Wideband 400 W Pulsed Power GaN HEMT Amplifiers Matthew J. Poulton, Karthik Krishnamurthy, Jay Martin, Bart Landberg, Rama Vetury, David Aichele Aerospace and Defense Business Unit, RF Micro Devices, Inc., Charlotte, NC 28269, USA. Abstract — RFMD has developed 400W pulsed output power GaN HEMT amplifiers operating over 2.9 – 3.5GHz band or 17% bandwidth. Under pulsed RF drive with 10% duty cycle and 100µ µs pulse width, the amplifier delivers output power in the range of 401 – 446 W over the band, with a drain efficiency of 48 – 55% when biased at drain voltage of 65V. The amplifier uses AlGaN/GaN HEMTs with a total device periphery of 44.4mm and advanced source connected field plates for high breakdown voltage. These wideband high power amplifiers are suitable for use in frequency agile pulsed applications such as military radar, air traffic control radar, and communications jamming. Index Terms — Power amplifiers, gallium nitride (GaN), highelectron-mobility transistors (HEMTs), broadband amplifiers, frequency agile. I. INTRODUCTION The high power and wide bandwidth potential of GaN HEMT devices is well known [1]. RFMD has been developing high power amplifiers using GaN HEMTs for various applications. A 250W amplifier in the 2.14 – 2.5GHz band for wireless infrastructure applications in the W-CDMA and WiMAX bands was reported earlier [2]. Such wide bandwidth is essential for next generation frequency agile software defined radio architectures that use reconfigurable radios to support multiple frequency bands and various standards [3]. The military and commercial community requires high power and broadband modules for pulsed radar surveillance and air traffic control applications. The market is looking for next generation devices that provide higher power and broader bandwidth able to support 1.2 to 1.4GHz L-band for IFF, TACAN, TCAS pulsed applications and 2.7 to 3.5GHz S-band pulsed applications. These devices will enable suppliers to power combine fewer devices and reduce size and weight for >1kW power modules used in radar systems. To obtain high power in existing power amplifier technologies, such as silicon laterally diffused metal oxide semiconductor (Si LDMOS) and gallium arsenide pseudomorphic high electron mobility (GaAs pHEMT), large periphery devices are required. The resulting inherent large device parasitic capacitances per watt of output power lead to low device input and output impedances. Matching to such low impedance from a 50Ω system severely limits the bandwidth achievable. Also this limits the maximum power obtainable in a package, and further power combining is needed on the board. Wide-band gap material systems like gallium nitride (GaN) can be operated at high drain voltage and have low parasitic capacitances per watt of output power. A combination of wider bandwidth and high output power can be achieved in a small package. This simplifies the design of kW transmitters for radar and jamming applications by reducing the number of devices required and minimizing losses in external combining networks. II. THEORY In theory purely real impedances can be matched to a 50Ω system over any bandwidth using an infinite number of matching elements. However actual devices have device optimum impedances with a reactive component. Complex loads can be matched only over a limited bandwidth as defined by the Fano’s limit [4]. The maximum bandwidth ratio achieved using an infinite lossless matching network is given by, Fhigh − Flow Fo = π − QL ln(Γ) . (1) where QL, is the Q-factor of the device optimum source or load impedance to be matched, and Γ is the minimum reflection coefficient needed over the band. This bandwidth is further limited in practice due to finite number of matching sections and the matching network losses. Hence low Q-factor for the optimum source and load impedances are critical to obtaining broad bandwidth. Thus a suitable figure of merit for high power broadband capability of a device technology is a low pF/W gate and drain capacitance. III. GAN HEMT TECHNOLOGY RFMD’s baseline AlGaN/GaN HEMT technology is based on devices with a standard 0.5µm gate length and an advanced source connected field plate to obtain breakdown voltages in excess of 150V. To be able to handle the high power densities in excess of 10W/mm, a SiC substrate is used that provides excellent thermal conductivity and minimizes temperature dependent memory effects. The device topology and the baseline fabrication process are detailed in an earlier publication [5]. Typical dc characteristics for RFMD’s gallium nitride HEMT process are provided in Table 1. A typical device biased at a drain voltage of 65V exhibits a peak current density of 0.9A/mm. The current and power gain cut-off frequencies (ft and fmax) as measured from small periphery devices are 11 GHz and 18GHz respectively (Figure 1). TABLE I SUMMARY OF GAN PROCESS D.C. PARAMETERS λ/4 Parameter Typical Value Units Idss 800 mA/mm Id-max 900 mA/mm Peak gm 225 mS/mm Vp -4.5 V Vbr(GD) >150 V 40 GMax (dB) Gain (dB) 30 |H(2,1)| (dB) 20 10 0 ft fmax -10 .1 1 10 Frequency (GHz) the impedance transformations required to provide the optimum source and load impedance to the devices. 