11.7 W/mm Continuous Wave Output Power in X-Band AlN-Buffer HEMTs

A study of the electrical performances of AlInN/GaN High Electron Mobility Transistors (HEMTs) on SiC substrates is presented in this paper. Four different wafers with different technological and epitaxial processes were characterized. Thanks to intensive characterizations as pulsed-IV, [S]-parameters, and load-pull measurements from S to Ku bands, it is demonstrated here that AlInN/GaN HEMTs show excellent power performances and constitute a particularly interesting alternative to AlGaN/GaN HEMTs, especially for high-frequency applications beyond the X band. The measured transistors with 250 nm gate lengths from different wafers delivered in continuous wave (cw): 10.8 W/mm with 60% associated power added efficiency (PAE) at 3,5 GHz, 6.6 W/mm with 39% associated PAE at 10.24 GHz, and 4.2 W/mm with 43% associated PAE at 18 GHz.

Two differential radio frequency (RF) power amplifier (PA) architectures are presented in this paper both of which can be used to implement a wideband Doherty PA. Classic PA architecture is compared to a SB (SB) PA in order to determine an architecture that offers the widest input impedance match without external component tuning. Both amplifiers presented in this paper have been designed to output an average of 10.5 dBm of continuous wave (CW) power with 1.8 V supply. The classic PA architecture is tuned to 1.9 GHz central frequency by external components and has a bandwidth of 500 MHz at S11=−10 dB. The SB PA architecture has an operating range spanning from 0.8 GHz to 2.2 GHz at S11=−10 dB. SB PA has a higher power added efficiency (PAE) at output compression point (OR-P1dB) in the whole 1.2 GHz–2.4 GHz range by 3 %–7 % with a peak efficiency of 31 %. The presented results indicate that a SB RF PA architecture can be implemented designing a wideband Doherty PA.

The monolithic microwave integrated circuit (MMIC) power amplifiers serve an essential and critical role in RF transmit/receive (T/R) modules of phased array radar systems, mobile communication systems and satellite systems. Over recent years, there has been an increasing requirement to develop wideband high-efficiency MMIC high power amplifiers (HPAs) to accommodate wideband operation and reduce power consumption. This paper presents a wideband high efficiency MMIC HPA for Sub-6-GHz applications using a 0.25-μm gate-length D-mode GaN/SiC high electron mobility transistor (HEMT) process. The amplifier consists of two stages with two HEMT cells for the driver stage and eight HEMT cells for the power stage. To obtain a flat gain while maintaining the wideband characteristic, a gain equalization technique is employed in the inter-stage matching circuit. Meanwhile, a low-loss output matching network is utilized to ensure high efficiency. The fabricated HPA occupies a compact chip area of 14.35 mm2 including testing pads. Over the frequency range of 2–6 GHz, measured results of this HPA show a saturated continuous wave (CW) output power of 44.4–45.2 dBm, a power added efficiency (PAE) of 35.8–51.3%, a small signal gain of 24–25.5 dB, and maximum input and output return losses of 14.5 and 10 dB, respectively.

Several mini-TWTs (traveling-wave tubes) were recently power-combined in the band 2.0 to 8.0 GHz to achieve 250 W of continuous-wave (CW) power at better than 90% combining efficiency. Graceful degradation was demonstrated by successively turning off each of the TWTs. The combining device, known as the spatial field power combiner, is especially suited for high-average-power applications. This approach has the potential for achieving CW powers in excess of 1 kW over multioctave frequency bands up to 20 GHz. This work focuses on a four-way combiner, and the results obtained in combining mini-TWTs are discussed. >

This study provides experimental evidence regarding the mechanism of gallium nitride (GaN) selective-area growth (SAG) on a polished plateau-patterned sapphire substrate (PP-PSS), on which aluminum nitride (AlN) buffer layers are deposited under the same deposition conditions. The SAG of GaN was only observed on the plateau region of the PP-PSS, irrespective of the number of growth cycles. Indirect samples deposited on the bare c-plane substrate were prepared to determine the difference between the AlN buffer layers in the plateau region and silicon oxide (). The AlN buffer layer in the plateau region exhibited a higher surface energy, and its crystal orientation is indicated by AlN [001]. In contrast, regions other than the plateau region did not exhibit crystallinity and presented lower surface energies. The direct analysis results of PP-PSS using transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) are similar to the results of the indirect samples. Therefore, under the same conditions, the GaN SAG of the deposited layer is related to crystallinity, crystal orientation, and surface energy.

