Patent Document

RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application No. 62/158,613, filed May 8, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     Embodiments of the present disclosure relate generally to the field of circuits, and more particularly to varactors. 
     BACKGROUND 
     A varactor is an electronic component with a capacitance that changes in response to an applied bias voltage. A varactor may be a diode under reverse bias that exhibits a capacitance that varies inversely with the applied voltage. As such, a varactor may be used for tuning electrical circuits. Varactors may be prone to self-modulation distortion resulting from applied radio frequency (RF) voltages. To reduce this nonlinearity a compound varactor may be formed having a number of individual varactors coupled in series to divide the RF voltage across each individual varactor. 
     SUMMARY 
     The present disclosure relates to varactors, and in particular to compound varactors. In certain embodiments a compound varactor may be formed as a semiconductor device having a substrate. The semiconductor device may include a first port having a cathode interface and a second port having a cathode interface. An anti-series string of varactors may be electrically coupled between the first port and the second port. 
     The anti-series string may include a first varactor, a second varactor having an anode electrically coupled to an anode of the first varactor, a third varactor having a cathode electrically coupled to a cathode of the second varactor, and a fourth varactor having an anode electrically coupled to an anode of the third varactor wherein the first and fourth varactors each have a first effective area and the second and third varactors each have a second effective area which is different than the first effective area. The effective area is defined as the area of a varactor layer in an epitaxial stack. In some embodiments, the first effective area is between 45% and 75% of the second effective area. In other embodiments, the first effective area is between 50% and 60% of the second effective area. 
     In other embodiments, the semiconductor device may have a first resistor electrically coupled between a third port and the anodes of the first and second varactors, and a second resistor electrically coupled between the third port and the anodes of the third and fourth varactors. A third resistor may electrically couple between a fourth port and the cathodes of the second and third varactors. 
     In another embodiment, the first anti-series string of varactors may be formed on a substrate, wherein each varactor of the first anti-series string of varactors is an epitaxial stack having an upper contact layer, a varactor layer, and a lower contact layer. 
     In other embodiments, the anti-series string may include a fifth varactor having a cathode electrically coupled to the cathode of the fourth varactor, a sixth varactor having an anode electrically coupled to an anode of the fifth varactor, a seventh varactor having a cathode electrically coupled to a cathode of the sixth varactor, and an eighth varactor having an anode electrically coupled to an anode of the seventh varactor wherein the fifth and eighth varactors each have the first effective area and the sixth and seventh varactors each have the second effective area. 
     In other embodiments, the semiconductor device may include a first port having an anode interface and a second port having an anode interface. An anti-series string of varactors may be electrically coupled between the first port and the second port. 
     The anti-series string may include a first varactor, a second varactor having a cathode electrically coupled to a cathode of the first varactor, a third varactor having an anode electrically coupled to an anode of the second varactor, and a fourth varactor having a cathode electrically coupled to a cathode of the third varactor wherein the first and fourth varactors each have a first effective area and the second and third varactors each have a second effective area which is different than the first effective area. In some embodiments the first effective area is between 45% and 75% of the second effective area. In other embodiments, the first effective area is between 50% and 60% of the second effective area. 
     In one embodiment, the first port is an input port, the second port is an output port, the third port is a v− bias port, and the fourth port is a v+ bias port. 
     In select non-limiting embodiments, the compound varactor may be incorporated in an impedance tuning circuit or an antenna tuning circuit for a radio front end (RFE) to reduce second order harmonic generation. In other embodiments, the modified compound varactor may be integrated with a voltage controlled oscillator, an electronically tuned filter, or an electronically controlled switch. 
     Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of this Specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
         FIG. 1  illustrates a compound varactor in a circuit having an example anti-series string, in accordance with various embodiments. 
         FIG. 2  illustrates a compound varactor in a circuit having a modified anti-series string, in accordance with various embodiments. 
         FIG. 3  illustrates a varactor formed in an epitaxial stack, in accordance with various embodiments. 
         FIG. 4  illustrates a graph of second harmonic generation, in accordance with various embodiments. 
         FIG. 5  illustrates an alternate embodiment of a compound varactor in a circuit having a modified anti-series string, in accordance with various embodiments. 
