Abstract:
The present disclosure provides a method for fabricating a compound varactor. The method includes steps of depositing a collector layer, depositing a first base layer arranged in a first plurality of parallel fingers directly onto the collector layer, and depositing a second base layer arranged in a second plurality of parallel fingers that are interleaved with the first plurality of parallel fingers directly onto the collector layer.

Description:
[0001]    This application is a Divisional filing of U.S. utility patent application Ser. No. 14/485,532, filed Sep. 12, 2014, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    Embodiments of the present disclosure relate generally to the field of circuits, and more particularly to reverse-biased diodes (varactors). 
       BACKGROUND 
       [0003]    A diode under reverse bias may exhibit a capacitance that varies inversely with the applied voltage. A component that behaves in this manner, e.g., as a variable capacitor, may be termed a varactor. The variable capacitance of the diode may be used for “tuning” electrical circuits. Generally, semiconductor varactors may have a wider tuning range (e.g., capacitance variance) and lower control voltage requirements than dielectric varactors realized on materials such as barium strontium titanate (BST). However, the semiconductor varactors may typically exhibit a lower capacitance per unit area than a dielectric varactor, thereby requiring a larger die area to implement a given capacitance. 
         [0004]    Generally, a varactor may be considered a two-port device, e.g., having a single input terminal and a single output terminal. As such, varactors may be prone to self-modulation distortion resulting from applied radio frequency (RF) voltages. This self-modulation distortion may introduce nonlinearity into a circuit using the varactors. To reduce this nonlinearity to acceptable levels, a number of individual varactors may be coupled in series to divide the RF voltage across them. If the number of varactors in the series is n, then the die area on the circuit board required to realize a desired net capacitance may be increased by a factor of n2 if the varactors are co-planar to one another. If a relatively large number of varactors are used, then this circuit may make the required die area prohibitively large for use in modern devices. 
         [0005]    In some cases such as oscillator or voltage-controlled oscillator (VCO) circuits in a mobile device, a high quality factor of greater than approximately 50 for the varactor may be desirable for increasing efficiency or reducing battery drain of the mobile device. Generally, the quality factor may be considered to be a measurement of the reactance of the varactor (e.g., the impedance presented to an RF signal propagating through the varactor) compared to the resistance of the varactor. However, if the collector or sub-collector of the varactor has a relatively high resistivity, such a quality factor may be difficult to achieve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
           [0007]      FIG. 1 a    illustrates an example compound varactor circuit, with a parallel resistive bias network, in accordance with various embodiments. 
           [0008]      FIG. 1 b    illustrates another example of a compound varactor circuit with a resistive bias network, in accordance with various embodiments. 
           [0009]      FIG. 1 c    illustrates another example of a compound varactor circuit with a resistive bias network, in accordance with various embodiments. 
           [0010]      FIG. 2 a    illustrates an example circuit diagram of a series-connected pair of reverse-connected varactors, in accordance with various embodiments. 
           [0011]      FIG. 2 b    illustrates an example overhead view of a series-connected pair of equal reverse-connected varactors that may be used in the circuit of  FIG. 2 a   , in accordance with various embodiments. 
           [0012]      FIG. 2 c    illustrates an example overhead view of a non-equal series-connected pair of equal reverse-connected varactors that may be used in the circuit of  FIG. 2 a   , in accordance with various embodiments. 
           [0013]      FIG. 3  illustrates a cut-away view of the series-connected pair of equal reverse-connected varactors of  FIG. 2 b   , in accordance with various embodiments. 
           [0014]      FIG. 4 a    illustrates an equivalent-circuit model that may be used to study varactor Q dependence on sub-collector resistivity, in accordance with various embodiments. 
           [0015]      FIG. 4 b    illustrates a varactor with a square footprint and associated quality factor versus frequency dependence, in accordance with various embodiments. 
           [0016]      FIG. 4 c    illustrates a varactor with a wide aspect ratio footprint and associated quality factor versus frequency dependence, in accordance with various embodiments. 
           [0017]      FIG. 5 a    illustrates a simplified example of a circuit that includes a series-connected pair of reverse-connected varactors, in accordance with various embodiments. 
           [0018]      FIG. 5 b    illustrates a series connection of two series-connected pairs of equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. 
           [0019]      FIG. 5 c    illustrates a series connection of two series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. 
           [0020]      FIG. 5 d    illustrates an alternative series connection of two series-connected pairs of equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. 
           [0021]      FIG. 5 e    illustrates an alternative series connection of two series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. 
           [0022]      FIG. 6 a    illustrates a simplified example of a circuit that includes a plurality of series-connected pairs of reverse-connected varactors, in accordance with various embodiments. 
           [0023]      FIG. 6 b    illustrates an example a plurality of series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 6 a   , in accordance with various embodiments. 
           [0024]      FIG. 6 c    illustrates an alternative example of a plurality of series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 6 a   , in accordance with various embodiments. 
           [0025]      FIG. 7  illustrates a process for constructing a compound varactor, in accordance with various embodiments. 
           [0026]      FIG. 8  illustrates an alternative overhead view of a compound varactor, in accordance with various embodiments. 
           [0027]      FIG. 9  illustrates an alternative overhead view of a system that includes a plurality of compound varactors, in accordance with various embodiments. 
           [0028]      FIG. 10  illustrates an alternative overhead view of a system that includes a plurality of compound varactors, in accordance with various embodiments. 
           [0029]      FIG. 11  is a block diagram of an exemplary wireless communication device, in accordance with various embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Embodiments include apparatuses and methods related to a compound varactor. Generally, a compound varactor may refer to a compact configuration of two varactors. A first varactor in the compound varactor may include a collector layer and a first base layer that is arranged in a first plurality of parallel fingers. A second varactor in the compound varactor may include a second base layer arranged in a second plurality of parallel fingers, and the base layer may be coupled with the collector layer. In embodiments, the fingers of the base layers of the first varactor and the second varactor may be interleaved with one another. 
         [0031]    In some embodiments, the fingers of the first varactor may have a first width, and the fingers of the second varactor may have a second width that may be the same as or different than the first width. In some embodiments, a plurality of compound varactors may be coupled with one another in series or parallel. 
         [0032]    Various aspects of the illustrative embodiments will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that alternate embodiments may be practiced with only some of the described aspects. For purposes of explanation, specific devices and configurations are set forth in order to provide a thorough understanding of the illustrative embodiments. However, it will be apparent to one skilled in the art that alternate embodiments may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative embodiments. 
         [0033]    Further, various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present disclosure; however, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. 
         [0034]    The phrase “in one embodiment” is used repeatedly. The phrase generally does not refer to the same embodiment; however, it may. The terms “comprising,” “having,” and “including” are synonymous, unless the context dictates otherwise. 
         [0035]    In providing some clarifying context to language that may be used in connection with various embodiments, the phrases “A/B” and “A and/or B” mean (A), (B), or (A and B); and the phrase “A, B, and/or C” means (A), (B), (C), (A and B), (A and C), (B and C) or (A, B and C). 
         [0036]    The term “coupled with,” along with its derivatives, may be used herein. “Coupled” may mean one or more of the following. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements indirectly contact each other, but yet still cooperate or interact with each other, and may mean that one or more other elements are coupled or connected between the elements that are said to be coupled with each other. 
