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Low Distortion Tunable RF Components, a Compound Semiconductor Opportunity

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Low Distortion Tunable RF Components, a Compound Semiconductor Opportunity Low Distortion Tunable RF Components, a Compound Semiconductor Opportunity Cong Huang1, Koen Buisman1, Peter J. Zampardi2, Lis K. Nanver1, Lawrence E. Larson3 and Leo C. N. de Vreede1 1Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, Mekelweg 4, 2628 CD, Delft, the Netherlands, [email protected], 0031(0)152787964 2Skyworks Solutions, Inc., 2427 W. Hillcrest Drive 889-A1, Newbury Park, CA 91320 3Center for Wireless Communications, Department of Electrical and Computer Engineering, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0407 Keywords: Compound semiconductor, low distortion, Q factor, tunable circuits and devices, varactors. Abstract An overview is given for the current state-of-the-art Theoretically, an ideal switch exhibits an “open” in the of semiconductor based tunable RF elements. For this OFF state and “short” in the ON state. When such a switch is purpose, semiconductor based capacitive switch banks series connected with a fixed capacitor (Cfix), a capacitive and varactors are reviewed and compared with the switch is created, which can alternate between an “open” recently developed ultra-low distortion semiconductor and a fixed capacitance value (Cfix). By combining N based varactors. It reveals that the latter solution, when capacitive switches as shown in Fig. 1(a), a capacitive compared against all existing technology platforms for switch bank is created, which can provide 2N different continuously tunable elements, can provide superior capacitance values. These types of capacitive switch banks performance in tuning range, linearity, and quality typically provide only discrete tuning resulting for factor. applications that require fine tuning, in a high number of capacitive switches with related control voltages. In contrast, INTRODUCTION tunable varactors [Fig. 1(b)] provide continuous capacitance tuning over a given range with only one control voltage. With the development of wireless communication, Generally speaking, the first solution is better suited to tunable elements like continuously variable reactors implement high capacitance tuning ranges, under the (varactors) and capacitive switch banks can play an assumption that an excellent switch is available, whereas important role to facilitate RF reconfigurability and phase- continuously tunable varactors can offer a more favorable diversity systems. Application examples include: adaptive combination of quality factor and tuning resolution. Both matching for multi-band/multi-mode power amplifiers and candidates are subject to intensive research to reduce their antenna mismatch correction, tunable filters, as well as device parasitics in order to achieve an improved quality adjustable “true” time delay phase shifters for smart antenna factor and capacitance tuning range. systems. An ideal tunable RF element for these applications will Cfix_1 VS1 exhibit very low loss, low DC power consumption, high single capacitive linearity, excellent ruggedness to high voltage and high switch Cfix_2 VS2 current conditions, large tuning range, high reliability, low Vcont cost, low area usage, and will be continuously tunable with Cfix_3 VS3 high tuning speed. … Compared to MEMS solutions, semiconductor based counterparts provide advantages in terms of low control voltage, high capacitance density, low packaging costs, high Cfix_N VSN reliability and technology compatibility. Within this work, conventional semiconductor based capacitive switch banks (a) (b) and varactors are reviewed for their benefits and Fig. 1. (a) Schematic of the capacitive switch bank composed of N shortcomings and compared with the recently developed capacitive switches. (b) Schematic of the varactor. ultra low-distortion semiconductor based varactors. CAPACITIVE SWITCH BANKS AND CONTINUOUSLY TUNABLE SEMICONDUCTOR BASED CAPACITIVE SWITCH BANK VARACTORS A capacitive switch can be composed by a series connection of a fixed capacitor with a semiconductor based CS MANTECH Conference, May 17th-20th, 2010, Portland, Oregon, USA switch (Fig. 2). When using (multiple stacked) diodes or degradation due to the parasitics of the connecting network transistors for the switching element, typically its behavior to the switch array will take place. Furthermore, the linearity can be approximated by a resistor (Ron) in the ON state and a of the semiconductor based capacitive switch bank is also a capacitor (Coff) in its OFF state. In order to achieve sufficient concern and challenge in their application. capacitance tuning range, the fixed capacitor Con must be significantly larger than the off-state capacitance (Coff.). TABLE I SURVEY OF STATE-OF-THE-ART