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A DC-6 GHz, Packaged 100 Watt GaN SPDT Switch MMIC

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A DC-6 GHz, Packaged 100 Watt GaN SPDT Switch MMIC

  1. 1. A DC-6 GHz, Packaged 100 Watt GaN SPDT Switch MMIC Ritu Jain, Sushil Kumar, Bernie Stocklingsky, Jeff Shealy RFMD, 105 E Tasman Drive San Jose, CA, USA ritu_jn@hotmail.com, skieee@gmail.com Abstract—A Low insertion loss, high isolation and high power DC-6GHz SPDT has been developed using GaN on SiC using 0.5um HEMT technology. The developed switch has CW P0.1dB as 100W and Pulsed Pmax >126W. Insertion loss of this SPDT is ≤0.8dB with isolation ≥30dB & RL ≥20dB across the band. Measured ON/OFF switching time was <20nS. Switch performed very well during hot switching test for pulsed control voltage (Vc) =-30 to -60V. No degradation in IL or Isolation was observed between hot and cold switching. The switch is packaged in a plastic 5X5 QFN package. Index Terms—SPDT switches, Gallium Nitride (GaN), HEMT, FETs, High Power, High Isolation, Low insertion loss, Stacked series/shunt FETs. I. INTRODUCTION RF and microwave switches find extensive application in wireless, radar and aerospace systems for signal routing and switching between transmit and receive chain. Switches are widely used in microwave and RF test systems as well for signal routing. There are two major types of switches available for RF & microwave application; one is electromechanical (MEMS) and other is solid-state (PIN/FET or Hybrid of PIN and FET) switches. Electromechanical switches rely on mechanical contacts for switching mechanism. Semiconductor materials used for FET based switches are GaAs, GaN, Si-On-Sapphire (SOS) and Si-On-Insulator (SOI). All these technologies and devices based switches have certain benefits relative to each other, like MEMS based switch has low insertion loss, high isolation, low power consumption, extreme linearity and the ability to be integrated with other electronics. Reliability, especially for hot switching is a major issue with MEMS switch and this technology is still emerging. So far GaAs based switches were workhorse for most of the RF and microwave solid-state circuit based low to moderate power applications. Recent advancement of CMOS SOS and SOI based switch has started gaining ground in RF and low microwave frequency range. Solid-state PIN/FET based switches are typically more reliable and exhibit longer lifetime than electromechanical switches, plus offer faster switching time. However, solid-state switches typically have higher intrinsic ON resistance and more harmonic distortion than mechanical switches. For moderate to high power and high frequency applications PIN diode based switches are widely used. GaAs based FET has low breakdown voltage and so power handling capability is limited to a few watts compared to PIN based switches. The emergence of new wideband gap device technology based on GaN material has significantly enhanced the power handling capability of GaN FET based switches due to the key properties, high electric field strength and low on resistance. GaN based switch provide very large power handling from almost DC to very high frequency and consumes negligible power. These are added benefit of GaN switches. GaN on SiC has been used for development of SPDT switch discussed in this paper. High thermal conductivity (>330 W/m.K) of SiC substrate prevents extreme channel temperature and reduce self-heating [1]-[2]. A DC- 6GHz High Power 100W (CW) MMIC SPDT switch has been developed using GaN on SiC. The developed switch has very low insertion loss (<0.8 dB) and high isolation (> 30 dB) over the entire DC-6GHz band. Switching speed is < 20ns. Switch did not show any degradation in RF performance during hot switching test with pulsed Vc from -30 to -60V range. To the author’s best knowledge, this switch has highest power, lowest insertion loss, highest isolation and fastest switching speed compared to any other reported commercially or published switch in the DC-6GHz frequency band. II. GAN SEMICONDUCTOR DEVICE PROCESS The developed switch MMIC was fabricated using Gallium Nitride HEMT technology on 4 mil SiC substrate. Source and Drain was formed using ohmic contacts and uses 0.5µm Schottky gate. For device passivation and MIM capacitors, thin layers of Si3N4 with cap-density of 135pF/mm2 were used. The gate resistance was defined by the bulk GaN mesa resistivity 500 Ω/sq. The switch MMICs were fabricated using a two-level metal interconnects with dielectric crossover technology. The wafers were thinned to 4 mil and uses back side via to DC and low parasitic RF ground. The wafer backsides were Au plated. Idmax for this process is 1.1A/mm PREPRESS PROOF FILE CAUSAL PRODUCTIONS1