         [0037]    Various figures may depict various vertical stacks of layers which may be epitaxially deposited. The sizes, widths, or heights of the various layers are not drawn to scale, and should not be assumed to be limited to being identical to, or different from, one another unless explicitly indicated to be so in the description below. 
         [0038]    As noted above, in certain applications a varactor or varactors with a high quality factor may be desirable to maximize signal quality while minimizing battery drain. It has been observed in legacy varactors that by increasing the width of a varactor while reducing the length, the signal quality may be increased. However, there may be limits to the amount in which the width or length of the varactor may be altered based on the amount of space available in a given system or on a given circuit board. Also, the propagation of a radio frequency (RF) signal through the varactor may cause the varactor to self-modulate, which may be undesirable. 
         [0039]      FIG. 1 a    illustrates an example compound varactor circuit  100  with a parallel resistive bias network, in accordance with various embodiments. The circuit  100  may include a plurality of varactors such as varactors  105   a ,  105   b ,  105   c ,  105   d ,  105   e , or  105   f  (collectively varactors  105 ) generally positioned between an input terminal  110  and an output terminal  115 . In embodiments, each of varactors  105  may be identical to one another, while in other embodiments one or more of the varactors  105  may be different from another of the varactors  105 . Examples of differences between varactors  105  are discussed in greater detail below with respect to  FIG. 1 b    and elsewhere. In some embodiments, the input terminal  110  may be configured to receive a radio frequency (RF) signal that may then propagate through the circuit  100  to the output terminal  115  (or vice versa). In some embodiments, one or more of the varactors  105  may be connected in parallel with the input terminal  110  and the output terminal  115 , in which case the RF signal may not propagate through the varactor to the output terminal  115 . In some cases, the circuit  100  may be used in shunt across an RF line in which case the output terminal  115  may be coupled with ground. 
         [0040]    In some embodiments, each of the varactors  105  may have a “front” side and a “back” side.  FIG. 1 a    depicts the front side  107  and back side  109  of varactor  105   a . In embodiments the front side  107  of varactor  105   a  may be referred to as the “cathode” of varactor  105   a , and the back side  109  of varactor  105   a  may be referred to as the “anode” of varactor  105   a . In  FIG. 1 , each of the varactors  105  may have a front side and a back side (or cathode and anode), though specific designators in  FIG. 1  are omitted for each varactor for the sake of clarity. 
         [0041]    In some embodiments, two or more of the varactors  105  may be coupled with one another in a back-to-back configuration. Specifically, the anodes of the varactors may be coupled directly to one another. For example, varactors  105   b  and  105   c  may be considered to be in a back-to-back configuration as shown in  FIG. 1 a   . In other embodiments, the varactors  105  may be coupled with one another in a front-to-front configuration as shown in  FIG. 1 a   . Specifically, the cathodes of the varactors may be coupled directly to one another. For example, varactors  105   a  and  105   b  may be considered to be in a front-to-front configuration as shown in  FIG. 1 . 
         [0042]    In embodiments, the back sides of one or more of the varactors  105  may be coupled with ground  120 . Additionally, the front sides of one or more of the varactors  105  may be coupled with a DC power source  125 . The DC power source  125  may be configured to provide a positive control voltage (VcTRL) to reverse bias the varactors  105 . In some embodiments, VcTRL may be between approximately 2 Volts (V) and approximately 18 V, while in other embodiments VcrnL may be between approximately −1.2 V and approximately 3 V. In other embodiments (not shown), the front sides of the varactors  105  may be coupled with ground  120 , and the back sides of the varactors  105  may be coupled with a DC power source  125 . In those embodiments, the DC power source  125  may be configured to provide a negative VcrnL to reverse bias the varactors  105 . Other more complicated circuits may be envisioned having multiple DC power sources that may each supply different or similar positive or negative voltages, or multiple ground connections. 
         [0043]    In embodiments one or more resistors such as resistors  135   a ,  135   b ,  135   c ,  135   d ,  135   e ,  135   g , and  135   f  (collectively resistors  135 ) may be positioned between the varactors  105  and the ground  120  or the DC power source  125 . In some embodiments, the resistance of each of the resistors  135  may be equal while in other embodiments certain of the resistors  135  such as outer resistors  135   f  or  135   g  may be greater than others of the resistors  135 . For example, in embodiments, the resistance of resistors  135   f  and/or  135   g  may be approximately 60 kilo-ohms (kO), while in other embodiments the resistance of resistors  135   f  and/or  135   g  may be between approximately 20 kO and approximately 60 kO. Similarly, in some embodiments the resistance of resistors  135   a ,  135   b ,  135   c ,  135   d , or  135   e  may be approximately 30 kO, while in other embodiments the resistance of resistors  135   a ,  135   b ,  135   c ,  135   d , or  135   e  may be between approximately 10 kO and approximately 30 kO. 
         [0044]    As shown above, the circuit  100  may include a number of varactors  105  and resistors  135 . Although only six varactors  105  and five resistors  135  are shown in  FIG. 1 , in other embodiments the circuit  100  may include a greater or lesser number of varactors  105  or resistors  135 . In some embodiments, it may be desirable for the circuit to include at least the resistors  135   a  and  135   e . In some embodiments, inductors may also be used in place of, or in combination with, the resistors  135 . As discussed above, as the number of varactors  105  in the compound varactor  100  increases, the area that the compound varactor  100  requires on a die may increase exponentially if all of the varactors  105  are co-planar to one another. 
         [0045]      FIG. 1 b    illustrates an example of a compound varactor circuit with an alternative resistive bias network. Specifically, the circuit  101  may include a series tree-type bias network in place of the parallel resistive bias network of  FIG. 1 a   . In embodiments, circuit  101  may include the input terminal  110 , DC power source  125 , ground  120 , and terminal  115 , and resistors  135  as described above with respect to  FIG. 1 a   . Circuit  101  may further include varactors  145  and  150 , which may be similar to varactors  105  of  FIG. 1 a   . In embodiments, each of varactors  145  and  150  may be similar to one another, while in other embodiments the varactors  145  may be similar to one another, but different from varactors  150 , as described below. 
         [0046]    Generally, circuits  100  and  101  may be more desirably used in shunt across the RF line, with output terminal  115  coupled with ground.  FIG. 1 c    illustrates an example of a compound varactor circuit  102  with an alternative resistive bias network. Specifically, circuit  102  may include a series tree-type resistive bias network with a symmetric bias feed that may be more suitable for a varactor to be used in series with the RF line. Specifically, circuit  102  may include the input terminal  110 , DC power source  125 , ground  120 , and terminal  115 , and resistors  135  as described above with respect to  FIG. 1 b   . Circuit  102  may further include varactors  145  and  150 , which may be similar to varactors  145  and  150  of  FIG. 1 b   . In embodiments, each of varactors  145  and  150  may be similar to one another, while in other embodiments the varactors  145  may be similar to one another, but different from varactors  150 , as described below. 
         [0047]    Generally, any of circuits  100 ,  101 , and  102  may be used if DC power source  125  is coupled to the back sides of varactors  105 ,  145 , and  150  and configured to provide a negative bias control voltage, as described above. 
         [0048]    Typically, in a varactor such as one of varactors  105 ,  145 , or  150 , the top region of the varactor (e.g., base or anode) may be coupled with a metalized layer of Aluminum (Al), Copper (Cu), Gold (Au), or some other metal or alloy with a relatively low resistivity. This metalized layer of the varactor may be used as an electrode that has a very low resistance, which results in very low loss of RF energy flowing through the varactor. 
