrate  61  (not as shown). 
     The bias electrode  55  is electrically thin, preferably less than about 0.01 microns so that it is less than about 0.1 skin depths. The added RF loss arising from the presence of the bias electrode is minimal and its effect is offset by the advantage gained in fabrication and improved tuning. In one implementation, the RF signal has a frequency equal to about 2.0 GHz, and the bias electrode  55  causes a field attenuation of about 0.28 percent in the RF signal. 
     The capacitor may be a tuning capacitor for use in a transceiver in a wireless communication device Preferably, the capacitor tunes a multiplexer or other filter-type device as described in U.S. Patent Application “Tunable Ferro-electric Multiplexer.” The method of tuning the capacitor as described herein advantageously eliminates the need for a DC blocking capacitor and optionally eliminates the need for a DC bias resistor. 
     Alternatively, the capacitor may be used in conjunction with, or as part of, a tunable matching circuit as described in U.S. Patent Application, “Tunable Matching Circuit.” Again, a DC blocking capacitor and a DC resistor may be eliminated. 
     It will be apparent to one of ordinary skill in the art that the tunable capacitor can be used in many other electronic circuits. Such uses are within the scope of the invention. 
     Referring now to  FIG. 5 , a diagram  100  that shows the top view of an interdigital capacitor will be discussed. The diagram includes a base substrate  104 . In some implementations the base substrate  104  may be, for example, a low loss substrate. An FE layer  107  is shown. The FE layer  107  is typically deposited on top of the base substrate, then metalized. Typically the interdigital capacitor is then fabricated using either etch-back or lift-off techniques. The diagram  100  also shows an output reference plane  110  and an input reference plane  112 . Advantages of using FE capacitors may include, for example, the ability to tune over more than one band using fewer components. 
     Referring now to  FIG. 6 , a wireless communication device  150  will be discussed. The mobile wireless device  150  includes a cellular portion and a PCS portion. FE material may be used in some cases to improve the performance of each portion. In some cases, FE materials may be used to allow one set of circuitry to function over both the cellular and PCS frequencies. 
     The wireless communication device  150  includes an antenna system  152 . The antenna system  152  includes antenna  154 . The antenna  154  may radiate energy into space, a transmit antenna, or receive energy from space, a receive antenna. In some cases a single antenna may perform both functions. In other words, in some cases the antenna  154  may be both a receive antenna and a transmit antenna. 
     The antenna  154  is coupled to a matching circuit  157 . The matching circuit  157  is used to maximize power transfer from the antenna to other components in the wireless communication device  160 , for example the diplexer. Alternatively, the matching circuit may maximize the power transmitted from the rest of the wireless communication device  150  to the antenna  157 . It will be clear that in some cases the matching circuit may perform both functions described above. Additionally, it will be understood that while matching circuit are typically used to attempt to optimize conditions for power transfer from one part of a system to another, in some cases, the matching circuit may not perform ideally. For example, in some cases differences in components used may vary the efficiency of power transfer between the antenna  154  and the rest of the wireless communication device  150 . Additionally, in some cases other design constraints may lead to less than optimal implementations. 
     The matching circuit  157  is coupled to a diplexer  159 . The diplexer  159  is a device typically provides a constant impedance at its input terminal, and directs to one of two ports depending on the frequency of the signal. It is thus a frequency-selective network with a typically well-defined resistive input impedance. One port of the diplexer is connected to the cellular duplexer  162 . In some cases a duplexer is a two channel multiplexer that uses a transmit-receiver switch so that one antenna may be used for both reception and transmission. The switch typically protects the receiver from the high power of the transmission. In another other example of a duplexer may be a pair of filters. One filter is a band pass filter with a pass band in the received frequency range of a communication system. Typically this filter connects an antenna to a receiver in the communication system. The other filter is a band pass filter with a pass band in the transmit frequency range of the communication system. Typically this filter connects an antenna to a transmitter in the communication system. Typically a duplexer using filters is used, for example, in CDMA communication systems. 
     The cellular duplexer  162  is coupled to a cellular isolator  164 . The cellular isolator is coupled to the cellular matching circuit  166 . The cellular matching circuit  166  may in many cases perform a similar function to the matching circuit  157 . In many, if not most cases, however, the cellular matching circuit  166  will not be required to operate over as wide a range of frequencies as the matching circuit  157 . The cellular matching circuit  168  is coupled to the cellular power amplifier  168 . The cellular power amplifier  168  is used to increase the transmit power of a signal that is transmitted by the wireless communication device  150 . A transmit cellular band-pass filter  171  filters the signal before is is amplified by the amplifier  171 . The signal is generated in the cellular transmit chain  173 . 
     The cellular receive block shown in the wireless communication device  150  includes an cellular LNA  176  coupled to the cellular duplexer  162 . The cellular LNA  176  amplifies low power. signals received at the mobile wireless device  150 . The cellular LNA  176  is coupled to a receive cellular band pass filter  179 . The filter  179  limits the frequencies received by the cellular at receiver block  182 . 
     Similar components arranged in a similar configuration are shown for the PCS portion of the mobile wireless device  150 . The PCS portion includes a PCS duplexer  185 , a PCS isolator  187 , and a PCS matching circuit  189 . The functions of each of these components is typically similar to the corresponding cellular components discussed above, however, in some cases the frequency ranges of the components may be different. The mobile wireless device  150  also includes a PCS power amplifier  192 , a PCS transmit band pass filter, and a PCS transmit chain  196 . Again, the functions of each of these components is typically similar to the corresponding cellular components discussed above, however, in some cases the frequency ranges of the components may be different. 
     The receive part of the PCS portion of the mobile wireless device  150  includes a PCS LNA  198 , a PCS receive band-pass filter  201  and a PCS receiver block  203 . Each may function similar to the corresponding cellular devices described above. 
     It should be noted that the mobile wireless device  150  is only one possible example of a mobile wireless device. The mobile wireless device  150  includes both PCS and cellular circuitry. In some cases a mobile wireless device may not include cellular, in other cases the mobile wireless device may only include PCS. Other mobile wireless device standards may be developed that are not cellular or PCS. Some mobile wireless devices may not include all of the components shown. Future implementations of mobile wireless devices may be designed to using fewer components. Specifically, as discussed above, through the use of FE components the functionality of both PCS and cellular may eliminate the need for separate PCS and cellular circuitry. Additionally, it should be understood that mobile wireless device may be both transmit and receive, as shown; transmit only; or receive only.
A tunable capacitor that introduces significantly less loss, if any, costs less and is smaller than previously available. A bias electrode is coupled to a FE material. The capacitor electrodes are electro-magnetically coupled to the FE material, such that the capacitor electrodes and the bias electrode are not touching. Only non-conductive material is in the gap defined by the capacitor electrodes. The bias electrode is used to apply a variable DC voltage to the FE material. A capacitor electrode serves as a DC ground for producing a variable DC field between the bias electrode and the capacitor electrodes.