X-Band T/R Module in state-of-the-art GaN Technology

A. Bettidi, A Cetronio, M. Cicolani, C. Costrini,C. Lanzieri, S. Maccaroni, L. Marescialli, M. Peroni, P. Romanini

SELEX Sistemi Integrati S.p.A., Via Tiburtina Km. 12,400, 00131 Rome, Italy lmarescialli@selex-si.com

Abstract— In this paper a first iteration X-band T/R module based on a GaN-HEMT MMIC Front-End chip-set, comprising a power amplifier, robust low-noise amplifier and power switch will be presented. Even though ultimate T/R module performance cannot be achieved with current GaN-HEMT technological maturity the impact that this technology can have at systems level in terms of performance/cost trade-off will be illustrated by means of a preliminary innovative module architecture which foresees the elimination of more traditional T/R module components such as ferrite circulator and limiter for front-end signal routing and protection.

I. INTRODUCTION To be competitive over conventional radars, AESA (Active

Electronically Steered Antenna) systems not only need to fulfil the continuous demand for increased performance, but above all must be competitive in terms of reliability and cost. For this reason dominance in innovative T/R module technology, the key enabling technology in AESA systems, is of paramount importance and as such many defence companies are developing independent capabilities to ensure competitive edge products in this field.

The enabling technologies for the realization of the T/R Module and more specifically, those related to the microwave Front End section are the technologies that more than others determine the feasibility, the performances, the reliability (due to high number of Modules, up to 10000 T/R Module/radiating surface), the cost and therefore the competitiveness of the entire system. The strategic importance of these enabling technologies arise from the fact that they are in full evolution and as such often are not available or subject to export restrictions. This particularly applies for GaN (Gallium Nitride) based technologies where restriction are not just limited to components, but are actually extended to the base materials. GaN HEMT MMICs (Monolithic Microwave Integrated Circuits) are strategic enabling components for both high performance and/or wideband T/R modules and for very high power solid state transmitter applications. In particular the impact of GaN technology will be evidenced at System level since it will enable reduced System size and costs, maximization of operational bandwidth, enhancement of detection, guidance and tracking abilities. Benefits will be significant for several military applications such as: radar, electronic warfare, high speed communications, multi-role and multi-domain functions, etc.

Based on self-sufficient technological capabilities within Selex Sistemi Integrati, for developing Phased Array radar

components (i.e. GaAs/GaN Foundry, MMICs and T/R modules Design, Microelectronics Integration and Test), L, C and X band T/R module families of components have been developed for new generation AESA products [1],[2],[3].

In this paper the design, realization and test of an innovative X-Band T/R Module Front End based on available GaN HEMT technology and utilizing the experience of GaAs T/R Modules, shall be presented. In particular this paper will focus on T/R Module architecture and on the evaluation of design simulations reliability as well as on predictable major advantages given by GaN technology application.

The presented T/R Module Front End includes the main microwave functions, that is: power amplification of the Transmitted signal HPA, Low noise amplification of the Received signal LNA, output power Switch to the radiating element for enabling Transmit or Receiving Mode functions, all in GaN-HEMT technology.

II. GAN-BASED T/R MODULE FRONT END ARCHITECTURE The general block diagram of a T/R Module is shown in Figure 1. The major functions of a T/R modules are: (i) generation of transmit power, (ii) low-noise amplification of received signals from the radiating element, (iii) phase-shift in transmit and receive modes for beam steering, and (iv) variable gain setting for aperture weighing during reception.

Figure 1 - General block diagram of a T/R Module

As visible from Figure 1, the general architecture of the Module also includes the Limiter function in order to protect components such as Low Noise Amplifier from high power signals, either accidental or intentional, and the Duplexer function to allow the use of the same antenna to Transmit and to Receive. From recent development in the field of Microwave Integrated Circuits based on GaN/AlGaN heterostructure epitaxy grown on semi-insulating SiC substrates, comes the possibility to apply this technology to design and manufacture MMIC components for RF applications. The main objective is not

RADIATING ELEMENT

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only to achieve higher performances in terms of Output Power and Efficiency (GaN HPA MMIC) in comparison to standard technology (i.e. GaAs), but also to make the most of their robustness and good noise figure performance and RF power switching. In particular, with the introduction of robust GaN LNAs it will be possible to eliminate the Limiter with its intrinsic insertion losses, allowing to increase the input power level of the LNA and an overall reduction of the receive chain noise figure. Furthermore, with GaN T/R RF Switch, by replacing the Circulator with a semiconductor device, the Duplexer function should be realized with reduced size and, in a medium time scale, at lower cost.