100 Figure 1 GaN unit cell current and power gain performance Under class-AB bias and CW operation at 3.3GHz a typical 2.2mm unit cell device obtains 56% peak power added efficiency (PAE) and a peak output power of 21.9W. This corresponds to a power density of 9.9 W/mm. This is about three times the 3.2W/mm power density obtained at 28V drain bias from a device without the field plate. The series equivalent optimum source and load impedances are Zs = 3.8 + j10.5 Ω and Zl = 30 + j47 Ω respectively. These indicate low gate and drain capacitances of ~0.46pF/W and 0.07pF/W respectively, which is about one fifth of equivalent silicon devices. Using this series equivalent source impedance, the theoretical maximum bandwidth ratio for a -15dB return loss can be calculated to be 57%. These low capacitances contribute to the higher bandwidth obtained compared to other device technologies. IV. CIRCUIT DESIGN The amplifier circuit (Figure 2) uses two 22.2mm periphery devices combined using a Wilkinson power divider / combiner [6] on the input and output respectively. This topology achieves wider bandwidth than would be obtained using a single 44.4mm device. Along with the power division / combination function, the Wilkinson combiners also performs λ/4 λ/4 λ/4 λ/4 λ/4 W = 22.2 mm Figure 2 Schematic of the amplifier circuit. The unit cell source and load pull impedance measurements from section III were used to estimate the large periphery devices source and load optimum impedances. Due to the higher gate capacitance, a two stage impedance transformation was used at the gate to obtain broader bandwidth. The drain section consists of an inductive element to provide the reactance needed for the optimum load and a single stage Wilkinson combiner / transformer. Electromagnetic field models were used extensively to model the frequency performance of the combiner transformer elements. Additionally extensive stability analysis and odd mode oscillation loop analysis were conducted. This type of combining network is prone to the formation of out of frequency band oscillation loops. Therefore the design needs significant analysis over a wide frequency range to determine if any potential odd mode oscillation loops exist. Previous work [7] provides detailed description applying stability analysis to multi device amplifiers using linear analysis and S parameters. Applying this methodology to gallium nitride based amplifiers an extensive analysis of odd mode oscillation loops is provided in [8]. For an odd mode loop to cause stability issues the following conditions must be met, Loop phase ang(Γ) = 0 degrees Loop gain mag(Γ) >1 (0dB) (2) (3) Generally however to provide adequate design margin loop gain less than -2dB should be maintained across a frequency band where Gmax is greater than 0dB. This similar methodology was applied to this amplifier design and example of the loop analysis is shown in Figure 3. Figure 3a) shows the amplifier’s loop gain and phase using Wilkinson combiner networks not employing an isolation resistor between the ports. It can be seen the loop phase angle meets the criteria for oscillation at two distinct frequencies 3.685GHz and 8.536GHz. Additionally loop phase is close to the criterion at low frequency. In all three cases the loop gain requirement is not met. However the loop gain margin is less than the adequate limit defined. Figure 3b) shows how the loop stability can be increased by adding an isolation resistor to the Wilkinson combiner and by optimizing the gallium nitride device layout. Loop gain has been reduced to levels meeting our requirements at all three frequency points. design margin Loop Analysis Without Isolation Resistor 0.1 GHz -6.256 Deg Loop Gain (dB) 6 8.536 GHz 0 Deg 3.685 GHz 0 Deg 200 150 4 100 2 50 0 0 -2 -50 -4 -100 -6 0.1 GHz -0.0653 dB 8.536 GHz -0.791 dB 3.685 GHz -1.29 dB -8 Loop phase (deg) 8 wave transformations were designed to obtain 35 Ω impedance at the package lead. The evaluation board used for testing further transforms the impedance to 50 Ω