Alexandros D. Margomenos

Scaling of GaN HEMTs and Schottky Diodes for Submillimeter-Wave MMIC Applications

In this paper, we report state-of-the-art high frequency performance of GaN-based high electron mobility transistors (HEMTs) and Schottky diodes achieved through innovative device scaling technologies such as vertically scaled enhancement and depletion mode (E/D mode) AlN/GaN/AlGaN double-heterojunction HEMT epitaxial structures, a low-resistance n+-GaN/2DEG ohmic contact regrown by MBE, a manufacturable 20-nm symmetric and asymmetric self-aligned-gate process, and a lateral metal/2DEG Schottky contact. As a result of proportional scaling of intrinsic and parasitic delays, an ultrahigh fT exceeding 450 GHz (with a simultaneous fmax of 440 GHz) and a fmax close to 600 GHz (with a simultaneous fT of 310 GHz) are obtained in deeply scaled GaN HEMTs while maintaining superior Johnson figure of merit. Because of their extremely low on-resistance and high gain at low drain voltages, the devices exhibited excellent noise performance at low power. 501-stage direct-coupled field-effect transistor logic ring oscillator circuits are successfully fabricated with high yield and high uniformity, demonstrating the feasibility of GaN-based E/D-mode integrated circuits with transistors. Furthermore, self-aligned GaN Schottky diodes with a lateral metal/2DEG Schottky contact and a 2DEG/ n+-GaN ohmic contact exhibited RC-limited cutoff frequencies of up to 2.0 THz.

92–96 GHz GaN power amplifiers

We report the test results of a family of 92-96 GHz GaN power amplifiers (PA) with increasing gate peripheries (150 µm to 1200 µm). The 1200 µm, 3-stage PA produces 2.138 W output power (Pout) with an associated PAE of 19% at 93.5 GHz (VD=14V). The amplifier offers Pout over 1.5W with associated PAE over 17.8% in the 92–96 GHz bandwidth. The measured data show that the maximum Pout scales linearly with increasing gate periphery at an almost constant PAE around 20%. This demonstrates the high efficiency of on-chip power combining and enables W-band high power single chip solid state power amplifiers.

GaN Technology for E, W and G-Band Applications

Highly scaled GaN T-gate technology offers devices with high ft/fMAX, and low minimum noise figure while still maintaining high breakdown voltage and high linearity typical for GaN technology. In this paper we report an E-band GaN power amplifier (PA) with output power (Pout) of 1.3 W at power added efficiency (PAE) of 27% and a 65-110 GHz ultra-wideband low noise amplifier (LNA). We also report the first G-band GaN amplifier capable of producing output power density of 296mW/mm at 180 GHz. All these components were realized with a 40 nm T-gate process (ft= 200 GHz, fMAX= 400 GHz, Vbrk > 40V) which can enable the next generation of transmitter and receiver components that meet or exceed performance reported by competing device technologies while maintaining > 5x higher breakdown voltage, higher linearity, dynamic range and RF survivability.

X band highly efficient GaN power amplifier utilizing built-in electroformed heat sinks for advanced thermal management

We report an X-band class-E GaN power amplifier with built-in electroformed heat sink. Our novel approach for packaging, cooling and interconnecting allows “known good die” GaN MMICs to be combined with other components (Si, SiGe, passives etc) and enable GaN based RF front-ends. The presented amplifier offers continuous wave (CW) output power (Pout) of 34 dBm (power density of 3.2W/mm) with associated power added efficiency (PAE) of 72% and drain efficiency (DE) of 82% when biased at 15V. At 21V the PA offers CW Pout of 36.4 dBm (power density of 5.5W/mm) and associated PAE of 57%. Compared to identical PAs mounted on Cu-W heat sinks with silver epoxy and AuSn eutectic solder this corresponds to a 2x and 1.5x improvement in Pout respectively.

A monolithic RF-MEMS filter with continuously-tunable center-frequency and bandwidth

A novel RF-MEMS structure to achieve tunable bandwidth is presented in this paper. A movable nickel electrode is employed to realize tunable broad-side coupling. The bottom electrode is fabricated with high resistivity-SiCr to avoid performance degradation. This tunable coupling section is implemented in a planar MEMS tunable filter that consists of λ/2 coplanar waveguide (CPW) resonators and nickel RF-MEMS varactors. The center frequency is tuned from 28.55 GHz to 27.22 GHz. The tuning range of the bandwidth is up to 5%. Simultaneous and continuous tuning of both center frequency and bandwidth is experimentally demonstrated.