  2. 2. and BV1mA is 200V. Peak Ft and Fmax are 12 GHz and 45GHz respectively for this process. Extracted on-state resistance is ~2.3Ohm-mm, Cds is ~0.078 pF/mm and off-state capacitance Cgd =Cgs ≈0.27 pF/mm and 0.11 pF/mm at -10V and -40V Vgs, respectively. High breakdown voltage (400V) enables operation at control voltage -60 V and excellent ruggedness. III. FET MODEL AND SWITCH DESIGN To design the SPDT switch a Verilog-A nonlinear passive FET model based on [1] has been developed using Agilent’s ICCAP and implemented in ADS. Various sizes of symmetric devices in CS and CG configuration have been layout and measured to extract proper model and check model device size scaling. Developed AlGaN/GaN PHEMT models are specifically fitted to address the switch design simulations. It provides the flexibility to model Hetero structure Field Effect Transistors for both switch and amplifier applications. Main features of these models are symmetry w.r.t. Drain and Source terminals, better description and fitting of sub-threshold regime and the continuous conduction and charge equations and their derivatives. It enables the elimination of discontinuities in current and capacitance formulations which plagues the typical FET models especially at VDS =0V. Model includes two gate diodes and two linear gate-drain and gate-source resistors to appropriately model the gate current. It also includes appropriate scaling relationships between device geometries including length, width and number of fingers of the device. Dispersion is treated as a frequency dependent output conductance in the form of a RC network between drain and source. Metallization capacitances are formulated in two parts, a gate-width-independent fringing portion and a width-dependent overlap portion. The design goals for developed SPDT switch were; insertion loss≤1 dB, isolation ≥ 30 dB, P0.1 dB ≈100Watt, I/O RL ≥ 10dB in the frequency range DC-6GHz and switching speed <30nS. The simplified switch design circuit configuration and the die picture are shown in Fig. 1a & 1b. Fig. 1a. Simplified Schematic of Fig. 1b. Layout of SPDT Switch SPDT Switch 1940µm x 1100µm x 100µm It consists of two stacked series and two stacked shunt transistors in each arm. Series transistors’ periphery has been selected to satisfy the power design requirements at common port for ON and OFF arm. For the high power switches, the power handling ability is determined by the ability of OFF state transistor to handle the max RF voltage without turning ON and the ability of series FETs to handle max RF current without burning. Following equations have been used to initially estimate series device size. Pmax = Imax 2 * Zo/2 (ON State) (1) And power delivered by single FET device shunt configuration in OFF state is Pmax = (Vbr+Vg) 2 / 2*Zo (OFF State) (2) Where Vg = (Vbr – Vp)/2, and Vbr, Vg and Vp, is the breakdown voltage, the bias voltage, and the pinch-off voltage of the FET. Zo is the characteristic impedance. Once series device size has been estimated, a proper large signal simulation for SPDT was performed with only series FETs in ON and OFF arm connected together without any shunt FET to ensure that both ON and OFF arm could handle >100W. Series FET’s sizes has been adjusted by this method. Same method has been used for shunt FETs. It was ensured that shunt FET could also handle Pin>100W independently. Advantage of stacked FETs is, it distributes RF swing and so higher power handling can be achieved. Disadvantage of stacked FETs is higher Ron. This increases IL and reduces Isolation. Coff of stacked FET is also lower as compared to a single device. This helps to improve isolation of OFF series stacked FETs but affects isolation of ON stack shunt FETs at lower frequencies. Shunt stack FETs also help to improve failures due to VSWR mismatch. The developed AlGaN/GaN PHEMT SPDT was housed in 5x5, 24 pin QFN plastic package. A proper package transition model has been developed and used during SPDT design. IV. MEASUREMENT RESULTS Fig.2. shows test fixture used to measure RF performance of SPDT switch, for thermal reason a large Aluminum heat sink was used. The switch has been characterized for Vc=-40 to - 60V and insignificant change in [S]-parameter and other parameters have been observed. Fig.3 shows measured insertion loss from DC-10GHz. It shows that @ 2GHz IL ~0.4dB and @ 6GHz IL ~0.8dB. Fig. 2. Test Fixture used for Fig. 3. Measured IL vs. Frequency Measurement 2
