W-Band, Broadband 2W GaN MMIC James Schellenberg, Bumjin Kim, and Trong Phan

QuinStar Technology, Inc., Torrance, CA, 90505, USA

Abstract — This paper reports the first high-power GaN MMIC covering 70 percent of W-band (75-110 GHz). Using an on-chip traveling-wave power combiner circuit, it achieves power levels of greater than 1 W CW over the 80 to 100 GHz band and a peak power of 2 W CW at 84 GHz. Operating in a pulsed mode (10% duty), this MMIC produces a peak power of 3.2 W at 86 GHz. This work represents the first application of an on-chip combining network to simultaneously achieve high power levels and wide bandwidths at W-band frequencies.

Index Terms — GaN, monolithic microwave integrated circuits (MMIC), solid-state power amplifier (SSPA), W-band.

I. INTRODUCTION

Current and projected W-band (75-110 GHz) systems require output power levels ranging from several watts to perhaps 50W or more. Many of these anticipated systems, particularly EW and high data-rate communications systems, require bandwidths of at least 10 GHz. Recent advances in wide bandgap gallium nitride (GaN) devices and materials have made high-power solid-state power amplifiers (SSPAs) a real possibility at these frequencies. Single-chip GaN based MMICs have been reported operating at frequencies up to 96 GHz with an output power level of typically 1.5W [1]. By using power combining techniques, W-band amplifiers with output power levels of up to 5W have been reported [2]. However, while these MMICs have produced respectable power levels at W-band, the bandwidth has been limited to less than 5% at the highest power levels. Lower power chips have been reported with bandwidths approaching full W-band [3], but the higher power versions, with a high degree of on-chip cell combining, tend to have much narrower bandwidths. For example, reference [4] reports a W-band MMIC with a peak output power of 1.7W, but its bandwidth is only 2-3 GHz or 3%. In addition, reference [1] reports a W-band GaN MMIC with a peak output power of 2.1W, but again its bandwidth is only 4%.

It is the objective of this paper to report the first wideband (>10 GHz), high-power (>1W) GaN MMIC operating at W-band frequencies with a bandwidth of greater than 20 GHz.

II. ON-CHIP COMBINER

A block diagram of the MMIC is shown in Fig. 1. The chip contains five, 3-stage amplifiers that are combined using an N-way, traveling-wave combiner [5]. In the literature, this approach is often referred to as a chain or serial combiner [6]. Conventional MMIC designs typically employ reactive

combining networks which effectively combine the unit cells in parallel. These networks also attempt to equalize the amplitude and phase paths for the combined cells. In the even mode, these networks function as impedance transformers with large transformation ratios, often 10 or more. To minimize size and loss, these networks are electrically short, and as a result, these networks exhibit a narrow bandwidth response. This is the primary reason for the limited bandwidth with large, multiple-cell MMICs, both GaAs and GaN.

Fig. 1. 5-way, traveling-wave power combining scheme employed on chip. Directional (Lange) couplers are used to couple the amplifiers to the input and output lines.

With this N-way power combining approach, the combining network is no longer the bandwidth limiting factor. The bandwidth of this combiner is an octave or more. Hence, the bandwidth of the chip is limited only by the bandwidth of the constituent unit amplifiers, labeled “3-stage PA” in the figure.

This combining network is a traveling-wave combiner, using directional couplers (Lange couplers) to couple the amplifiers to the input and output transmission lines. This combiner is fully isolated and matched at all ports. The isolation is only limited by the directivity of the Lange couplers. Furthermore, it is a true N-way combiner, not limited to binary combinations. In this work we use a 5-way version of this combiner.

This combiner is easily expandable. For example, to expand the power combiner from a 5-way to a 6-way, it is only necessary to add two 9-dB couplers and, of course, the additional unit amplifier. All other components remain the same.

At lower frequencies, the use of directional couplers in MMICs is usually prohibitive due to the expensive real-estate they occupy. However, at these W-band frequencies, this is no longer the case. The size of the coupler is compatible with the spacing between the unit amplifiers and can therefore be used

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in place of a line length to connect the sections together. Furthermore, the topology of the combiner lends itself to minimum interconnecting line lengths, which help to minimize line losses and phase errors.

III. MMIC DESIGN

A photograph of the W-band MMIC is shown in Fig. 2. It was fabricated by HRL in Malibu, CA using their 0.14 µm GaN on SiC process [7]. This process produces a DHEMT device with a typical fMAX and fT of 214 and 87 GHz, respectively. Operating at W-band frequencies and 12 V, the device saturated output power density is typically 1.6W/mm.

Fig. 2. Broadband W-band GaN MMIC. The MMIC is configured as a 3-stage, 5-section combiner using a traveling-wave combiner. The chip dimensions are 3.2 x 5.4 mm2.

The MMIC design consists of five, 3-stage unit amplifiers combined on chip using the combining network described above. The 3-stage amplifier uses 4 x 37.5 µm unit cells in the final two stages and 2 x 37.5 µm unit cells in the first stage. The 4 x 37.5 µm unit cell contains 2 vias (one internal source island). There are two unit cells in each stage, resulting in a stage gate periphery ratio (or drive ratio) of 1:2:2 instead of the common 1:2:4 ratio. This unusual drive ratio was chosen to ensure that the driver stage has enough power to drive the output stage even under high levels (3 dB or more) of gain compression. This also helps to prevent premature power saturation at the high end of the band where the device gain is limited. The negative consequence of this drive ratio is that it results in poorer efficiency. However, for the intended application (EW), efficiency is not a prime system requirement; output power and bandwidth are the main considerations.