         [0049]    In contrast, the bottom region of the varactor (e.g., collector or cathode) may be coupled with a sub-collector doped region. Typically, the resistivity of the collector and/or sub-collector may have a resistivity that is significantly higher than that of the metalized layer. For example, in some embodiments the resistivity of the collector and/or sub-collector may be an order of magnitude greater than the resistivity of the metalized layer. 
         [0050]    The quality factor Q of a varactor may be defined in terms of the angular frequency ω (2π*f, where f may be the frequency of the RF signal passing through the varactor), C (the varactor capacitance), and R (a resistive component). Specifically, the quality factor Q may be 
         [0000]    
       
         
           
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         [0051]    Generally, R may be an equivalent series resistance value that may account for dissipative losses in the varactor. In high Q varactors, this resistive component may arise primarily based on resistance in the bottom electrode of the varactor, e.g., the collector/sub-collector. 
         [0052]    To model the losses associated with the high resistivity of the collector/sub-collector region, an equivalent circuit model  400  such as that illustrated in  FIG. 4 a    may be used. Specifically, the model  400  may include a plurality of capacitors C and resistors R. Using the model  400 ,  FIG. 4 b    shows the predicted quality factor dependence over frequency of a varactor  405  with a square footprint. 
         [0053]    The model prediction for the quality factor dependence over frequency of a varactor  410  with an equivalent footprint, but having a wide aspect ratio, is shown for comparison in  FIG. 4 c   . It can be seen that a varactor layout with a wider form factor may have a substantially improved quality factor Q. This improved quality factor Q may be due to the shorter propagation distance in the collector/sub-collector between ports  1  and  2 . In general, if the varactor area and capacitance is kept constant, the quality factor Q of the varactor may have the following dependence on the width of the varactor: QαW 2 . Therefore, making the varactor as wide as possible in a direction normal to the input and output ports may maximize the quality factor Q of the varactor. However, in practice there may be limits on how wide the varactor can be made. 
         [0054]      FIG. 2 a    illustrates an example circuit for a compound varactor  290  that includes a pair of reverse-biased varactors having a series connection with an increased quality factor Q. Specifically, the compound varactor  290  may include a first varactor  291  and a second varactor  292  in a front-to-front configuration. Additionally, in some embodiments circuit  200  may include one or more resistors such as resistors  135 , a ground connection such as ground  120 , a DC power source such as power source  125 , an input terminal such as input terminal  110 , and/or an output terminal such as output terminal  115  as shown in  FIG. 1   a.    
         [0055]      FIG. 2 b    illustrates an overhead view of a compound varactor  200  that may correspond to the compound varactor  290  and exhibit an increased quality factor Q, in accordance with various embodiments. As will be recognized, the compound varactor  200  may include a pair of front-to-front varactors that both include a plurality of electrode fingers. As noted above, the collector/sub-collector of the compound varactor  200  may have a relatively high resistance compared to the resistance of the electrode fingers. By implementing two front-to-front varactors with a plurality of electrode fingers, the distance that the signal is required to travel through the collector/sub-collector layer may be reduced or minimized, which may result in a significant reduction in resistance experienced by an RF signal traveling through the compound varactor  200 . This reduction in resistance may result in an increase in the quality factor Q. 
         [0056]    An additional advantage of the compound varactor  200  may be that in certain legacy embodiments only a single varactor may be implemented such that electrode fingers are used for a first port of the varactor, and the second port of the varactor may be coupled directly with the collector/sub-collector layer through one or more vias. This legacy design may exhibit a significantly decreased capacitance per unit area, and an increased quality factor Q, because the second port that is coupled directly with the collector/sub-collector layer may not result in a measurable capacitance because the corresponding base layer in that region may be missing. In other words, the space where the second port is coupled directly with the collector/sub-collector layer may not have a significant electrical effect on the RF signal traveling through the varactor. By contrast, in embodiments herein, the base layer may be continuous under all of the electrode fingers. As such, the compound varactor  200  may experience only a minimal reduction in the capacitance per unit area based on the fingers of the electrodes, resulting in a significant advantage in terms of both size and cost of the invention. This reduction in capacitance per unit area may be a result of the small but necessary gaps between the electrodes. 
         [0057]    In embodiments, the compound varactor  200  may include two separate varactors  205  and  210  (that may respectively correspond to varactors  291  and  292 ), and have a Length and a Width as designated in  FIG. 2 b   . In embodiments, varactors  205  and  210  may be similar to one of varactors  105 . Specifically, the first varactor  205  may include a collector layer  215 . The collector layer  215  may be composed of or include a semiconductor material such as gallium arsenide, silicon, germanium, aluminum phosphide, aluminum arsenide, indium phosphide, gallium nitride, combinations or alloys thereof, or some other semiconductor. In some embodiments, the collector layer  215  may be doped or heavily doped with one or more impurities such as carbon, zinc, beryllium, or some other dopant. In embodiments, the collector layer  215  may be doped with an “n+” dopant. 
         [0058]    The varactor  205  may include a base layer  220  directly coupled with the collector layer  215 . The base layer  220  may be composed of or include one or more of the semiconductor materials discussed above, but the base layer  220  may be doped with a “p+” dopant. The varactor  205  may further include a first metal layer  225  directly coupled with the base layer  220 . The first metal layer  225  may be composed of or include titanium, platinum, gold, zinc, nickel, beryllium, or combinations or alloys thereof. The varactor  205  may further include a second metal layer  230  directly coupled with the first metal layer  225 . The second metal layer  230  may be composed of or include one or more of the same materials listed above as the first metal layer  225 . In some embodiments, the first metal layer  225  and second metal layer  230  may be composed of identical materials, while in other embodiments the first metal layer  225  and second metal layer  230  may be composed of different materials. In some embodiments, vias  235  may communicatively connect two or more of the base layer  220  or the metal layers  225  and  230 . In some embodiments, one or both of the metal layers  225  or  230  may not be included in compound varactor  200 . 
         [0059]    The second varactor  210  may be similar to the first varactor  205 , and include the collector layer  215 , a base layer  240 , a first metal layer  245 , and a second metal layer  250 , which may be similar to base layer  220 , first metal layer  225 , and second metal layer  230 , respectively. In embodiments, vias  235  may electrically couple one or more of base layer  240 , metal layer  245 , and metal layer  250 , as described above. In some embodiments, the collector layer  215  may include a sub-collector layer (not shown) that is directly coupled with the collector layer  215  on a side of the collector layer  215  directly opposite the base layers  220  and  240 . It will be understood that although each of the layers is depicted in  FIG. 2  as smaller than or inside of one or more other layers, such depiction is done for the ease of understanding the relative positioning of the layers, and in some embodiments different layers such as the base layer  220  and metal layer  225  may have approximately similar or identical lengths or widths. In some embodiments, the collector layer  215  may be coupled with a bias input  280  that may be coupled with and configured to receive a voltage bias from, for example, DC power source  125 . 