  3. 3. Fig.4. shows return loss of all the ports. It shows that RF input return loss and ON arm output return loss are >20dB over entire frequency range from DC-6GHz. Fig.5 shows Isolation of OFF arm. It shows that @ 2GHz Isolation ~38dB and @ 6GHz Isolation ~30dB. Fig. 4. Measured RL vs. Frequency Fig. 5. Measured Isol vs. Frequency Fig.6 shows measured Pin vs. IL (Pin-Pout) for switch operation in CW mode @ 2GHz. This plot shows that input P0.1dB of the switch is > 50dBm (100W). Fig.7 shows pulsed power compression at 10, 20, 50 & 75% duty cycle @ 3.1GHz, 200µs pulse width was used for testing. Measured pulsed P0.2dB was >126W. CW and Pulsed measurement were done on two different location and unit. The unit used for pulsed has non-optimal bond wire length and so it has slightly higher IL compared to unit used for CW test. With optimum bond wire length pulsed power is supposed to improve. This work is still in progress. Fig. 6. Pin-Pout vs. Pin (P0.1dB) Fig. 7. Pulsed Pout vs. Duty Cycle Fig.8 shows measured IIP3 @ 2GHz and 'f =50GHz. It was most difficult measurement of all. With the use of several isolators and filtering, system intermodulation could not be suppressed enough to ensure a highly accurate DUT intermodulation measurement. Measured IIP3 is ~ +82dBm. Fig.9 shows hot Switching performance measured for RFin= 1 GHz at 0dBm using spectrum analyzer in time domain at different Vcontrol. Measured insertion loss is - 0.4dB and isolation is -42dB or less for all Vc= -30 to -60V. Fig.10 (a) & (b) Switching time is measured using 100 KHz, Vc= -40 V control pulse, for RF= 1GHz. Rise time of control pulse is 32 ns. Measured switching ON and OFF time is ~ 20ns. Fig. 8. IIP3 vs. Pin Fig. 9. Hot Switching IL & ISOL @ Vc= -30 to -60 V, RFin= 1 GHz Fig. 10a. Measured Switch ON Fig. 10b. Measured Switch OFF Time Time TABLE I COMPARATIVE PERFORMANCE OF SPDT GaN SWITCHES Parameters This work [2], [3] [4], [5] [6] Freq. (GHz) DC-6 DC-6 DC-4 DC-3.5 P0.1dB (W) 100 40 25 100 Pulsed Pmax >126W - - - I L (dB) ≤0.8 ≤1.0 ≤0.7 ≤0.97 Isolation (dB) ≥ 30 ≥ 40 ≥ 30 ≥ 18.7 Switch. Speed (nS) On 20 50 30 - Off 20 50 30 - I/O RL (dB) >20 >18 >16 >17.5 IIP3@2GHz 81dBm 66dBm >60dBm - Package 5x5 4x4 5x5 die V. CONCLUSION This paper discusses design and measured performances of a DC-6GHz, 100W SPDT MMIC in a QFN package. To the best of author’s knowledge as shown in Table-I, measured performance in terms of small signal, power, IIP3 and switching speed are one of the best in comparison to any published or commercially available similar SPDT switch. ACKNOWLEDGMENT Authors are thankful to RFMD fab team for helping to make this circuit into real part, Dain Miller and Dan Henry for useful discussion. Authors are also thankful to Gorden Cook, Rick Montgomery for their market feedback, Eric Carpenter for helping as PM, Kalaya Gilstrap for testing the parts. Lin Ng and Lili Gong (assemblers) for doing all assembly work. Gautham Heeraguppe and Gunjan Pandey for helping with 3
  4. 4. PCB design. We are thankful to Trishul VP, Carl Hinshaw and Rob Dry for package design and selection. REFERENCES [1] F. Kharabi, “Modeling of FET Switches” CSICS2012 IEEE, pp.1-4. [2] Charles F. Cambell, and Deep C. Dumka, “Wideband High Power GaN Sic SPDT Switch MMICs,”2010 IEEE MTT-S Int. Microwave Symp., pp.145-148. [3] TGS2351-SM, “DC–6 GHz High Power SPDT Switch,” TriQuint Semiconductor Data Sheet. [4] Raymond S. Pengelly, et al., “A Review of GaN on SiC High Electron- Mobility Power Transistors and MMICs,” IEEE Trans. MTT., vol. 60, no. 6, pp. 2397-2402, June 2012. [5] CMSA30025S, “25 Watt, 0.1-3.0 GHz GaN SPDT Reflective Switch” CREE Data Sheet [6] Hangai, M., Nishino, T., Kamo, Y. and Miyazaki, M., "An Sband 100W GaN Protection Switch," IEEE MTT-S Int. Microwave Symp. Dig., 2007, pp. 1389-1392. 4

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