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Figure 2 - General block diagram of the T/R Module based on GaN technology: in blue the Front End in GaN, in green the GaAs Core Chip for the modulation of RF signal phase and amplitude.

In Figure 2 is reported the block diagram of the GaN based T/R Module. The primary advantage is the highest power density that GaN devices can provide. This will imply that a GaN-based HPA chip shall have a footprint that is one quarter with respect to a GaAs-based HPA chip under the same condition of generated power, which results in the production of a greater number of components per single productive cycle. From the module architecture point of view, the main consequences are: the possibility to reduce the number of MMIC chips, the reduction of the number of interconnections, the reduction of the dimensions (size) of the Module itself, the simplification of the routing of the Module. The cost reduction of the single T/R Module can be estimated in about 40% with respect to the equivalent module based on GaAs technology. The application of GaN technology in future T/R Modules shall allow to:

• Obtain greater power performances with respect to GaAs-based components;

• Realize greater level of integration. This approach is relevant to evolve towards a T/R Module organized in two main chips: the first based on GaN technology including Front-End functionalities, and the second based on Silicon or Silicon Germanium (SiGe) technology for the Back-End functionalities (phase and amplitude control, up and down conversion, A/D and

D/A conversion, direct digital conversion from and to the Signal Processor)

III. GALLIUM NITRIDE TECHNOLOGY AlGaN/GaN HEMT is a new pacing technology mainly

targeted for high power applications at microwave and millimetre-wave frequencies, owing to its high critical breakdown field, its one-order of magnitude higher power density, its saturation drift velocity and the availability of a high thermal conductivity SiC substrate [4]. Apart from their exceptional power performance [5]-[6], GaN HEMTs are promising candidates for robust low-noise applications due to their low-noise performance combined to their high power handling capability, that provides large advantages in terms of linearity and robustness [7].

Although some possibility for RF switching devices has been investigated, there have been few studies to date on the use of GaN high electron mobility transistors (HEMTs) for high power microwave and RF control applications [8], [9]. For this applications low-loss/high power RF switches are necessary, especially for multifunction antenna systems where a single aperture is used to serve multiple applications by simply switching between transmission and receive channels of the component.

The AlGaN/GaN MMICs reported in this paper have been fabricated with the current SELEX Sistemi Integrati GaN-HEMT MMIC technology. Said process is based on an epi-layer structure of GaN/AlGaN/GaN deposited on semi insulating SiC substrates by either MOCVD or MBE techniques. The mask levels for MMIC fabrication are based on a mix and match procedure utilising both I-Line Stepper and Electron Beam Lithography (EBL) processes. The latter used only for the fabrication of the high resolution quarter micron Gate dimensions necessary for the HEMT devices.

IV. GAN-BASED T/R MODULE FRONT END DESIGN AND PERFORMANCES

The main objective of the design of the GaN-based T/R Module was to approach the new available GaN MMICs technology, the evaluation of Module design methodology in relation with needed dc bias levels, thermal management and new duplexer configuration. The available CPW MMICs characteristics are the result of a first design and technology iteration whose input requirements were technology driven and could not consider T/R Module target performance.

As regards packaging solution for T/R Modules, generally there are different technologies available, with different levels of integration. For present application the chosen packaging technology solution is the LTCC. This technology allows an Integrated Single Package (ISP) approach based on a Multilayer Ceramic substrate: by simply brazing a proper metal frame and a cover lid it can afford the required hermeticity. Moreover, the assembly and packaging of the RF MMICs and related bias and control circuitry requires a certain number of interconnections and as such for this section multi-layer substrates such as LTCC/HTCC are

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attractive to minimize both module footprint and coupling between microwave and control signals.

Starting from the block diagram shown in Figure 2, the design of the complete T/R module has been performed using the said approach.

It must be underlined that, in order to achieve the desired HPA compression level, a GaAs driver amplifier has been included in the Transmitting path. The phase and amplitude modulation is performed by a multifunction GaAs Core Chip working in X Band.