Wafer-level packaging method incorporating embedded thermal management for GaN-based RF front-ends

We report a new wafer-level, low-cost, scalable RF front-end packaging approach that enables heterogeneous integration of GaN integrated circuits (IC) with other ICs (Si, SiGe, InP, GaAs etc) and RF passives in a 3D package that includes RF/DC interconnects and thermal management. This is achieved by forming a composite substrate utilizing double-side polished alumina wafers with embedded electroformed heat spreaders and through substrate vias. We call this composite substrate Integrated Thermal Array Plate (ITAP). Compared to conventionally mounted GaN power amplifier (PA) using AuSn and silver epoxy the ITAP pckaged X-band PA demonstrated 1.42x and 2x improvement in output power respectively (36.4 dBm, with 57% associated power added efficiency). By using a junction temperature (Tj) evaluation circuit we demonstrated that the ITAP reduces the Tj by 40° when the dissipated power is 2W/mm or increases the power handling by 1.45x when the Tj is held at 150°C. Using the same approach we are also reporting wafer-level packaged GaN power switches as well as thermal cycling and thermal shock test data that show no performance degradation.

W-Band GaN Receiver Components Utilizing Highly Scaled, Next Generation GaN Device Technology

We report the first W-band GaN receiver components using a next generation, highly scaled GaN device technology. This technology (40nm, fT= 220 GHz, fmax= 400 GHz, Vbrk > 40V) enables receiver components that meet or exceed performance reported by competing device technologies while maintaining > 5x higher breakdown voltage, higher linearity, dynamic range and RF survivability. This paper includes results for a 4 and a 5 stage low noise amplifier (LNA) (gain over 5 dB/stage at 110 GHz), a single-pole single-throw (SPST) and a single-pole double-throw (SPDT) switch with loss of 0.9 dB and 1.3 dB respectively and a reflective type phase shifter

Silicon-packaged GaN power HEMTs with integrated heat spreaders

We present the fabrication and experimental characterization of wafer-level-packaged GaN power HEMTs incorporating embedded copper thermal heat spreader and microfabricated interconnects for GaN-based RF front-ends. The packaging fabrication technology mainly relies on silicon micromachining, metal electroplating, and thermocompression bonding. The presented packaging approach simultaneously addresses thermal management, electrical interconnects, performance, and has size and cost advantages over conventional assembly approaches. Silicon-packaged GaN-on-SiC power switches with slanted field plate technology demonstrated comparable DC IV characteristics with on-wafer measurements (threshold voltage = 0.3 V, static on-resistance = 2 Ω.mm measured at gate bias voltage of 1.5V, and drain and gate leakage current <; 10-6 A/mm at gate bias voltage of -2 V). The performance results of Si-packaged GaN devices were consistent with on-wafer measurements, indicating compatibility of the packaging technology with GaN power HEMTs.

Deeply-Scaled E/D-Mode GaN-HEMTs for Sub-mm-Wave Amplifiers and Mixed-Signal Applications

In this paper, we report state-of-the-art high-frequency performance of GaN-based HEMTs achieved through innovative device scaling technologies such as vertically-scaled AlN/GaN/AlGaN double-heterojunction (DH) HEMT epitaxial structure, low-resistance n+-GaN ohmic contacts regrown by MBE, and manufacturable 20-nm self-aligned sidewall gate process. Engineering top barrier layer structure enabled both enhancement- and depletion-mode (E/D) device operations with record cutoff frequencies while maintaining Johnson figure of merit (JFoM) breakdown performance. Furthermore, E/D-mode devices were monolithically integrated using a full epitaxial regrowth technique with a successful demonstration of DCFL ring oscillator circuits. Deeply-scaled E/D-mode GaN-HEMTs with an unprecedented combination of high-frequency and high-breakdown characteristics offer practical advantages in circuit applications such as sub-millimeter-wave power amplifiers, ultra-linear mixers, and increased output power digital-to-analog converters.

Low Loss, Cu damascene interconnects and passives compatible with GaN MMIC

We report a Cu damascene process on multi-layer benzocyclobutene (BCB) dielectrics compatible with GaN MMICs that enables the creation of low-loss, high power handling and high current carrying interconnects. The reported process uses two thick Cu traces (7 μm and 3 μm) and two supplemental Au layers. 7 μm thick Cu traces with width and pitch of 4 μm were realized. Reported circuits showed 0.11 dB/mm loss at 50 GHz, inductor Q-factors of 30 and 57, 3:1 planar balun with less than 2 dB insertion loss, 100% yielding RF and DC via chains.