(Invited) Integration of Self-Biased Isolators with III-V MMICs

Abstract

The technology presented in this work combines several critical features to enable intimate integration of isolators with III-V MMIC amplifier circuits. These critical features are self-biased hexaferrite material optimization and processing, sophisticated design capability to accurately optimize and predict circulator performance, and heterogeneous integration capability to combine the self-biased hexaferrite material with semiconductor MMICs. The use of self-biased ferrite material integrated onto semiconductor substrates enables significant size, weight, and cost savings as compared to state-of-the-art technologies that use bulky magnets to externally bias the devices and also require tuning to achieve high performance when used in modules along with MMIC amplifiers. Figure 1 shows an example drawing of a Ka-band isolator on a GaAs substrate. The isolator junction is fabricated on the ferrite material and integrated onto the semiconductor substrate. The input and output matching networks and the isolator 50 Ω termination are fabricated on the semiconductor substrate. The small signal performance of the Ka-band isolator was simulated with Ansys HFSS. Figure 2 shows a comparison of simulated and measured small-signal RF performance. This design is able to achieve less than 0.7 dB insertion loss and over 20 dB isolation. The process and integration technique used to fabricate the stand-alone isolators shows excellent repeatability. Four separate parts have been measured and the small signal performance variation is shown in Figure 3. All four devices have been integrated with the same GaAs substrate. Figure 4 shows Ka-band isolator performance over a temperature range reflective of what the part may experience during its lifetime. Peak insertion loss and isolation vary predictably over temperature and return to initial performance values upon a return to room temperature. The same fabrication and integration technique was used to precisely align and integrate an isolator on the RF output of a MMIC amplifier as shown in Figure 5. The integrated device did not require any RF tuning to achieve predicted performance. To assess the performance of the integrated MMIC-isolator device, measurements were taken of a standalone amplifier and isolator. The scattering parameters for the standalone devices were analytically combined and compared against measured data from the integrated device. These results in Figure 6 show excellent agreement between the ideally simulated case and the as-built hardware. The approach demonstrated here is applicable to isolator and MMIC designs spanning the frequency range from K-band to W-band. Furthermore, this integration approach allows these designs to be transferred to any other semiconductor technology in a straightforward manner. Future work will demonstrate these capabilities. The above described approach demonstrates intimate integration of self-biased isolators and circulators with III-V MMIC amplifiers and enables aggressive reductions in space, weight, and cost that are not possible with existing state of the art isolators and circulators which require external magnets, complicated packaging, and post-assembly tuning to achieve high performance. The measured performance of standalone isolators as well as integrated MMIC-isolator devices show excellent agreement with simulated behavior.This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions, and/or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government. Figure 1