As anticipated above, the Front End includes available Monolithic Microwave Integrated Circuits (MMICs) designed in Coplanar Wave (CPW) configuration in the band 9÷10 GHz. In the meantime promising results have been also obtained with Selex Sistemi Integrati Foundry Micro-strip (MS) technology[10][11]. For this reason the integration of a 10 watt MS GaN HPA (instead of the CPW chip) has been chosen as a better chance to test the Front End performances.

The power amplification of the transmitted signal is performed by a two stage amplifier with a total gate periphery of 6 mm (2mm gate width for the first stage and 4mm of gate width for the output stage) fabricated using a 0.5 μm gate-length process. The HPA Output Power is about 40 dBm when biased with 20 volt of Drain Voltage, with an associated Gain of 14 dB @3dB Compression point. By increasing drain bias up to 35 volt the MS HPA can provide 20 watt of Output Power.

The Low Noise Amplification of the received signal is performed by a three-stage amplifier with a total gate periphery of 1.2 mm yielding a NF performance better than 2.5 dB and an associated gain of 16 dB respectively.

The Duplexer function is performed by a T/R switch using a non reflective SPDT configuration with shunt FETs to enable high power handling capabilities. The T/R switch shows an I.L. < 2 dB, Isolation>40 dB and ports matching >10 dB on the operative bandwidth. The input Power @1dB compression point of the T/R switch is better than 37 dBm @10 GHz.

In Figure 3 the photograph of the X Band GaN chip-set is shown.

Design activity showed that the Output Power of the module turned out to be limited by the T/R Switch 1dB compression point and related Insertion Losses. For this reason levels less than 33.6 dBm are expected. The future requirements for the SPDT 1 dB compression point must be increased up to 4-5 dB more than the HPA Output Power. In Receive Mode, the simulation provides a module Noise Figure forecast better than 5.7 dB. When considering only the results obtained with MS SPDT, in a future second iteration[12], the Module Noise Figure shall be improved to better than 5 dB and with further expected improvements in LNA noise-figure (less than 2 dB) and power switch insertion loss, the overall noise figure of the receive chain is expected to be better than 4 dB.

In Figure 4 the first iteration GaN-based T/R Module fully integrated has been shown. It is evident from obtained

preliminary results (Figure 5), that they are in line with the expected behaviour.

Figure 3 – Photograph of the GaN MMIC X band chip-set

Figure 4 – Photograph of the fully integrated GaN-based T/R Module: size 45

mm x 15.5 mm .

Figure 5 – Output Power behaviour (under compression) in the bandwidth,

as measured on the GaN HPA output on the T/R Module.

The complete characterisation of the T/R Module, both in Tx and Rx mode, has been obtained using a dedicated test jig (Figure 6). In the following figures, characterisation results are presented and briefly commented.

Figure 6 – Photograph of Test Jig used to evaluate the TRM

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Pout with PW=10us, Duty=1%, Tback=45°C, Pin=+2dBm

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Figure 7 – Output Power behaviour of TR Module (under compression) in

the bandwidth. As expected, the output power of T/R module is actually

limited by switch performances.

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Figure 8 – Noise Figure of TR Module in Rx mode. The behaviour in Noise figure and Output Power around 9

GHz is due to mismatch effects in the common path (connector or Alumina transition on the test jig)

Figure 9 – Main Bits of Attenuation.

Figure 10 – Main Bits of Phase. Attenuation and Phase bits exhibit the expected behaviour.

V. CONCLUSIONS In present paper the design, realization and test of a GaN-

based X band T/R module has been presented. The main aspects in terms of cost reduction, future improvements of T/R Module products and relevant impacts on System main performances have been shown.

ACKNOWLEDGMENT This research activity has been carried out in the

KORRIGAN RPT N° 102.052 Project funded within the EUROPA framework in the CEPA 2 priority area, in the Italian MIUR project on Advanced TRM Enabling Technologies within the Consorzio OPTEL framework and in FINMECCANICA R&D Corporate Project.

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[4] R. J. Trew, “SiC and GaN – Is There One Winner for Microwave Power Applications?”, Proceedings of the IEEE, vol. 90, no. 6, pp. 1032–1047, June 2002.

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[7] D. Krausse, R. Quay, R. Kiefer, A. Tessmann, H. Massler, A. Leuther, T. Merkle, S. Muller, C. Schworer, M. Mikulla, M. Schlechtweg, and G. Weimann, “Robust GaN HEMT low-noise amplifier MMICs for X-band applications”, Gallium Arsenide applications Symposium, October 2004, pp. 71-74.

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