         [0060]    In some embodiments, one or more of the base layer  220  or metal layers  225  or  230  of the first varactor  205  may be constructed as a plurality of generally parallel fingers  260  that define one or more lateral cavities or spaces  265 . Similarly, one or more of the base layer  240  or metal layers  245  or  250  of the second varactor  210  may be constructed as generally parallel fingers  270  that define a plurality of spaces  275 . Fingers  260  and  270  may be the “electrode fingers” discussed above. As shown, fingers  270  may be positioned in spaces  265 , and fingers  260  may be positioned in spaces  275  such that the fingers  260  and  270  are considered to be interspersed or interleaved with one another. 
         [0061]    In embodiments, the collector layer  215  may act as the cathode of varactors  205  and  210 , while the base layers  220  and  240  may act as the anodes of varactors  205  and  210 , respectively. That is, varactor  205  may be communicatively coupled with an input terminal such as input terminal  110  that is configured to provide an RF signal. The RF signal may propagate through the layers of varactor  205  to the collector layer  215 . The RF signal may then propagate through the collector layer  215  and back up through the various layers of varactor  210 , which in turn is communicatively coupled with an RF output terminal such as output terminal  115 . 
         [0062]    In the example compound varactor  200 , the signal may propagate along the length of the compound varactor  200 . Specifically, as compound varactor  200  is depicted in  FIG. 2 b   , the RF signal may propagate from the top to the bottom of  FIG. 2 b    or vice-versa. As noted above, the collector layer  215  may have a relatively high resistance, for example, on the order of 6 Ohms per square. By contrast, the resistance of the base layers or metal layers of varactors  205  or  210  may be very low. Therefore, it may be more desirable for a signal to propagate primarily through the base layers  220  and  240  or metal layers  225 ,  230 ,  245 , and  250  of varactors  205  or  210  such that the distance that the signal has to propagate through collector layer  215  is minimized. By constructing varactors  205  and  210  to include a plurality of fingers  260  and  270 , the signal may only travel through the collector layer  215  a short distance to one of fingers  270 , or from one of fingers  260 . 
         [0063]    In some embodiments, improved performance may be realized by using a combination of different valued varactors in a compound varactor or plurality of compound varactors in series and/or parallel with one another.  FIG. 2 c    illustrates an example overhead view of a non-equal series-connected pair of equal reverse-connected varactors in a compound varactor  201 . The compound varactor  201  may have a relatively high Q compared to, for example, compound varactor  200 . 
         [0064]    The compound varactor  201  may include first and second varactors  206  and  211 , which may be similar to first and second varactors  205  and  210 . Specifically, varactor  206  may include a collector layer  216 , a base layer  221 , a first metal layer  226 , and a second metal layer  231 , which may be respectively similar to collector layer  215 , base layer  220 , metal layer  225 , and metal layer  230 , respectively. Varactor  211  may include the collector layer  216 , base layer  241 , metal layer  246 , and metal layer  251 , which may be respectively similar to base layer  240 , metal layer  245 , and metal layer  250 . In embodiments, varactors  206  and  211  may include vias similar to vias  235 , and a bias input that may be similar to bias input  280 , both of which are not shown in  FIG. 2 c    for the sake of clarity. In embodiments, varactor  206  may include fingers  261 , and varactor  211  may include fingers  271 . However, as can be seen in  FIG. 2 c   , in embodiments fingers  261  may be significantly narrower than fingers  271 . It will be understood that in other embodiments fingers  271  may be significantly narrower than fingers  261 . 
         [0065]    In compound varactor  201 , the difference in widths of fingers  261  and  271  may result in an unequal capacitance between varactors  206  and  211 . It should be noted that the number, width, and length of the fingers  261  and  271  may not be uniquely constrained, but may be flexible parameters that may be optimized to best achieve desired performance parameters. In  FIG. 2 c   , for example, each of the arrays is depicted as having three fingers. However, the capacitance of each of the varactors  206  and  211  may be solely dependent upon the total area of the array of fingers. Thus, the two series-connected capacitances may be equally-well achieved with five fingers, seven fingers, or some other number of fingers in each of the arrays in other embodiments. In the case where there are more fingers in a given varactor, the fingers may be narrower than shown in either compound varactors  200  or  201 . Alternatively, in cases where there are less fingers in a given varactor, the fingers may be wider than shown in either compound varactors  200  or  201 . In these embodiments, the wider fingers may result in lower series resistance in the fingers, which may benefit performance. However, the wider electrodes may also mean an increase in the mean propagation distance in the collector/sub-collector, which in turn may result in an effective increase in the collector resistance. This increased resistance may result in a negative impact on the quality factor Q of the device. Thus, the optimum device layout may be a compromise between loss mechanisms in the varactor, as described above. Factors that may change based on this compromise may include the materials used in the compound varactor, the level of dopant of the collector/sub-collector layer, the length or width of the fingers, the overall size of the compound varactor, or other factors. 
         [0066]      FIG. 3  illustrates an example side view of a compound varactor  300  such as compound varactor  200 , taken along line A-A of  FIG. 2 b   . It will be understood that the compound varactor  300  of  FIG. 3  is intended as an example to show the relative positions of certain elements of  FIG. 2 b    along the z-axis. As such, relative heights, lengths, or widths of elements of  FIG. 3  should not be considered as definitive unless explicitly defined as such below. 
         [0067]    In embodiments, the compound varactor  300  may include two varactors  305  and  310 , which may be similar to compound varactors  205  and  210 . In embodiments, the varactors  305  and  310  may both include a collector layer  315 , which may be similar to collector layer  215  of  FIG. 2 b   . Varactor  305  may include a base layer  320  and metal layers  325  and  330 , which may be respectively similar to base layer  220  and metal layers  225  and  230  of  FIG. 2 b   . In embodiments, varactor  305  may further include one or more vias  335 , which may be similar to vias  235  and configured to communicatively couple one or more of the base layer  320  or metal layers  225  or  230  to one another. 
         [0068]    Similarly to varactor  305 , varactor  310  may include a base layer  340  and metal layer  345  and  350 , which may be respectively similar to base layer  240  and metal layers  245  and  250 . In some embodiments, the compound varactor  300  may include a sub-collector layer  355  coupled with the collector layer  315 , which may be similar to the sub-collector layer that is described above, but not shown, with respect to compound varactor  200 . 
         [0069]    In some cases, as illustrated in  FIG. 1 a , 1 b   , or  1   c , stacking of more than two varactor diodes may be desirable to meet linearity requirements of a circuit or apparatus using compound varactors. In some cases, more than one pair of inerdigitated varactors may readily be stacked in series to achieve multi-varactor configurations such as those shown in  FIG. 1 a , 1 b   , or  1   c.    
         [0070]      FIG. 5 a    shows a simple circuit diagram of a circuit  500  that includes four varactors  505 ,  510 ,  515 , and  520 , which may be similar to varactors  105 ,  145 , or  150 . Specifically, varactors  505  and  510  may be a first compound varactor that includes an interdigitated varactor pair, and varactors  515  and  520  may be a second compound varactor that includes an interdigitated varactor pair. In some embodiments, varactors  505 ,  510 ,  515 , and  520  may be identical to one another. In other embodiments, one or more of varactors  505 ,  510 ,  515 , and  520  may be different from another of the varactors. For example, in some embodiments varactor  505  may have relatively narrow fingers such as those shown with respect to varactor  206  in  FIG. 2 c   , while varactor  510  may have relatively wide fingers such as those shown with respect to varactor  211  in  FIG. 2 c    (or vice-versa). Similarly, if varactor  510  has relatively wide fingers, then varactor  515  may have relatively wide fingers and varactor  520  may have relatively narrow fingers (or vice-versa). In some embodiments, if varactor  510  has relatively wide fingers, then varactor  515  may have relatively narrow fingers and varactor  520  may have relatively wide fingers. 