         FIG. 6  illustrates a wireless device having the compound varactor, in accordance with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element, or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or “extending directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the accompanying Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the accompanying Figures. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Related art here and in  FIG. 1  discloses a circuit diagram  10  having a compound varactor  12  and bias circuitry  14 . The compound varactor  12  may be a semi-conductor device and may have an input port  16 A, an output port  16 B, a v+ bias port  16 C, and a v− bias port  16 D. The input port  16 A may have a cathode interface and the output port  16 B may have a cathode interface. An example anti-series string  18  of varactors D 1 -D 8  may be coupled between the input port  16 A and the output port  16 B. In an anti-series string, adjacent varactors may be coupled as cathode-to-cathode or anode-to-anode. As such, cathode of varactor D 1  may be coupled with the input port  16 A and coupled with the v+ bias port  16 C via resistor RP 1 . Anodes of varactors D 1  and D 2  may be coupled together and coupled with v− bias port  16 D via resistor RN 1 . Cathodes of varactors D 2  and D 3  may be coupled together and coupled with v+ bias port  16 C via resistor RP 2 . Anodes of varactors D 3  and D 4  may be coupled together and coupled with v− bias port  16 D via resistor RN 2 . Cathodes of varactors D 4  and D 5  may be coupled together and coupled with v+ bias port  16 C via resistor RP 3 . Anodes of varactors D 5  and D 6  may be coupled together and coupled with v− bias port  16 D via resistor RN 3 . Cathodes of varactors D 6  and D 7  may be coupled together and coupled with v+ bias port  16 C via resistor RP 4 . Anodes of varactors D 7  and D 8  may be coupled together and coupled with v− bias port  16 D via resistor RN 4 . Cathode of varactor D 8  may be coupled with output port  16 B. Varactors D 1 -D 8  may be formed as individual epitaxial stacks on a substrate. Each epitaxial stack may be equal in size and doping profile, wherein each of the varactors D 1 -D 8  have approximately equal capacitance-voltage (C-V) tuning characteristics. 
     Resisters RP 1 -RP 4  may provide a bias voltage connection between each of the cathodes of varactors D 1 -D 7  and the v+ bias port  16 C. Resistors RN 1 -RN 4  may provide bias connections between each of the anodes of the varactors D 1 -D 8  and the v− bias port  16 D. In some embodiments, the resistors RP 1 -RP 4  and RN 1 -RN 4  may be equal in resistance value (ohms or Ω). In other embodiments, certain resistors of resistors RP 1 -RP 4  and RN 1 -RN 4 , such as outer resistor RP 1 , may be greater than the other resistors RP 2 -RP 4  and RN 1 -RN 4 . In some embodiments resistor RP 1  may be approximately 60 kΩ and resistors RP 2 -RP 4  and RN 1 -RP 4  may be approximately 30 kΩ. In other embodiments, RP 1  may be between approximately 20 kΩ and approximately 60 kΩ while resistors RP 2 -RP 4  and RN 1 -RP 4  may be between approximately 10 kΩ and approximately 30 kΩ. In some embodiments, inductors may also be used in place of, or in combination with, the resistors RP 1 -RP 4  and RN 1 -RN 4 . In other embodiments, a resistor (not shown) may be positioned between the output port  16 B and the v+ bias port  16 C. 
     Bias circuitry  14  may be coupled between the v+ bias port  16 C and ground  20 , and ground  20  may be coupled with v− bias port  16 D to provide a reverse bias voltage to each of the varactors D 1 -D 8 . The bias circuitry  14  may set the effective capacitance of the example anti-series string  18 . By increasing the bias voltage between the v+ bias port  16 C and the v− bias port  16 D, the effective capacitance of the example anti-series string  18  may be reduced. By decreasing the bias voltage between the v+ bias port  16 C and the v− bias port  16 D, the effective capacitance of the example anti-series string  18  may be increased. 
     In some embodiments, a radio frequency (RF) signal may propagate between the input port  16 A and the output port  16 B. In an ideal anti-series arrangement, each varactor D 1 -D 8  may have negligible parasitic capacitance to other circuitry and/or ground  20 . Varactors D 1  and D 2  may reduce self-modulation caused by propagation of the RF signal from the input port  16 A to the output port  16 B. As the RF signal increases, reverse bias voltage on D 1  increases, and reverse bias voltage on D 2  decreases. As such, as the capacitance of D 1  decreases, the capacitance of D 2  increases. By providing additional varactors D 3 -D 8  in the example anti-series string  18 , RF signal is reduced across each varactor D 1 -D 8  of the example anti-series string  18  further reducing self-modulation. 
