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Ka Band High Power AlGaAs PIN Diode Switches Andrzej Rozbicki, James Brogle, Nitin Jain*, Timothy Boles, David Hoag MA-COM Technology Solutions – a Cobham Company, Lowell, MA, 01851, USA * Anokiwave Inc., San Diego, CA, 92130, USA SP3T switch manufactured in GaAs and AlGaAs shows difference of 0.12 dB at 20GHz. Further, it has been proven that AlGaAs PIN diode switches demonstrate improvements in terms of reduced insertion loss with no degradation of isolation. Also for many years MA-COM PIN based switch circuits fabricated from this material have been tested and have demonstrated broadband RF performance from 50 MHz to greater than 77GHz. A photograph of a shunt PIN diode and series PIN diodes used in these MMIC circuits are depicted in Fig 1. Abstract — In this paper we present the design and performance of millimeter wave MMIC switches in the patented MA-COM AlGaAs heterojunction PIN Diode process that allow us to produce high power and low insertion loss devices. The design process from a reflective SPDT switch to a non-reflective version of the switch, with intense use of HFSS and ADS software, is presented. These switches were designed to meet demanding requirements: low insertion loss less than 0.8 dB, 40dBm peak power and 37dBm CW power, and 30dB isolation. Index Terms — AlGaAs, PIN diode, monolithic microwave integrated circuit; switch, Ka Band. I. INTRODUCTION Monolithic microwave integrated circuits (MMICs) based on high power PIN diodes are increasingly used for many transmit/receive systems in advanced defense electronics and telecommunications applications. Examples of such systems include radar, half-duplex data links, Internet-protocol-based (IP-based) wireless LAN’s, and millimeter-wave imagers. For such applications, switches are required to have high power handling (several watts), low insertion loss, good matching, fast switching (several nanoseconds) and for non-reflective switches good return loss in off state is required. Also these devices have to be small in size to support tight-fitting and portable applications. In the design process intense modeling including ADS and HFSS software was implemented to ensure first-pass success. The switches presented here use AlGaAs/GaAs based heterojunction technology [1] that provides PIN diodes with reduced series resistance. The design and performance of the reflective SPDT switch and of the non-reflective switch are described. The process of simulation and matching with measured results are presented. Air Bridge Series Diodes Air Bridge Fig. 1. diodes Cathode connection Photographs of a shunt PIN diode and two series PIN The shunt diode anode diameter, used in this application, is 46um and has Rs = 1.12 Ohm @ 1 GHz with a bias current of 10mA, and Rs = 0.88 Ohm @ 1 GHz with a bias current of 20mA, and Coff = 0.12pF. These parameters allowed us to design a low insertion loss switch with high isolation. The series diode used in the non-reflective version of the SPDT switch has an anode diameter of 20um. This diode has Rs=4.7 Ohm @ 1 GHz with a bias current of 20mA, and Coff = 0.08 pF. In order to correctly predict the performance of the circuit, first an ADS equivalent circuit of the PIN diode of previously manufactured diodes was created. For the shunt diode a lumped-element equivalent circuit model was used. The model consist of tree main elements in series: 1) a diode with junction parameters in parallel Rj, Cj, and diffusion capacitance Cd; 2) Rs that represents the resistance of the bulk semiconductor regions plus the resistance of the contacts; and 3) i-region parameters Ri and Ci in parallel. Also the via inductance Lvia and a distributed parasitic capacitance Cpar were included in this model. Detailed PIN diode equivalent circuits parameters were derived from S-parameters measured on existing PIN diodes. Then S-parameters were transformed to Z-parameters, a resistance versus frequency was extracted for forward biased II. DISCUSSION The MA-COM patented technology used to manufacture AlGaAs hetero-junction PIN diodes use the advantage of the P+ AlGaAs junction over comparable P+ GaAs to I region junction. As Hoag et. al describes “It was found that the recombination rate for electrons at the P+ GaAs - I-region junction is sizable in comparison to the P+ AlGaAs heterojunction. The combination of lower recombination rates and higher carrier injection will yield a greater number of carriers thus lowering the effective resistivity of the I-region’’ [1]. Indeed average measured insertion loss response for a 978-1-4244-2804-5/09/$25.00 © 2009 IEEE Anode Shunt Diode 453 Authorized licensed use limited to: SOUTHWEST JIAOTONG UNIVERSITY. Downloaded on January 31, 2010 at 03:49 from IEEE Xplore. Restrictions apply. IMS 2009 diode and shunt capacitance versus frequency for reverse biased diode. The size of the shunt diode was selected so that the diode is capable to handle the maximum peak power of 10W through the switch. Simulated power dissipation (Fig. 2) for diodes in Cathode connection Anode 1000 Pdiss_diode Pdiss_on_diode Pdiss_off_diode mW Fig. 4. Total Diss Power 800 The series diode was not used in the reflective switch design because the topology chosen was the typical shunt connected PIN diode switches with quarter wavelength