         [0071]    Additionally, in some embodiments circuit  500  may include one or more resistors such as resistors  135 , a ground connection such as ground  120 , a DC power source such as power source  125 , an input terminal such as input terminal  110 , and/or an output terminal such as output terminal  115 . 
         [0072]      FIG. 5 b    illustrates a series connection of two series-connected pairs of equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. Specifically,  FIG. 5 b    illustrates a compound varactor  501  that may include four varactors  506 ,  511 ,  516 , and  521  in a series connection with one another. Varactor  506  may include collector layer  531  and base layer  526 , which may be similar to collector layer  215  and base layer  220  of  FIG. 2 b   . Varactor  511  may include collector layer  531 , and base layer  536 , which may be similar to base layer  240  of  FIG. 2 b   . Varactor  516  may include collector layer  541 , which may also be similar to collector layer  215 , and base layer  536 . Finally, varactor  521  may include collector layer  541  and base layer  546 , which may be similar to base layer  240  of  FIG. 2 b   . In embodiments, one or more of varactors  506 ,  511 ,  516 , and  521  may include one or more metal layers, a sub-collector layer, or vias, which are not illustrated in  FIG. 5 b    for the sake of clarity. In operation, a signal may flow from varactor  506  through the compound varactor  501  and exit the compound varactor  501  at varactor  521  (or vice-versa). Although each of the varactors in compound varactor  501  are shown as having only three fingers, in other embodiments the varactors in compound varactor  501  may have more or less fingers. 
         [0073]      FIG. 5 c    illustrates a series connection of two-series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. Specifically,  FIG. 5 c    illustrates a compound varactor  502  that may include four varactors  507 ,  512 ,  517 , and  522  in a series connection with one another. Varactor  507  may include collector layer  532  and base layer  527 , which may be similar to collector layer  216  and base layer  221  of  FIG. 2 c   . Varactor  512  may include collector layer  532 , and base layer  537 , which may be similar to base layer  241  of  FIG. 2 c   . Varactor  517  may include collector layer  542 , which may also be similar to collector layer  216 , and base layer  537 . Finally, varactor  522  may include collector layer  542  and base layer  547 , which may be similar to base layer  241  of  FIG. 2 c   . In embodiments, one or more of varactors  507 ,  512 ,  517 , and  522  may include one or more metal layers, a sub-collector layer, or vias, which are not illustrated in  FIG. 5 c    for the sake of clarity. In operation, a signal may flow from varactor  507  through the compound varactor  502  and exit the compound varactor  502  at varactor  522  (or vice-versa). Although the fingers of varactors  507  and  522  are shown as relatively narrow and the fingers of varactors  512  and  517  are shown as relatively wide, in other embodiments the fingers of varactors  507  and  522  may be relatively wide, and the fingers of varactors  512  and  517  may be relatively narrow. Although each of the varactors in compound varactor  502  are shown as having only three fingers, in other embodiments the varactors in compound varactor  502  may have more or less fingers. 
         [0074]    While functional, the simple stacking architectures of compound varactors  501  and  502  may experience energy from a signal flowing through the compound varactors  501  and  502  flowing transversely along the horizontal busbars of the varactors  501  and  502 . This energy may flow transversely along the horizontal busbars because of the vertical discontinuities of base layer  537 . Specifically, a signal flowing vertically (as seen in the  FIG. 5 b    or  5   c ) may flow into varactor  511  or  512 , but then have to flow horizontally through base layer  536  or  537  to the fingers of varactors  516  or  517 . 
         [0075]      FIG. 5 d    illustrates an alternative series connection of two-series-connected pairs of equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. Specifically,  FIG. 5 d    illustrates a compound varactor  503  that may include four varactors  508 ,  513 ,  518 , and  523  in a series connection with one another. Varactor  508  may include collector layer  533  and base layer  528 , which may be similar to collector layer  215  and base layer  220  of  FIG. 2 b   . Varactor  513  may include collector layer  533 , and base layer  538 , which may be similar to base layer  240  of  FIG. 2 b   . Varactor  518  may include collector layer  543 , which may also be similar to collector layer  215 , and base layer  538 . Finally, varactor  523  may include collector layer  543  and base layer  548 , which may be similar to base layer  240  of  FIG. 2 b   . In embodiments, one or more of varactors  508 ,  513 ,  518 , and  523  may include one or more metal layers, a sub-collector layer, or vias, which are not illustrated in  FIG. 5 d    for the sake of clarity. In operation, a signal may flow from varactor  508  through the compound varactor  503  and exit the compound varactor  503  at varactor  523  (or vice-versa). Although each of the varactors in compound varactor  503  are shown as having only three fingers, in other embodiments the varactors in compound varactor  503  may have more or less fingers. 
         [0076]    In embodiments, the signal may experience less loss in compound varactor  503  than, for example, compound varactor  501  because the fingers of varactors  513  and  518  may be vertically aligned with one another, as shown in  FIG. 5 d   . Therefore, if the signal is flowing vertically through compound varactor  503 , then the signal may not have to flow transversely through base layer  538  to move from the fingers of varactor  513  to the fingers of varactor  518 . 
         [0077]      FIG. 5 e    illustrates an alternative series connection of two-series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 5 a   , in accordance with various embodiments. Specifically,  FIG. 5 e    illustrates a compound varactor  503  that may include four varactors  509 ,  514 ,  519 , and  524  in a series connection with one another. Varactor  509  may include collector layer  534  and base layer  529 , which may be similar to collector layer  216  and base layer  221  of  FIG. 2 c   . Varactor  514  may include collector layer  534 , and base layer  539 , which may be similar to base layer  241  of  FIG. 2 c   . Varactor  519  may include collector layer  544 , which may also be similar to collector layer  216 , and base layer  539 . Finally, varactor  524  may include collector layer  544  and base layer  549 , which may be similar to base layer  241  of  FIG. 2 c   . In embodiments, one or more of varactors  509 ,  514 ,  519 , and  524  may include one or more metal layers, a sub-collector layer, or vias, which are not illustrated in  FIG. 5 e    for the sake of clarity. In operation, a signal may flow from varactor  509  through the compound varactor  504  and exit the compound varactor  504  at varactor  524  (or vice-versa). Although the fingers of varactors  509  and  524  are shown as relatively narrow and the fingers of varactors  514  and  519  are shown as relatively wide, in other embodiments the fingers of varactors  509  and  524  may be relatively wide, and the fingers of varactors  514  and  519  may be relatively narrow. Although each of the varactors in compound varactor  504  are shown as having only three fingers, in other embodiments the varactors in compound varactor  504  may have more or less fingers. 
         [0078]    Similarly to compound varactor  503 , a signal flowing through compound varactor  504  may experience less loss in compound varactor  504  than, for example, compound varactor  502  because the fingers of varactors  514  and  519  may be vertically aligned with one another, as shown in  FIG. 5 e   . Therefore, if the signal is flowing vertically through compound varactor  504 , then the signal may not have to flow transversely through base layer  539  to move from the fingers of varactor  514  to the fingers of varactors  519 . 