     However, in actual anti-series arrangements each varactor D 1 -D 8  may have non-negligible parasitic capacitance to other circuitry and/or ground  20 . In some embodiments, certain layers of each of the epitaxial stacks of the varactor D 1 -D 8  may have an effective area over the substrate. This effective area may be proportional to the parasitic capacitance, wherein as the effective area is made larger the parasitic capacitance is larger. For example, the parasitic capacitance to ground  20  may be approximately 0.002 pico-Farads (pF) for each varactor D 1 -D 8  having a capacitance between approximately 6 pF and 20 pF when reversed biased from approximately 18 volts to approximately 2 volts. In other embodiments, the parasitic capacitance may be between approximately 0.001 pF and 0.005 pF. As an RF signal such as a 900 Mega-Hertz (MHz) sine wave propagates from the input port  16 A to the output port  16 B, second harmonic signal may be generated from self-modulation. For example, a +35 decibel-milliwatts (dBm) sine wave may generate a second harmonic of approximately −54 dBm. In other embodiments, the second harmonic may be between approximately −50 dBm and approximately −60 dBm. This second harmonic signal may be unacceptable for unfiltered varactor applications such as directly coupled impedance matching of an antenna to a radio front end (or RFE) circuit. For example, the second harmonic from a transmitted signal of the RFE may violate the required frequency spectrum mask for the transmitted signal. 
       FIG. 2  illustrates a circuit diagram  22  having a compound varactor  24  and bias circuitry  14 . The compound varactor  24  may be a semi-conductor device and may have an input port  26 A, an output port  26 B, a v+ bias port  26 C, and a v− bias port  26 D. The input port  26 A may have a cathode interface and the output port  26 B have a cathode interface. Compound varactor  24  replaces the compound varactor  12  shown in  FIG. 1 . The compound varactor  24  has a modified anti-series string  28  including varactors D 1 ″ through D 8 ″. Modified anti-series string  28  replaces the anti-series string  18  of compound varactor  12  shown in  FIG. 1 . Cathode of varactor D 1 ″ may be coupled with the input port  26 A and coupled with the v+ bias port  26 C via resistor RP 1 . Anodes of varactors D 1 ″ and D 2 ′ may be coupled together and coupled with v− bias port  26 D via resistor RN 1 . Cathodes of varactors D 2 ′ and D 3 ′ may be coupled together and coupled with v+ bias port  26 C via resistor RP 2 . Anodes of varactors D 3 ′ and D 4 ″ may be coupled together and coupled with v− bias port  26 D via resistor RN 2 . Cathodes of varactors D 4 ″ and D 5 ″ may be coupled together and coupled with v+ bias port  26 C via resistor RP 3 . Anodes of varactors D 5 ″ and D 6 ′ may be coupled together and coupled with v− bias port  26 D via resistor RN 3 . Cathodes of varactors D 6 ′ and D 7 ′ may be coupled together and coupled with v+ bias port  26 C via resistor RP 4 . Anodes of varactors D 7 ′ and D 8 ″ may be coupled together and coupled with v− bias port  26 D via resistor RN 4 . Cathode of varactor D 8 ″ may be coupled with output port  26 B. Varactors D 1 ″, D 4 ″, D 5 ″, and D 8 ″ may each have a first effective area and each have a first parasitic capacitance to other circuitry and/or ground  20 . Varactors D 2 ′, D 3 ′, D 6 ′, and D 7 ′ may each have a second effective area that is smaller than the first effective area and each have a second parasitic capacitance that is smaller than the first parasitic capacitance. In a preferred embodiment, the second effective area may be 55% of the first effective area. In other embodiments, the second effective area may be between 50% and 60% of the first effective area, while in other embodiments, the second effective area may be between 45% and 75% of the first effective area. 
     Resisters RP 1 -RP 4  may provide parallel bias voltage connections between each of the cathodes of varactors D 1 ″ through D 8 ″ of the modified anti-series string  28  and the v+ bias port  26 C. Resistors RN 1 -RN 4  may provide parallel bias connections between each of the anodes of varactors D 1 ″ through D 8 ″ and the v− bias port  26 D. In some embodiments, a resistor (not shown) may be positioned between the output port  26 B and the v+ bias port  26 C. 