The total MMIC gate periphery in the output stage is 1.5 mm, and hence with output matching and combining losses, we expect an output power of about 2 W from this MMIC.

IV. CHIP MEASUREMENTS

To test this MMIC, we first mounted it on a Thermkon (W-Cu composite) plate and then used wafer probing techniques

to characterize it. Fig. 3 shows the test setup, and Fig. 4 illustrates the small-signal test results over the 70 to 110 GHz band. The small-signal gain is greater than 8 dB across the 75 to 102.7 GHz band and reaches a peak of 14.4 dB at 83 GHz. The input and output match has an average return loss of about 15 dB across this same band.

Fig. 3. Probe test system for W-band MMIC tests.

Fig. 4. Small-signal measurements of GaN MMIC chip showing gain response and input/output return loss over the 70 to 110 GHz band. Bias: Vds = 10V, Vgs = -2.2V and Ids = 1A.

For characterizing the power performance of this chip, we used the same probe test setup shown in Fig. 3 with a pulsed drain supply. To minimize self-heating, we used a 10% duty pulse train and a pulse width ranging from 10 to 100 µS. The results are shown in Fig. 5 for 3 different drain voltages: 12, 14, and 16 volts. For this test, the input power was fixed at 27 dBm. Due to test system power limitations (driver power), the test frequency range is limited to 83-100 GHz. The output power flatness is ±1.1 dB across the band. With 12 V bias (the nominal voltage), the output power varies between 2.6 W at the low end of the band to 1.5 W at the high end.

Due to reduced self-heating, the pulsed test system allows us to test the chip at higher drain voltages than would be otherwise possible under CW conditions. Biased at 16 V, the peak output power at the low end increased to 3.2 W. This

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represents an outstanding power density including MMIC (power combiner) losses.

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V. MODULE PERFORMANCE

To evaluate the MMIC performance underwe mounted these MMICs in Thermkon carriFig. 6. In addition to the chip, this carrier or the waveguide input/output transitions and thnetworks.

Fig. 6. W-band power module with integral transitions. The module dimensions are 0.74 x 0.44

The CW performance of a typical module isover the 80 to 100 GHz band. This data inclosses in the module, including the losses oftransitions and the test fixture that it mounts The total loss between the MMIC output and plane is estimated to be 0.5-0.6 dB. Biasemodule produces a minimum output power ofband and a peak output power of 33 dBm (2With a small-signal gain of 10 to 14 dB acrogain compression characteristics are also ifigure.

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Fig. 7. Frequency response of a typicaloutput power of 1.1 W CW across the 8is biased at 12 V.

The performance of 16 of these Fig. 8. This data represents the satmodules and again includes all thethe module test fixture. All units aCW data. With a fixed input powpower of any one module varies abwith the best units producing 33 dBof the band. The spread in power bmodules is about 2 dB. Ten of thoutput power of 1 W (30 dBm) or PAE (not shown) is typically 10%due to test system limitations (i.e. thend of the test band was limited to 8indicate that the module performaGHz.

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Comparing the CW results showpulsed results shown in Fig. 5, we care about 1 dB lower than the 12 V3 factors: 1) the CW data is moincludes the loss of the input/ou

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fixture, 2) the input drive level is lower for the CW data, and 3) under CW conditions the MMICs run hotter and this higher temperature degrades the gain and output power.

VI. CONCLUSION

This paper presents new levels of performance for GaN MMICs at W-band frequencies. Employing a broadband combining network on-chip, this MMIC has demonstrated an unprecedented combination of power and bandwidth. It has produced an output power of greater than 1 W CW over the 75 to 100 GHz band, and operating in a puled mode (10% duty), it has produced a peak power of 3.2 W at 86 GHz. These results demonstrate the potential of GaN technology to produce broadband, high-power amplifiers at millimeter-wave frequencies.

ACKNOWLEDGEMENT

This work was supported by the Naval Research Laboratory and the Office of Naval Research under Contract No. N00173-09-C-2025.

REFERENCES

[1] M. Micovic, A. Kurdoghlian, A. Margomenos, D. F. Brown, K. Shinohara, S. Burnham, I. Milosavljevic, R. Bowen, A.J. Williams, P. Hashimoto, R. Grabar, C. Butler, A. Schmitz, P. J. Willadsen, D. H. Chow, “92-96 GHz GaN Power Amplifiers,” 2012 IMS Digest, June 2012.

[2] J. Schellenberg, E. Watkins, M. Micovic, B. Kim, and K. Han., “W-band, 5W Solid-State Power Amplifier/Combiner,” IEEEIMS Symp. Dig., May 2010.

[3] BAL-WPA data sheet. http://mmics.hrl.com/GaNPA/[4] A. Brown, K. Brown, J. Chen, K. C. Hwang, N. Kolias, and R.

Scott., “W-band GaN Power Amplifier MMICs,” IEEE IMS Symp. Dig., June 2011.

[5] R. J. Mohr, “A Microwave Power Divider,” IRE Trans. Microwave Theory Tech., vol. M’IT-9, p. 573, Nov. 1961.

[6] K.J. Russell, “Microwave Power Combining Techniques,” IEEE Trans. Microwave Theory Tech., vol. MTT- 27, pp. 472-478, May 1979.

[7] D. F. Brown, A. Williams, K. Shinohara, A. Kurdoghlian, I. Milosavljevic, P. Hashimoto, R. Grabar, S. Burnham, C. Butler, P. Willadsen, and M. Micovic, “W-Band Power Performance of AlGaN/GaN DHFETs with Regrown n+ GaN Ohmic Contacts by MBE,” IEEE IEDM. Dig., December 2011.

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