         [0079]    In some embodiments, an anti-parallel pair of stacked diodes may be beneficial as shown in  FIG. 6 a   . Specifically, as mentioned above, in some uses dual interconnected varactor stacks may be advantageous. Such a circuit could be desirable, for example, because in some embodiments parallel asymmetric varactor stacks with non-equal capacitance ratios may increase linearity. Additionally, an interdigitated reverse-connected varactor pair configuration may be particularly space-efficient in realizing such dual stacked pairs. 
         [0080]      FIG. 6 a    depicts a high-level circuit diagram of a circuit  600  that includes two sets of series-stacked varactors. Specifically, the circuit  600  may include varactors  605 ,  610 ,  615 ,  620 ,  625 ,  630 ,  635 , and  640 , which may be similar to varactors  105 ,  145 , and/or  150 . In embodiments, certain of the varactors such as varactors  605  and  610  may be front-to-front with one another, while others of the varactors such as varactors  610  and  615  may be back-to-back with one another. In embodiments, the stacks may be connected to one another via interconnects such as interconnects  645 ,  650 , and  655 . 
         [0081]    In some embodiments, each of the varactors  605 ,  610 ,  615 ,  620 ,  625 ,  630 ,  635 , and  640  may be similar to one another, for example having similar finger width or constructed of the same materials. In other embodiments, at least one of varactors  605 ,  610 ,  615 ,  620 ,  625 ,  630 ,  635 , and  640  may be different from another one of the varactors, for example having a different finger width or being constructed of a different material from the other varactor. In some embodiments, varactors  605 ,  620 ,  630 , and  635  may be similar to one another, but different from varactors  610 ,  615 ,  625 , and  640  (which may be similar to one another). 
         [0082]    Although not shown for the sake of simplicity, in some embodiments circuit  600  may include more or fewer varactors than are shown in  FIG. 6 a   . Additionally, in some embodiments circuit  600  may include one or more resistors such as resistors  135 , a ground connection such as ground  120 , a DC power source such as power source  125 , an input terminal such as input terminal  110 , and/or an output terminal such as output terminal  115 . 
         [0083]      FIG. 6 b    illustrates an example a plurality of series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 6 a   , in accordance with various embodiments. Specifically,  FIG. 6 b    depicts a compound varactor  602  that includes two varactor stacks  662  and  672  of varactors that are in series with one another as shown in  FIG. 6 a   . In embodiments, varactor stacks  662  and  672  may be in parallel with one another. 
         [0084]    Specifically, stack  662  may include varactors  607 ,  612 ,  617 , and  622 , which may be similar to varactors  509 ,  514 ,  519 , and  524 , respectively. Similarly, stack  672  may include varactors  627 ,  632 ,  637 , and  642 , which may also be similar to varactors  509 ,  514 ,  519 , and  524 , respectively. In embodiments, the base layers of stacks  662  and  672  may be coupled with one another as shown in  FIG. 6 b   . Varactors  607  and  612  may share collector layer  646 . Varactors  617  and  622  may share collector layer  651 . Varactors  627  and  635  may share collector layer  656 . Varactors  637  and  642  may share collector layer  661 . 
         [0085]    As shown, in some embodiments the configurations of the stacks  662  and  672  may be different. For example, the “inner” varactors  612  and  617  of stack  662  may have relatively wide fingers, while the “outer” varactors  607  and  622  of stack  662  may have relatively narrow fingers. By contrast, in stack  672  the “inner” varactors  632  and  637  may have relatively narrow fingers while the “outer” varactors  627  and  642  may have relatively wide fingers. In other embodiments, the widths of the “inner” fingers of the fingers of stack  662  and the “outer” fingers of stack  672  may be relatively narrow while the widths of the “inner” fingers of the stack  672  and the “outer” fingers of the stack  662  may be relatively wide. As described herein, “inner” and “outer” are only intended as descriptive elements to identify the different fingers of the different varactors in  FIG. 6 b   , and are not intended as limiting or definitional elements. 
         [0086]    In some embodiments, the collector layers of the compound varactors may be coupled with one or more DC power sources such as DC power source  125  that may be configured to provide a DC voltage bias. Specifically, collector layers  646 ,  651 ,  656 , and  661  may be coupled with a DC power source via interconnects  645 ,  655 ,  650 , and  660 , respectively. In some embodiments, one or more resistors such as resistors  135  may be positioned between one or more of the collector layers  646 ,  651 ,  656 , and  661  and the DC power source. In embodiments, the various varactors of compound varactor  602  may include one or more of vias, metal layers, or sub-collector layers, which are not shown in  FIG. 6 b    for the sake of clarity. Although the varactors of compound varactor  602  are shown with three fingers each, in other embodiments the varactors may have a greater or lesser number of fingers. Similarly, it can be seen that the fingers of the base layers of varactors  612 ,  617 ,  632 , and  637  are wider than the fingers of the base layers of varactors  607 ,  622 ,  627 , and  642 . In other embodiments, the fingers of the base layers of varactors  612 ,  617 ,  632 , and  637  may be narrower than the fingers of the base layers of varactors  607 ,  622 ,  627 , and  642 . 
         [0087]      FIG. 6 c    illustrates an alternative example of a plurality of series-connected pairs of non-equal reverse-connected varactors that may be used in the circuit of  FIG. 6 a   , in accordance with various embodiments. Specifically, the dual stack architecture of  FIG. 6 a    or  6   b  may result in significantly increased linearity for a signal propagating through the compound varactor. Non-linear artifacts from the two stacks may cancel each other via the interconnects between the two stacks. However, if each of the stacks has a wide aspect ratio, which may be desirable as required for increasing or maximizing the quality factor Q of the compound varactor, then resistance and inductance in the horizontal connections between the stacks may inhibit the cancellation of the spurious artifacts. To reduce any such degradation in performance, the left and right stacks may be broken up into sub-sections, or segmented, and interspersed with one another as illustrated in  FIG. 6   c.    
         [0088]    Specifically,  FIG. 6 c    illustrates a compound varactor  603  that may consist of four stacks of series varactors. The stacks may be similar to stacks  662  or  672 , but they may be segmented versions of stacks such as stacks  662  or  672 . Specifically, stacks  695  and  697  may be a segmented version of a stack such as stacks  662  or  672 , and stacks  696  and  698  may be a segmented version of a stack such as stacks  662  or  672 . 
         [0089]    For example, even though the varactors of stacks  695 ,  696 ,  697 , or  698  are shown as having three fingers each, the number of fingers is shown simply as an example and is not intended to be determinative. In some embodiments, because stacks  695  and  697  are segmented portions of one of the stacks shown in  FIG. 6 a   , the number of fingers of varactors in stacks  695  and  697  combined may be equal to the number of fingers of varactors in stack  662 . In other words, stack  662  may be segmented to form stacks  695  and  697 . Similarly, stacks  696  and  698  may be a segmented version of other stacks described herein. 
         [0090]    In embodiments, the collector layers (not labeled for the sake of clarity) of elements of a single segmented stack may be coupled to one another. For example, stacks  695  and  697  may be segmented elements of a stack such as stack  662 , as described above. The collector layers of stacks  695  and  697  may be coupled together by interconnect  699   a , which may be similar to one of interconnects  645 ,  650 ,  655 , or  660 , and further coupled with a DC power source as described above with respect to  FIG. 6 b   . Similarly, the collector layers of stacks  696  and  698  may be coupled together by interconnect  699   b . By coupling the collector layers of stacks  695  and  697 , or  696  and  698 , together, a similar DC voltage bias may be applied to both segmented elements of a stack. 