and is shown in Figure 5. -150 -200 0 1 2 3 4 5 6 7 8 Frequency (GHz) 9 10 11 12 Figure 3a Example of initial amplifier loop gain and phase Figure 5 Photograph of the 400 W 50Ω test fixture Loop Analysis with Isolation Resistor 8.323 GHz 0 Deg 200 V. PULSE POWER PERFORMANCE 150 RF performance of the device was evaluated after optimizing the on-board matching. The amplifier was biased in class A-B mode at a fixed drain voltage of 65V and a drain current of 440mA. The RF input was pulsed using a 100µs wide pulse with 1ms period and the output power was measured at the center of the pulse. The drain current pulse waveform was monitored to calculate the drain efficiency. The amplifier was tested over the frequency range of 2.9 to 3.5GHz. 4 100 2 50 0 0 -2 -50 -4 -100 0.1 GHz -3.17 dB 4.477 GHz -5.19 dB 8.323 GHz -2.78 dB -8 -150 60 -200 0 1 2 3 4 5 6 7 8 Frequency (GHz) 9 10 11 12 Figure 3b Example of improved amplifier loop gain and phase 60 Pout Drain Efficiency Gain 55 Pout (dBm) -6 50 50 40 45 30 40 20 35 10 30 DrainEff (%), Gain (dB) 6 Loop Gain (dB) 4.477 GHz 0 Deg 0.1 GHz -7.354 Deg Loop phase (degrees) 8 0 20 25 30 35 40 45 50 Pin (dBm ) Figure 6 RF output power at the mid-point of the pulse, Drain Efficiency and Gain measured at 3.4GHz. Figure 4 Photograph of the 400 W GaN HEMT amplifier The devices are packaged in a 15 mm x 17 mm package (Figure 4) [9]. The combiner / dividers were implemented on high dielectric constant substrates to achieve the small dimensions required to fit inside the package. The quarter- Figure 6 shows the measured output power at the mid-point of the pulse, drain efficiency, and gain at 3.4GHz as a function of the input power. A peak output power of 434W was obtained at 3.4GHz with a drain efficiency of 52.6%. Figure 7 shows the measured output power over frequency for a range of input power. The broadband power capability of the device is apparent from the plot. pulse, and less than 0.15dB across the middle 50% of the pulse. This confirms the excellent thermal capability of the GaN on SiC dies in the package, even under the high power density they are operating at. 60 Pout(dBm) 55 50 45 40 35 2.9 3.0 3.1 3.2 3.3 3.4 3.5 Frequency (GHz) 58 60 57 50 56 40 55 30 Psat Drain Efficiency 54 20 53 2.9 3.0 3.1 3.2 3.3 3.4 Drain Efficiency (%) Psat(dBm) Figure 7 RF output power at the mid-point of the pulse over the 2.9 – 3.5GHz band for an input power of 25, 30, 35, 40, and 45dBm. 10 3.5 Frequency (GHz) Figure 8 Measured saturated output power, and drain efficiency over the 2.9 – 3.5GHz frequency band. Figure 8 shows the peak saturated output power and drain efficiency over the frequency band and Table II summarizes the data. Output power in excess of 401.5W was obtained over the entire band, with better than 48.4% drain efficiency. TABLE II SUMMARY OF RF PERFORMANCE Frequency Pk Pout (GHz) (W) 446.4 2.9 432.9 3.0 414.8 3.1 419.3 3.2 405.4 3.3 434.1 3.4 401.5 3.5 Drain Eff (%) 55.5 55.1 50.8 54.0 48.4 52.6 53.8 PAE (%) 49.1 49.0 44.6 47.3 42.4 46.5 47.9 The pulse droop performance at 56.4dBm output power (Figure 9) shows about 0.25dB droop over the complete 100µs Figure 9 Measured power droop of 0.25dB over a 100 µs pulse at 56.4 dBm output power (at the center of the pulse) with 10% duty cycle. VI. CONCLUSION RFMD has demonstrated a compact > 400 W wideband AlGaN/GaN HEMT power amplifier operating at 65V with better than 48.4 % drain efficiency over a 600 MHz bandwidth from 2.9 to 3.5GHz, under pulsed condition with 10% duty cycle and 100 µs pulse width. The combination of GaN HEMT device technology and the impedance matching topology, achieves high power and broad bandwidth in a small package. The design incorporates Wilkinson combiner/transformers that provide excellent low loss, wide bandwidth performance in a compact design. Loop stability is a critical design consideration that was addressed to ensure stability over all frequencies. These amplifiers are well suited for pulsed applications including advanced radar systems. The small size and high power obtained in a single package greatly simplifies the design of kW transmitters by reducing the number of devices needed in parallel. In addition improvement to overall system bandwidth and efficiency is achieved as the combiner losses are further reduced. Additional savings in inventory and critical real estate needs are further advantages of using this device. ACKNOWLEDGEMENT The authors wish to acknowledge their colleagues at RFMD for their continued and timely support in device fabrication and assembly. REFERENCES [1] L. F. 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