Doyle T. Nichols

Substrate agnostic monolithic integration of the inline phase-change switch technology

Omni-directional GeTe inline phase-change switches (IPCS) have been fabricated and heterogeneously integrated with commercial SiGe BiCMOS technology to create a reconfigurable receiver. The reconfigurable receiver required integrating thirteen (13) 8-port and two (2) 4-port omni-directional switch circuits with a commercial SiGe IC, requiring very stable and repeatable performance from the 112 integrated GeTe IPCS devices. Insertion loss, isolation, and cycling data will be presented, as well as performance issues encountered during the heterogeneous integration process. A new monolithic integration scheme is briefly discussed that is independent of the substrate and semiconductor technology used. This integration plan enables the monolithic fabrication of GeTe IPCS devices on any semiconductor technology, allowing low-loss, low-power, broadband reconfigurable RF systems and SoCs (system-on-chip) to be realized in any technology.

Examination of the temperature dependent electronic behavior of GeTe for switching applications

The DC and RF electronic behaviors of GeTe-based phase change material switches as a function of temperature, from 25u2009K to 375u2009K, have been examined. In its polycrystalline (ON) state, GeTe behaved as a degenerate p-type semiconductor, exhibiting metal-like temperature dependence in the DC regime. This was consistent with the polycrystalline (ON) state RF performance of the switch, which exhibited low resistance S-parameter characteristics. In its amorphous (OFF) state, the GeTe presented significantly greater DC resistance that varied considerably with bias and temperature. At low biases (<1u2009V) and temperatures (<200u2009K), the amorphous GeTe low-field resistance dramatically increased, resulting in exceptionally high amorphous-polycrystalline (OFF-ON) resistance ratios, exceeding 109 at cryogenic temperatures. At higher biases and temperatures, the amorphous GeTe exhibited nonlinear current-voltage characteristics that were best fit by a space-charge limited conduction model that incorporates the effect of a defect band. The observed conduction behavior suggests the presence of two regions of localized traps within the bandgap of the amorphous GeTe, located at approximately 0.26–0.27u2009eV and 0.56–0.57u2009eV from the valence band. Unlike the polycrystalline state, the high resistance DC behavior of amorphous GeTe does not translate to the RF switch performance; instead, a parasitic capacitance associated with the RF switch geometry dominates OFF state RF transmission.

Morphological analysis of GeTe in inline phase change switches

Crystallization and amorphization phenomena in indirectly heated phase change material-based devices were investigated. Scanning transmission electron microscopy was utilized to explore GeTe phase transition processes in the context of the unique inline phase change switch (IPCS) architecture. A monolithically integrated thin film heating element successfully converted GeTe to ON and OFF states. Device cycling prompted the formation of an active area which sustains the majority of structural changes during pulsing. A transition region on both sides of the active area consisting of polycrystalline GeTe and small nuclei (<15u2009nm) in an amorphous matrix was also observed. The switching mechanism, determined by variations in pulsing parameters, was shown to be predominantly growth-driven. A preliminary model for crystallization and amorphization in IPCS devices is presented.

Optically controlled GeTe phase change switch and its applications in reconfigurable antenna arrays

Characterization of GeTe-Based RF Switches under direct optical laser excitation is shown with the ON state DC electrical resistivity and OFF state capacitance. Based on our tightly-coupled dipole array with performance in excess of 4 to 1 bandwidth over wide-scan angle up to 60 degrees, and with in-band rejection capability using reconfigurable baluns, the antenna aperture is shown to exhibit reconfiguration flexibility with the integration of optically-controlled GeTe-Based RF switches.

Reconfigurable inline phase-change switches for broadband applications

Improvements to the GeTe inline phase-change switch (IPCS) technology have resulted in a 10× increase in the figure-of-merit (FOM) for radio-frequency (RF) switches. An ON-state resistance of 0.9 Ω (0.027 Ω·mm) with an OFF-state capacitance and resistance of 14.1 fF and 30 kΩ, respectively, were measured. This results in a switch cutoff frequency (Fco) of 12.5 THz, with an OFF/ON resistance ratio of 10,000:1. Passive RF circuits have been built with these switches to compare their frequency performance to state-of-the-art technologies, as well as to demonstrate fine-grain reconfigurability in RF circuits. Further analysis of 8-port omni-directional IPCS switches used in a reconfigurable transceiver demonstrates less than 2dB degradation in gain and 1dB in noise figure when reconfiguring a single chip for 4 different receiver chain frequencies (S-band, X-band, Iridium, and CDL-Ku), demonstrating the feasibility of IPCS devices for low-power, broadband reconfigurable RF systems.

Recent advances in fabrication and characterization of GeTe-based phase-change RF switches and MMICs

Recent progress in germanium telluride (GeTe) based phase-change RF switch technology development has resulted in switches with tens of thousands of switching cycles and μs-level switching times. A highly compact (0.33 × 0.61mm) series-shunt, single-pole double-throw (SPDT) switch based on 3rd generation inline phase change switch (IPCS) devices was built and characterized. The SPDT switch exhibited less than 1.1dB insertion loss and greater than 39dB isolation in the DC-65GHz bandwidth. It achieved <2μs settling time for RF switching between the throws. In addition, 30,000 switching operations were demonstrated with <0.05dB variation in insertion loss and < 2dB variation in isolation.

Origin and Optimization of RF Power Handling Limitations in Inline Phase-Change Switches

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Improvements in GeTe-based Phase Change RF Switches

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Experimental Demonstration of AlN Heat Spreaders for the Monolithic Integration of Inline Phase-Change Switches

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Moving Beyond 3D Hetero-Integration and Towards Monolithic Integration of Phase-Change RF Switches with SiGe BiCMOS

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