         [0091]    It may further be seen that the varactors of the various stacks of compound varactor  603  may be unequal, that is having different finger widths, similarly to compound varactor  602  of  FIG. 6 b   . Additionally, the fingers widths of the varactors of the stacks may not vary in the same pattern. Specifically, as can be seen the outer varactors of stacks  695  and  697  have relatively narrow fingers, while the inner varactors of stacks  695  and  697  have relatively wider fingers. By contrast, the outer varactors of stacks  696  and  698  have relatively wide fingers, while the inner varactors of stacks  696  and  698  have relatively narrow fingers. By segmenting the various stacks of the compound varactor circuit of  FIG. 6 a   , and interleaving the segmented stacks, the second-order non-linear components of the RF signal may be reduced or minimized. 
         [0092]      FIG. 7  depicts an example process for generating a compound varactor such as compound varactor  200 . Initially, a collector layer such as collector layer  215  may be deposited at  700 . Next, a base layer such as base layer  220  may be deposited on the collector layer  215  at  705 . Finally, a base layer such as base layer  240  may be deposited on the collector layer  215  at  710 . 
         [0093]    As described herein, the deposition of the base layers at  710  and  715  may include depositing the base layer to form fingers such as fingers  260  or  270 . In some embodiments, the base layer may be deposited and then etched to form the fingers by mechanical, electrical, or chemical etching. In some embodiments, only a single base layer may be deposited and then etched to form the fingers of the two varactors  205  and  210  in the compound varactor  200 . In some embodiments, additional layers such as the sub-collector layer or one or more of the metal layers as described above may be deposited and/or etched. 
         [0094]      FIG. 8  depicts an alternative embodiment of a compound varactor  800  that may include elements that are similar to elements of compound varactor  200 , and are labeled similarly. For example, compound varactor  800  may include varactors  805  and  810 . Varactor  805  may include the collector layer  815 , one or more base layers  820 , one or more metal layers  825 , and a second metal layer  830  which may be respectively similar to collector layer  215 , base layer  220 , metal layer  225 , and metal layer  230 . In embodiments, vias  835  may electrically connect one or more of base layer  820 , metal layer  825 , and metal layer  830 . 
         [0095]    Similarly, varactor  810  may include the collector layer  815 , one or more base layers  840 , one or more metals layers  845 , and a metal layer  850  that may be respectively similar to base layer  240 , metal layer  245 , and metal layer  250 . In embodiments, vias  835  may electrically connect one or more of base layers  840 , metal layer  845 , and metal layer  850 . As can be seen in  FIG. 8 , in embodiments the base layers  820  and  840  may not be formed as fingers, but instead be formed as discrete elements that are arranged generally opposite one another. In embodiments, the base layers  820  and  840  may be generally joined by metal layers  825 ,  830 ,  845 , and/or  850 , as shown in  FIG. 8 . 
         [0096]    The collector layer  815  may be coupled with a bias input  880  that may be coupled with and configured to receive a voltage bias from, for example, DC power source  125 . In some embodiments, the collector layer  815  may further include or be coupled with a sub-collector layer (not shown). Also, in some embodiments, the different sizes or number of elements may be different than depicted in  FIG. 8 . For example, the metal layer  830  may have a generally similar width to the total width of the base layer  820 , rather than being slightly narrower as depicted in  FIG. 8 . Additionally, in some embodiments the compound varactor  800  may include more or fewer base layers  820  or  840 , or vias  835 . 
         [0097]      FIG. 9  depicts an alternative embodiment that includes a compound varactor  900 . Elements of  FIG. 9  may be similar to elements of  FIG. 2 , and numbered similarly. Varactor  905  may include collector layer  915 , base layer  920 , metal layer  925 , and metal layer  930 , which may be respectively similar to collector layer  215 , base layer  220 , metal layer  225 , and metal layer  230 . In embodiments, vias  935 , which may be similar to vias  235 , may electrically connect one or more of base layer  920 , metal layer  925 , and metal layer  930 . Varactor  910  may include collector layer  915  and base layer  940 , which may be similar to collector layer  215  and base layer  240 . Varactor  910  may further include metal layers  995  and  990 , as described in further detail below. In embodiments, vias  935  may electrically connect one or more of base layer  940 , metal layer  995 , and metal layer  990 . 
         [0098]    Varactor  906  may include collector layer  916  and base layer  921 , which may be respectively similar to collector layer  215  and base layer  220 . Varactor  906  may further include metal layers  990  and  995 , as described in further detail below. In embodiments, vias  935  may electrically connect one or more of base layer  921 , metal layer  990 , and metal layer  995 . Varactor  911  may include collector layer  916 , base layer  941 , metal layer  946 , and metal layer  950 , which may be respectively similar to collector layer  215 , base layer  240 , metal layer  245 , and metal layer  250 . In embodiments, vias  935  may electrically connect one or more of base layer  941 , metal layer  946 , and metal layer  950 . 
         [0099]    In some embodiments the collector layers  915  and  916  may be coupled with a bias input  980  that may be coupled with and configured to receive a voltage bias from, for example, DC power source  125 . Additionally, in some embodiments each of metal layers  930 ,  990 , and  950  may include a bias tab  985  that is configured to be coupled with, and receive a voltage bias from, a DC power source. In some embodiments, the collector layers  915  or  916  may include or be coupled with a sub-collector layer, as described above with reference to collector layer  215 . In some embodiments, the number of different elements may be different than depicted in  FIG. 9 . For example, in embodiments the compound varactor  900  may include more or fewer collector layers, base layers, or metal layers. Additionally, in some embodiments the relative sizes of elements may be different than depicted in  FIG. 9 . For example, in some embodiments the metal layer  925  may be the same length or width as base layer  920 . 
         [0100]    As can be seen in  FIG. 9 , the metal layer  995 , which may be similar to one or both of metal layers  225  or  245 , may be an element of both varactors  910  and  906 , and configured to allow an RF signal to propagate from varactor  910  to varactor  906 , or vice versa. Similarly, metal layer  990 , which may be similar to one or both of metal layers  230  or  250 , may be an element of both varactors  910  and  906 , and configured to allow an RF signal to propagate from varactor  910  to varactor  906 , or vice versa. 
         [0101]    Therefore, as shown in  FIG. 9 , an RF signal may enter compound varactor  900  at metal layer  930 , where it may propagate through varactor  905  to collector layer  915 . From collector layer  915 , the RF signal may propagate through varactor  910  to metal layer  990  to varactor  906 . The RF signal may then similarly propagate through compound varactor  901  to metal layer  950 , where it may then exit system  900 . It will be understood that this description of how an RF signal may propagate through compound varactor  900  is only intended as an example, and in other embodiments the RF signal may enter, exit, or propagate through different layers or in a different direction dependent on the specific construction of the compound varactor  900  or a circuit utilizing compound varactor  900 . 
         [0102]      FIG. 10  depicts an alternative embodiment that includes a compound varactor  1000 . Elements of compound varactor  1000  may be similar to compound varactor  200  or compound varactor  900 , and be numbered similarly. 
