Modern military systems require the highest performance radio frequency (RF) semiconductor technology. For example, in radars in transmit mode, RF energy needs to be distributed to each active antenna array element, precisely phase shifted and then greatly amplified before being radiated, and in receive mode, the small return signal must be amplified with great fidelity. Likewise, electronic warfare (EW) and communication systems require the same sort of transmit and receive functionality with even more emphasis on signal fidelity in both transmit and receive modes. The final active array amplification of transmitted RF signal at each element is performed by a compact RF power amplifier (PA) circuit chip that is a type of monolithic microwave integrated circuit (MMIC). This PA technology must be small enough to fit within the radar’s element spacing, be able to generate high RF output power and also be very efficient in converting direct current (DC) power into RF power to minimize prime power consumption and waste heat generation. In receive mode, the first stage of amplification of the reflected signal is performed by a low noise amplifier (LNA) MMIC which must increase the desired signal strength while minimizing additional noise degradation and the introduction of nonlinear distortions. The phase shifter MMIC within each radar element allows dynamic beam steering of the radiated RF energy.
Raytheon, leveraging its own trusted foundry (Figure 1), has a long and successful legacy of developing next generation, high performance semiconductor processes for PA, LNA and phase shifter MMICs as well as for inserting them into highly reliable, fielded, phased array radars. This MMIC design and fabrication capability coupled with expertise in semiconductor material growth, MMIC-based module development, and integration and test, provide a leading edge RF MMIC capability that supports current and emerging Department of Defense (DoD) system needs at a lower cost and shorter timeline than alternative approaches. Specifically, today’s commercial RF capabilities, that are driven primarily by commercial technology demands, cannot meet the most stressing DoD system requirements.
Traditionally, gallium arsenide (GaAs) has been the semiconductor of choice for efficiently amplifying and phase shifting RF signal in radars. Throughout the 1990s, Raytheon was a pioneer in inserting GaAs-based MMICs into the first modern phased array radars. As the performance requirements of these military systems have increased to meet ever-growing threats, so too have the power, efficiency and low noise requirements on these MMICs. During that time, Raytheon has continued to customize and optimize its semiconductor processes for each specific radar function (Table 1). One such Raytheon GaAs pseudomorphic high electron mobility transistor (pHEMT) technology, customized for multifunction MMICs containing phase shifters, attenuators and gain stages, combines RF and logic functions on the same MMIC, providing a serial or parallel logic interface to a separate silicon (Si) controller chip. By designing a process that allowed the combining of some logic functions on the RF MMIC circuitry, the number of off-chip components and interfaces between chips was minimized, reducing size and cost while improving radar manufacturability.
During the past six years under independent research and development (IRAD) and Defense Advanced Research Projects Agency (DARPA) funding, Raytheon has taken the integration of RF and logic functions to the next level. Raytheon has developed technology to directly integrate GaAs, indium phosphide (InP) or gallium nitride (GaN) RF devices with high density Si complementary metal oxide semiconductor (CMOS) logic on a common Si substrate. This heterogeneous integration technology enables greater levels of functionality and digital control of RF circuits as well as a reduction in cost over using the traditional separate RF and logic chips.
Using this integration approach, Raytheon demonstrated the world’s first GaN HEMT and Si CMOS heterogeneously integrated chip; a GaN RF amplifier with in-situ Si CMOS gate bias control (Figure 2). This circuit was a proof of concept that demonstrated that GaN HEMT and Si CMOS devices could be integrated on the same Si substrate with minimal performance impact to the Si CMOS and GaN HEMT technologies. The circuit also serves as a building block for digitally assisted RF and mixed signal circuits, such as amplifiers with on-chip digital control and calibration, reconfigurable or linearized PAs with in-situ adaptive bias control, high-power digital-to analog converters (DACs) and on-chip power distribution networks.
For low noise amplifications such as what is used in radar receivers, Raytheon developed high indium-content metamorphic HEMT (mHEMT) technology and manufacturing processes for higher gain performance with even lower noise contribution than traditional GaAs HEMT technology that is limited to 19 percent indium in the channel region where the electrons travel. Previously, only InP HEMT devices with high indium content (53 percent) in their InGaAs quantum well channels could fill this ultimate low noise function, but at a higher cost due to the challenges associated with manufacturing MMICs on fragile InP wafers.
mHEMT technology, with up to 60 percent indium composition in the channel, offers the gain and low noise performance advantage of InP HEMTs and the manufacturability and cost advantages of GaAs MMIC wafers. Through the growth of a metamorphic material layer on a GaAs substrate, the lattice constant of GaAs can be transformed to allow the growth of higher indium content InP HEMT devices on GaAs wafer substrates. Additionally, metamorphic growth allows classically forbidden indium contents to be grown, i.e., those not lattice matched to GaAs or InP substrates, thus enabling the device designer to explore and exploit the properties of a new set of HEMT devices. Figure 3a shows that the measured mobility of channel electrons increases with increasing channel indium content, due mainly to the reduction in electron effective mass and reduced scattering in the deeper quantum well. These improvements give rise to higher electron channel velocities, allowing operation at higher gain, higher frequency and lower noise figure than GaAs devices. Figure 3b shows the reduction in minimum noise figure, (Fmin1, achieved as one moves from a traditional GaAs pHEMT device with 19 percent indium content to that of a mHEMT with a 60 percent indium content InGaAs channel. When fabricated into LNAs, these mHEMT MMICs greatly improve the signal-to-noise ability of the system, extending its range while reducing prime power over GaAs technology as the high mobility enables them to operate at lower DC bias.
For improvement on the transmit side, Raytheon developed and matured GaN technology for PAs, enabling the next generation of DoD systems. The power, efficiency and bandwidth performance of GaN-based MMICs is unsurpassed, revolutionizing the design of radars by creating not only higher performance but also lower system cost. With over 5 Watts/millimeter (W/mm) of RF output power density compared with GaAs at 1 W/mm, GaN RF amplifiers deliver five times the power per element compared to GaAs within the same footprint, providing up to 50 percent more radar range or the ability for the radar to search five times the volume of space in the same amount of time (Figure 4). Fewer high-power GaN MMICs could be used to replace many low-power GaAs MMICs, decreasing radar size by half while maintaining search performance and increasing efficiency or, alternatively, equal-power GaN chips can be made dramatically smaller in size. Thus, GaN can reduce overall system costs while enabling new smaller size-constrained systems. The higher drain current that GaN offers makes the broadband matching of high-power EW MMICs simpler and more efficient than GaAs, while the approximately seven-fold improvement in the thermal conductivity enables efficient amplifier cooling. Finally, the wide bandgap that is intrinsic to GaN material provides large critical breakdown fields and voltages, making a more robust amplifier and easing system implementation.