     Bias circuitry  14  may be coupled between the v+ bias port  26 C and ground  20 , and ground  20  may be coupled with v− bias port  26 D to provide a reverse bias voltage across each of the varactors D 1 ″ through D 8 ″. The bias circuitry  14  may adjust the effective capacitance of the modified anti-series string  28 . 
     As the RF signal described in  FIG. 1  propagates from the input port  26 A to the output port  26 B, a second harmonic signal from self-modulation may be reduced from the second harmonic signal of the compound varactor circuit  12 . In some embodiments, the second harmonic signal may be reduced by approximately 20 decibels (dB). This reduced level of second harmonic signal level generation may be acceptable for use of the compound varactor  24  in antenna matching and other unfiltered varactor applications. 
     The compound varactor  24  as compared with compound varactor  12  may have negligible change in the following varactor parameters: 
     input third order intercept point (IIP3), 
     output third order intercept point (OIP3), and 
     quality factor (Q). 
     The total effective areas of modified anti-series string  28  have a less than 10% increase over the total effective areas of example anti-series string  18 . In a non-limiting example for the compound varactor  12 , the example anti-series string  18  may provide 1 unit of capacitance for a given bias voltage. Each varactor D 1 -D 8  may have 8 units of effective area and may each provide 8 units of capacitance. The total effective area of the example anti-series string  18  may be 64 units. For the modified anti-series string  28  to provide 1 unit of capacitance for a given bias voltage, the effective area of each varactor D 1 ″, D 4 ″, D 5 ″, and D 8 ″ may have 11.3 units of effective area and may each provide 11.3 units of capacitance, while each varactor D 2 ′, D 3 ′, D 6 ′, and D 7 ′ may have 6.2 units of effective area and may each provide 6.2 units of capacitance. The total effective area of anti-series string  28  may be approximately 70 units. For this example the total increase in effective area of the modified anti-series string  28  is less than 10%. 
       FIG. 3  illustrates an epitaxial stack  30  forming a single varactor such as varactor D 1 ″, D 2 ′, D 3 ′, D 4 ″, D 5 ″, D 6 ′, D 7 ′, or D 8 ″, as shown in  FIG. 2 . In embodiments, the epitaxial stack  30  may comprise a plurality of layers formed on a substrate  32 . The substrate  32  may be constructed of a semiconductor material that is relatively inert with respect to the epitaxial stack  30 . The substrate  32  may be an undoped or a lightly doped semiconductor material having a relatively high resistivity as compared with the other layers. 
     A lower contact layer  34  may be positioned over the substrate  32 . A varactor layer  36  may be positioned over the lower contact layer  34 . An upper contact layer  38  may be position over the varactor layer  36 . An ohmic contact  40  may be positioned over the upper contact layer  38 . An ohmic contact  42  may be positioned over the lower contact layer  34 . In some embodiments, the lower contact layer  34  may be a heavily doped P+ anode layer, while the upper contact layer  38  may be a heavily doped N+ cathode layer. In this embodiment, ohmic contact  40  may be a cathode contact and ohmic contact  42  may be an anode contact. In other embodiments, the lower contact layer  34  may be a heavily doped N+ cathode layer, while the upper contact layer  38  may be a heavily doped P+ anode layer. In this embodiment, ohmic contact  40  may be an anode contact and ohmic contact  42  may be a cathode contact. 
     The varactor layer  36  may be a lightly doped N− cathode layer and may be created with an abrupt, hyper abrupt, or linear doping profile. The area of the varactor layer  36  is the effective area  44  for parasitic capacitance effects as described in  FIG. 2 . 
     In some embodiments, portions of the modified compound varactor  24  may be configured as shown in FIG. 3 of U.S. Pat. No. 9,484,471, entitled “COMPOUND VARACTOR”, which is hereby incorporated by reference in its entirety. Materials and manufacture processes for compound varactor  24  may be equivalent to compound varactor  12 . 