elements. This shunt diode topology offers relatively high isolation and lowest insertion loss as well as it is capable of handling more RF power then a diode in a series type switch because the shunt diode is electrically and thermally grounded at one electrode. The insertion loss of a shunt PIN diode switch is a function of diode off-capacitance CT and defined by: 600 Diode ON 400 200 Diode OFF 0 24 26 28 30 32 34 36 38 40 42 44 46 Frequency (GHz) Fig. 2. Power IL = 10Log[1+( fCTZ0)2] (1) Power dissipation in shunt diodes with 10W Incident As previously specified the shunt diode used in this design has very low capacitance CT = 0.12pF in Off state. The shunt diodes designed with the MA-COM process are rated at 78V breakdown voltage that guaranties high linearity at high power applications. The breakdown voltage was measured at 10uA. A simple thermal analysis was included to ensure that the diode selected would handle high power conditions. Combinations of silicon nitride layers of 4000Å thickness have been applied as the MIM capacitors dielectric layer and the diode passivation layers. The silicon nitride layer forms capacitors with a breakdown voltage higher than 250V [2]. Also it was proven that the incorporation of thicker silicon nitride films improve the switch performance over the 30 GHz to 40Ghz frequency range. Circuit insertion losses were optimized by properly selecting the elements for a low ripple Chebyshev response. Lower insertion loss of a system with a shunt diode could be achieved by lowering the system impedance. At 50 ohm, for a given shunt diode with Ct of 0.12pf, the theoretical insertion loss is 1.5dB. If the system impedance would be 40 ohm the insertion loss would be lowered to 1.0dB, and at 30ohm would be 0.63dB. Such an approach was applied in this design. The quarter wavelength section of each arm was carefully simulated and the impedance was lowered so that at 35GHz the impedance is 38 Ohms. In order to maintain relatively high return loss and low VSWR the common arm was designed also as a quarter wavelength section with lower impedance in range of 36ohms. To compensate the lower impedance of the input there is a small section of transmition line connecting the quarter wavelength section with the input test pad that represents high impedance. Each part of these transmission lines were modeled using HFSS and simulation of the whole system was done in ADS software. Examples of the HFSS models are presented in Fig. 3, 4, 5, and 8. the OFF state should not exceed 300mW and for both diodes in the circuit at any given state should not exceed 550mW at 40GHz. Such equivalent circuits were used in further design process to determine the accurate size of the diode. As the size and parameters of the diode were established, then an HFSS model, Fig. 3, of the shunt diode was designed and used in further simulations. ON OFF Anode Ground Via Fig. 3. HFSS model of AlGaAs series diode HFSS Model of the shunt PIN diode in ON and OFF state. In case of series diodes, existing diode S-parameter measurements as well as an HFSS model of the series diode Fig. 4 were used for simulations. The small series diode has relatively low isolation 9dB @ 35GHz and during the process of design it became evident that there was a need for higher isolation in order to maintain lower insertion loss and VSWR of the outputs. The isolation of two diodes in series was increased to 20dB @ 35GHz. Circuit simulations indicated that adding one additional diode into the circuit did not have a significant influence on the return loss of the termination circuit. There was a need to adjust the L-C balance of the circuit that was changed. 454 Authorized licensed use limited to: SOUTHWEST JIAOTONG UNIVERSITY. Downloaded on January 31, 2010 at 03:49 from IEEE Xplore. Restrictions apply. 0.0 Isolation [dB] Air bridges -10.0 Insertion Loss -20.0 -0.2 -0.4 -0.6 -30.0 -40.0 30.0 Fig.5. High impedance section with a blocking capacitor, HFSS model (left) and on-wafer photograph (right) 0.0 Isolation 32.0 34.0 36.0 38.0 -0.8 -1.0 40.0 Insertion Loss [dB] Capacitor Freq [GHz] Fig. 7. III. REFLECTIVE SPDT SWITCH In result of this process, a reflective SPDT switch was designed and manufactured. Fig. 6 shows a photograph of the single chip. BIAS RF Low Z IV. NON REFLECTIVE CIRCUIT The second phase was required to redesign this basic configuration of the SPDT switch into multi-operational switch that partially included the SPDT design with the requirements of providing non reflective outputs. Because this switch was solely a custom design it is not possible to present details of the switch configuration. But for purposes of this publication it is sufficient to present and discuss a part of the circuit that is in scope of this paper. The following describes the termination circuit as seen in Fig. 8 and its performance. BIAS High Z RF RF DC Ctrl Bias GND Via Shunt Diode Fig. 6. Measured insertion loss and isolation Photograph of a Die of the reflective SPDT switch RF output To achieve optimal results and small dimensions of 2.2 mm x 1.2 mm the switch represents a typical T with bias circuits between the common arm and output arm. The trace connecting the bias pad and its capacitor to the arm represents a high pass filter and works very well in the desired range of 30GHz to 40 GHz