         [0103]    Varactor  1005  may include collector layer  1015 , base layer  1020 , metal layer  1025 , and metal layer  1030 , which may be respectively similar to collector layer  215 , base layer  220 , metal layer  225 , and metal layer  230 . In embodiments, vias  1035 , which may be similar to vias  235 , may electrically connect one or more of base layer  1020 , metal layer  1025 , and metal layer  1030 . Varactor  1010  may include collector layer  1015  and base layer  1040 , which may be respectively similar to collector layer  215  and base layer  240 . Varactor  1010  may further include metal layer  1095  and metal layer  1090 , which will be described in greater detail below. Vias (not labeled for the sake of clarity) may electrically connect base layer  1040 , metal layer  1095 , and metal layer  1090 . 
         [0104]    Varactor  1006  may include collector layer  1016  and base layer  1021 , which may be respectively similar to collector layer  215  and base layer  220 . Varactor  1006  may further include metal layers  1095  and  1090 , as described in further detail below. Vias  1035  (not labeled for the sake of clarity) may electrically connect one or more of base layer  1021 , metal layer  1095 , and metal layer  1090 . Varactor  1011  may include collector layer  1016  and base layer  1041 , which may be respectively similar to collector layer  215  and base layer  240 . In embodiments, varactor  1011  may further include metal layer  1096  and metal layer  1091 , as described in further detail below. In embodiments, vias  235  (not labeled for the sake of clarity) may electrically connect one or more of base layer  1041 , metal layer  1096 , and metal layer  1091 . 
         [0105]    Varactor  1007  may include collector layer  1017  and base layer  1022 , which may be respectively similar to collector layer  215  and base layer  220 . Varactor  1007  may further include metal layers  1096  and  1091 . Vias  1035  (not labeled for the sake of clarity) may electrically connect one or more of base layer  1022 , metal layer  1096 , and metal layer  1091 . Varactor  1012  may include collector layer  1017 , base layer  1042 , metal layer  1047 , and metal layer  1052 , which may be respectively similar to collector layer  215 , base layer  240 , metal layer  245 , and metal layer  250 . In embodiments, vias  1035  may electrically connect one or more of base layer  1042 , metal layer  1047 , and metal layer  1052 . 
         [0106]    In some embodiments the collector layers  1015 ,  1016 , and  1016  may be coupled with a bias input  1080  that may be coupled with and configured to receive a voltage bias from, for example, DC power source  125 . Additionally, in some embodiments each of metal layers  1090  and  1091  may include a bias tab  1085  that is configured to be coupled with, and receive a voltage bias from, a DC power source. In some embodiments, the collector layers  1015 ,  1016 , and  1017  may include or be coupled with a sub-collector layer, as described above with reference to collector layer  215 . In some embodiments, the number of different elements may be different than depicted in  FIG. 10 . For example, in embodiments the compound varactor  1000  may include more or fewer collector layers, base layers, or metal layers. Additionally, in some embodiments the relative sizes of elements may be different than depicted in  FIG. 10 . For example, in some embodiments the metal layer  1025  may be the same length or width as base layer  1020 . 
         [0107]    As can be seen in  FIG. 10 , the metal layer  1095 , which may be similar to one or both of metal layers  225  or  245 , may be an element of both varactors  1010  and  1006 , and configured to allow an RF signal to propagate from varactor  1010  to varactor  1006 , or vice versa. Similarly, metal layer  1090 , which may be similar to one or both of metal layers  230  or  250 , may be an element of both varactors  1010  and  1006 , and configured to allow an RF signal to propagate from varactor  1010  to varactor  1006 , or vice versa. Similarly, metal layer  1091 , which may be similar to metal layer  1090 , may be an element of both varactors  1011  and  1007 , and configured to allow an RF signal to propagate from varactor  1011  to varactor  1007 , or vice versa. Similarly, metal layer  1091 , which may be similar to one or both of metal layer  230  or  250 , may be an element of both varactors  1011  and  1007 , and configured to allow an RF signal to propagate from varactor  1011  to varactor  1007 , or vice versa. 
         [0108]    Therefore, as shown in  FIG. 10 , an RF signal may enter the system  1000  at metal layer  1030 . The RF signal may propagate through varactor  1005  to collector layer  1015 , where it may then propagate back up through varactor  1010  to metal layer  1090 . The RF signal may propagate through metal layer  1090  to varactor  1006  where it may propagate through the compound varactor  1002  to metal layer  1091 . From metal layer  1091 , the RF signal may propagate to varactor  1007  to metal layer  1052  where it may then exit the compound varactor  1000 . It will be understood that this description of how an RF signal may propagate through compound varactor  1000  is only intended as an example, and in other embodiments the RF signal may enter, exit, or propagate through different layers or in a different direction dependent on the specific construction of the compound varactor  1000  or a circuit utilizing compound varactor  1000 . 
         [0109]    Compound varactors  200 ,  201 ,  300 ,  501 ,  502 ,  503 ,  504 ,  601 ,  602 ,  603 ,  800 ,  900 , or  1000  may be incorporated into a variety of systems. A block diagram of an example system  1100  is illustrated in  FIG. 11 . As illustrated, the system  1100  includes a power amplifier (PA) module  1102 , which may be a radio frequency (RF) PA module in some embodiments. The system  1100  may include a transceiver  1104  coupled with the PA module  1102  as illustrated. The PA module  1102  may include one or more of compound varactors  200 ,  201 ,  300 ,  501 ,  502 ,  503 ,  504 ,  601 ,  602 ,  603 ,  800 ,  900 , or  1000 . In various embodiments, the compound varactors  200 ,  201 ,  300 ,  501 ,  502 ,  503 ,  504 ,  601 ,  602 ,  603 ,  800 ,  900 , or  1000  may additionally/alternatively be included in the transceiver  1104  to provide, e.g., up-converting, or in an antenna switch module (ASM)  1106  to provide various switching functions. 
         [0110]    The PA module  1102  may receive an RF input signal, RFin, from the transceiver  1104 . The PA module  1102  may amplify the RF input signal, RFin, to provide the RF output signal, RFout. The RF input signal, RFin, and the RF output signal, RFout, may both be part of a transmit chain, respectively noted by Tx-RFin and Tx-RFout in  FIG. 11 . 
         [0111]    The amplified RF output signal, RFout, may be provided to the ASM  1106 , which effectuates an over-the-air (OTA) transmission of the RF output signal, RFout, via an antenna structure  1108 . The ASM  1106  may also receive RF signals via the antenna structure  1108  and couple the received RF signals, Rx, to the transceiver  1104  along a receive chain. 
         [0112]    In various embodiments, the antenna structure  1108  may include one or more directional and/or omnidirectional antennas, including, e.g., a dipole antenna, a monopole antenna, a patch antenna, a loop antenna, a microstrip antenna or any other type of antenna suitable for OTA transmission/reception of RF signals. 
         [0113]    The system  1100  may be suitable for any one or more of terrestrial and satellite communications, radar systems, and possibly in various  1100  and medical applications. More specifically, in various embodiments, the system  1100  may be a selected one of a radar device, a satellite communication device, a mobile computing device (e.g., a phone, a tablet, a laptop, etc.), a base station, a broadcast radio, or a television amplifier system. 
         [0114]    Although the present disclosure has been described in terms of the above-illustrated embodiments, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. Those with skill in the art will readily appreciate that the teachings of the present disclosure may be implemented in a wide variety of embodiments. This description is intended to be regarded as illustrative instead of restrictive.