       FIG. 4  illustrates a graph  44  of second harmonic signal generation for compound varactors  12  and  24 . Power level (dBm) of the second harmonic is represented on the vertical axis and varactor bias (volts) is represented on the horizontal axis. The dashed line plots the second harmonic of compound varactor  12  at output port  16 B using the circuit diagram  10  of  FIG. 1 . Bias circuitry  14  is varied between 2 and 18 volts. A +35 decibel-milliwatts (dBm) sine wave at 900 MHz is coupled to input port  16 A. The second harmonic is shown to vary between approximately −52 dBm and approximately −56 dBm. The solid line plots the second harmonic of compound varactor  24  at output port  26 B using the circuit diagram  22  of  FIG. 2 . Bias circuitry  14  is varied between 2 and 18 volts. A +35 decibel-milliwatts (dBm) sine wave at 900 MHz is coupled to input port  26 A. The second harmonic is shown to vary between approximately −72 dBm and approximately −77 dBm. 
       FIG. 5  illustrates a circuit diagram  46  having a compound varactor  48  with modified anti-series string  50  and bias circuitry  14 . Modified anti-series string  50  is an alternate embodiment of modified anti-series string  28  wherein cathodes are coupled to input port  52 A and output port  52 B. Compound varactor  48  also includes a v+ bias port  52 C and a v− bias port  52 D. The modified anti-series string  50  comprises varactors D 1 ″ through D 8 ″ and replaces the anti-series string  28  of compound varactor  24 . Anode of varactor D 1 ″ may be coupled with the input port  52 A and coupled with the v− bias port  52 D via resistor RN 1 . Cathodes of varactors D 1 ″ and D 2 ′ may be coupled together and coupled with v+ bias port  52 C via resistor RP 1 . Anodes of varactors D 2 ′ and D 3 ′ may be coupled together and coupled with v− bias port  52 D via resistor RN 2 . Cathodes of varactors D 3 ′ and D 4 ″ may be coupled together and coupled with v+ bias port  52 C via resistor RP 2 . Anodes of varactors D 4 ″ and D 5 ″ may be coupled together and coupled with v− bias port  52 D via resistor RN 3 . Cathodes of varactors D 5 ″ and D 6 ′ may be coupled together and coupled with v+ bias port  52 C via resistor RP 3 . Anodes of varactors D 6 ′ and D 7 ′ may be coupled together and coupled with v− bias port  52 D via resistor RN 4 . Cathodes of varactors D 7 ′ and D 8 ″ may be coupled together and coupled with v+ bias port  52 C via resistor RP 4 . Anode of varactor D 8 ″ may be coupled with output port  52 B. Varactors D 1 ″, D 4 ″, D 5 ″, and D 8 ″ may each have the first effective area and each have the first parasitic capacitance to other circuitry and/or ground  20 . Varactors D 2 ′, D 3 ′, D 6 ′, and D 7 ′ may each have the second effective area that is smaller than the first effective area and each have the second parasitic capacitance that is smaller than first parasitic capacitance. In a preferred embodiment, the second effective area may be 55% of the first effective area. In other embodiments, the second effective area may be between 50% and 60% of the first effective area, while in other embodiments, the second effective area may be between 45% and 75% of the first effective area 
     Resisters RP 1 -RP 4  may provide parallel bias voltage connections between each of the cathodes of varactors D 1 ″ through D 8 ″ of the modified anti-series string  50  and the v+ bias port  52 C. Resistors RN 1 -RN 4  may provide parallel bias connections between each of the anodes of varactors D 1 ″ through D 7 ′ and the v− bias port  52 D. In some embodiments, a resistor (not shown) may be positioned between the output port  52 B and the v− bias port  52 D. 
     In some embodiments, the modified anti-series string  28  and  50  may have only four varactors. In other embodiments the modified anti-series string  28  and  50  may have twelve varactors. 
     Compound varactors  24  and  48  may be incorporated into a variety of devices and/or systems. 
       FIG. 6  illustrates a wireless device  54  having a radio front end (RFE) module  56  coupled with a processor  58  and an antenna  60 . Compound varactor  24  may be incorporated within or coupled with the RFE module  56  and may be used to provide impedance matching of the antenna  60  with other circuitry of the RFE module  56 . Such circuitry may be a transmission line, a filter, a transmit power amplifier (PA), or a receive low noise amplifier (LNA). In other embodiments of the RFE module  56 , the modified compound varactor  24  may be configured to control the frequency of a voltage controlled oscillator (VCO), control the frequency and/or phase response of a filter, or be incorporated within an electronically controlled switch. In other embodiments, compound varactor  48  may be incorporated within or coupled with the RFE module  56 . 
     Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.

Technology Category: 5