frequency. Each arm was designed as a low impedance quarter-wave transmission line which ends in a 25um width air-bridge (see Fig. 1) connected to the shunt diode anode. The width of the bridge was chosen so that it can handle high RF current density and is in balance with the Rs and Cj of the diode. To lower the diode’s connection inductance to ground as well as to balance the diode’s symmetry the cathode is connected to the ground over two vias symmetrically placed on each side. The RF outputs are connected with the shunt diode over a high impedance transmition line that includes a blocking capacitor (see Fig. 5). The line was analyzed in HFSS for best performance. The typical insertion loss of 0.55dB was achieved at 35GHz but not more than 0.67 dB from 30 to 40GHz. Typical isolation of 27 to 30 dB in this frequency range is shown in Fig. 7, in addition to insertion loss. Series Diodes Terminatio n Resistor Fig. 8. RF Output with the termination circuit, HFSS model (left) and on-wafer photograph (right) As previously described the two series diodes were implemented as switch elements to the RF output in order to control the termination resistor. The HFSS model and the actual layout of these two series diodes with the termination resistor are presented in Fig. 8. Because of space limitation the connection between the shunt diode and the RF output pad was achieved through two chamfered 90 degree bends including a DC blocking capacitor. Also the bias pads were relocated to opposite side of the arms and this change was included in the HFSS simulation, but it is not presented in this paper. Intensive analysis using HFSS and ADS resulted in achieving good performance of the new design. The typical insertion loss of one arm in ON state increased from 0.55dB to 0.7dB at 35GHz but not more than 0.8dB from 30 to 40GHz. This 455 Authorized licensed use limited to: SOUTHWEST JIAOTONG UNIVERSITY. Downloaded on January 31, 2010 at 03:49 from IEEE Xplore. Restrictions apply. remarkable insertion loss is the best that is known for this type of switch [3]. As presented in Fig. 9, the measured insertion loss is similar to HFSS/ADS simulation results. Samples of this chip underwent some power tests. The tests demonstrated an insertion loss of 0.78dB at 35GHz with a 10% duty cycle pulse with 0.87W input power. The insertion loss increased to 0.92dB at an input level of 8.41W (39.2dBm). The minimal amount of 0.14dB compression indicates that this circuit can likely handle 10W (40dBm) peak power. dB(S(6,5)) dB(S(4,3)) 0.0 -0.2 Measured -0.4 HFSS/ADS V. CONCLUSIONS -0.6 The two Ka band PIN diode switches described here have demonstrated excellent RF characteristics and high power capabilities. The reflective switch has an insertion loss of 0.65dB at 35GHz and good flatness between 30 and 40 GHz. Isolation is better than 27dB from 30 to 40 GHz. The switch is capable of handling 40dBm peak power and 37dBm CW. The non-reflective switch has typical insertion loss of 0.75dB at 35GHz that does not exceed 0.8dB between 30 and 40 GHz. The isolation is better than 30dB from 30 to 40GHz. The switch is capable of handling 40dBm peak pulsed power and 37dBm under CW conditions. The performance was achieved due to two main factors: MA-COM’s unique manufacturing process and intensive involvement of high quality simulation tools like HFSS and ADS. The design process shown in this paper proves that use of available tools like HFSS and ADS make it possible to design and achieve complicated high frequency MMIC circuit and be successful in the first run. The authors believe that some adjustments are needed to the simulation process including better correlation of the simulation tool with the real product. In case of insertion loss and isolation the achieved results are very good, only return loss of the terminated port should be carefully analyzed for possible improvement. -0.8 -1.0 30 32 34 36 38 40 freq, GHz Fig. 9. Measured vs. simulated insertion loss VSWR does not exceed 1.5:1 including a 3x1mil ribbon connection at ON state. The return loss (Fig 10) of the terminated output is 10dB at 34 GHz and 13dB at 39 GHz including the ribbon connection. dB(S(6,6)) dB(S(4,4)) 0 Measured -5 HFSS/ADS -10 -15 -20 30 32 34 36 38 40 REFERENCES freq, GHz [1] D. Hoag, J. Brogle, T. Boles, D. Curcio, and D. Russell, “Heterojunction PIN diode switch,” 2003 IEEE MTT-S International Microwave Symposium Digest, vol. 1, pp. 255258, 8-13 June 2003. [2] D. Hoag, D. Curcio, T. Boles, “Development of High Voltage mmW GaAs PIN Diode Switch”, GaAs MANTECH, 2001 [3] O. Levy, A. Madjar, D. Kryger, S. Matarasso, “Fully terminated Ka band high isolation, high power MMIC SPDT switch in GaAs PIN technology,” IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, pp. 2019-2022, June 2003. Fig. 10. Measured vs. simulated return loss of the terminated port The return loss of a terminated port below 34GHz is lower then 10dB and it is required to perform additional analysis in order to get better performance. The isolation is 32dB at 35GHz but not less then 30dB from 30Ghz to 40GHz. The expected isolation was around 35dB in HFSS/ADS simulations. 456 Authorized licensed use limited to: SOUTHWEST JIAOTONG UNIVERSITY. Downloaded on January 31, 2010 at 03:49 from IEEE Xplore. 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