RON-COFF PRODUCTS FROM DIFFERENT PROCESS TECHNOLOGY AND CORRESPONDING MOS Q-TTUNE PRODUCTS AT 2 GHZ AND 5 GHZ transistor Coff Ron RC4 QT4(1) QT4(1) Process Technology on off tune tune (fs) at 2 GHz at 5 GHz Con Con Con 0.5 m 10 nm Tox SOS 756 [1] 105 42 0.25 m 5 nm Tox SOS 448 [1] 178 71 (a) (b) (c) 0.5 m pHEMT 360 [2] 221 88 0.15 m pHEMT 435 [2] 183 73 Fig. 2. (a) Schematic of a capacitive switch composed by a series connection of a fixed capacitor (Con) with a MOS transistor. (b) Equivalent circuit of the capacitive switch in the OFF state. (c) Equivalent circuit of the CONVENTIONAL SEMICONDUCTOR VARACTORS capacitive switch in the ON state. Semiconductor based varactors, like the p-n diode, Since semiconductor based capacitive switches can Schottky diode and MOS varactors, are basically voltage- provide a large capacitance density, it is relatively easy to controlled capacitances, which provide continuously tuning integrate a large number of capacitive switches to achieve a making use of the voltage-dependent depletion layer fine tuning resolution. However, there is a stringent tradeoff thickness of the space charge region. Due to the relatively between the tuning range and quality factor for a high dielectric constants of semiconductor materials, semiconductor based capacitive switch. To understand this, capacitance densities are typically at least 10 times larger we consider Fig. 2. The best operation of such a capacitive than their MEMS counterparts. In addition, semiconductor switch is achieved when both Ron and Coff are small, yielding based solutions have advantages in terms of integration, higher quality factor and capacitance tuning range. Note that reliability, tuning speed (1-100 ns), low-control voltage and lowering Ron through up-scaling of the switching device will ruggedness. However, their inherently nonlinear behavior typically increase the OFF-state capacitance (Coff), which in and low quality factor at microwave frequencies are turn limits the resulting capacitance ratio (Ttune). In view of considered to be incompatible with the requirements of this, care is normally taken to improve the Ron-Coff product of modern wireless communication systems, which require the switching device by special process technologies, e.g. high linearity and good quality factors. silicon-on-sapphire (SOS) CMOS [1] and pseudomorphic high-electron mobility transistors (pHEMT) [2]. Table I SEMICONDUCTOR BASED LOW-DISTORTION VARACTORS provides an overview of the current state-of-the-art Ron-Coff products of semiconductor based capacitive switches and As mentioned above, for conventional semiconductor their related Q(Ttune -1) product, which can be found as based varactors, tradeoffs are normally made between 1 capacitance tuning range and linearity. This can be QT4(1) (1) tune intuitively understood as follows. For a single varactor RCon off diode, increasing the tuning range for a constant breakdown where is the angular RF frequency and voltage will yield bigger capacitance changes for an applied QRC1/ on on (2) RF voltage, consequently its linearity will degrade. To represents the quality factor of the capacitive switch in the overcome this conflict, recently several specific varactor ON state, while diode topologies have been developed and CSCC C implemented [3]-[11]. These proposed varactor TC/1DTon off on (3) configurations act as variable capacitors between their tune on DTCC C EUon off off RF terminals with ideally zero, or extremely low is the capacitance ratio between ON and OFF states. Closer distortion, while a third terminal is used for a “low- inspection of Table I indicates that the room for frequency” control voltage. A brief description of these compromising between the quality factor and tuning range is low-distortion varactor configurations is given below. in fact very limited, especially at RF frequencies. This The distortion-free varactor stack (DFVS) [4], [6] is clearly highlights the current bottleneck for semiconductor based on an anti-series connection of two identical based capacitive switches. Consequently, their applications uniformly doped diodes [Fig. 3(a)]. This uniform doping are mostly found below 2 GHz. In addition, note that when results in a capacitance power law coefficient of n=0.5 fine tuning of the capacitive device is needed, performance [12]. Furthermore, an “infinitely” high impedance is used CS MANTECH Conference, May 17th-20th, 2010, Portland, Oregon, USA to connect to the center-tap of the diode configuration. avoided. In (adaptive) transmitting systems, however, the in- Under these conditions, all distortion components at the band linearity will be the biggest concern. For these RF terminals are perfectly cancelled, yielding a applications the NTSVS [7] is recommended, since it distortion-free operation. provides the highest linearity for
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