Since 1976, the Laser Hardened Materials Evaluation Laboratory (LHMEL) has been characterizing material responses to laser energy in support of national defense programs and the aerospace industry. Now, that capability is available to commercial industry as well. LHMEL’s power levels, beam uniformity, and diagnostics capabilities make it an ideal facility for proof-of-concept testing or process development.Located at Wright-Patterson Air Force Base, Ohio, LHMEL is managed by the Wright Laboratory Materials Directorate and operated by Lawrence Associates, Incorporated. The facility’s advanced hardware is centered around two carbon dioxide laser systems: the LHMEL I laser producing 15 kW of continuous wave (CW) power on target and LHMEL II producing 150 kW of CW power on target. Both lasers operate at 10.6 microns and can deliver stable, full power irradiations for durations of up to 80 seconds.Both LHMEL devices use a multi-mode resonator configuration producing a flat-top spatial intensity distribution. This beam profile delivers a uniform amount of energy over the entire beam diameter and ensures a controlled and repeatable laser exposure on target.The singular capabilities of the LHMEL facility provide unique opportunities for commercial materials processing. Uses of LHMEL might include demonstration of the feasibility of a new material processing technique before investing capital in its development, simulating the high service life temperatures experienced by parts within an automobile or jet engine, performing a limited production run to laser cut thick steel plates for a special custom order, or using LHMEL’s high power to perform deep-penetration welds of steel, aluminum, or titanium. LHMEL has already performed extensive work in the area of thermal load simulation and has begun initial testing efforts in other areas of materials processing.LHMEL’s capabilities are perhaps most beneficial in the surface treatment of materials. Applications requiring uniform irradiation of large surface areas to produce the ideal heat treatment with limited or single passes could be satisfied by LHMEL. Initial heat treatment tests have been performed at LHMEL. Other techniques to increase wear resistance, extend service life, or even inhibit surface corrosion might be developed using LHMEL.Tests are performed on a cost-reimbursement basis or, if appropriate, a Cooperative Research and Development Agreement (CRDA) can be negotiated.Since 1976, the Laser Hardened Materials Evaluation Laboratory (LHMEL) has been characterizing material responses to laser energy in support of national defense programs and the aerospace industry. Now, that capability is available to commercial industry as well. LHMEL’s power levels, beam uniformity, and diagnostics capabilities make it an ideal facility for proof-of-concept testing or process development.Located at Wright-Patterson Air Force Base, Ohio, LHMEL is managed by the Wright Laboratory Materials Directorate and operated by Lawrence Associates, Incorporated. The facility’s advanced hardware is centered around two carbon dioxide laser systems: the LHMEL I laser producing 15 kW of continuous wave (CW) power on target and LHMEL II producing 150 kW of CW power on target. Both lasers operate at 10.6 microns and can deliver stable, full power irradiations for durations of up to 80 seconds.Both LHMEL devices use a multi-mode resonator configuration producing a flat-top spatial intensity distribution. T...

High wall-plug efficiency and a wide range of available wavelengths make laser diode arrays preferable for many high-power applications, including optical pumping of solid state lasers. Recently, we designed and fabricated InGaAsP/InP arrays operating at 1.5-μm and In(Al)GaAsSb/GaSb arrays operating at 2.3-μm. We have demonstrated a high continuous-wave (CW) output power of 25 W from a one dimensional laser array and a quasi-CW (q-CW) output power of 110 W from a two dimensional laser array both operating near 1.5-μm. We have obtained a CW output power of 10 W from the 2.3-μm laser array. The 1.5-μm arrays are suitable for resonant pumping of erbium doped solid-state lasers, which require high power optical sources emitting in the narrow erbium absorption bands. Long current-injection pulses produce a considerable temperature increase within the diode laser structure which induces a red-shift of the output wavelength. This thermal drift of the laser array emission spectrum can lead to misalignment with the erbium absorption bands, which decreases pumping efficiency. We have developed an experimental technique to measure the time dependence of the laser emission spectrum during a single current pulse. From the red-shift of the laser emission, we determine the temperature of the laser active region as a function of time. The spacing between the individual laser emitters has an effect on the array heating. In steady state operation, this spacing is a contributing factor in the non-uniformity of the thermal field within the bar, and thus to the overall thermal resistance of the laser bar. Under pulse operation, the transient heating process can be divided into three time periods; each with its own heat transport condition. It was shown that in the initial period of time the heat propagates within the laser bar structure and the laser bar design (fill factor) strongly affects the active region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor. This analysis has been verified in experimental studies using the 1.5-μm laser arrays.

AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) with thick (>30 nm), high-κ (TiO2/NiO), submicron-footprint (0.4 μm) gate dielectric on SiC substrate are demonstrated, which are found to exhibit low gate leakage current (~ 1 nA/mm of gate periphery), high IMAX (1 A/mm), and high drain breakdown voltage (188 V). The derived current gain cutoff frequency is 30 GHz (from S-parameter measurements). The output power density is 6.6 W/mm, and the associated power-added-efficiency is 46% at 2.5 GHz frequency and 50 V drain bias. This high performance submicron-footprint MOSHEMT is highly promising for microwave power amplifier applications in communication and radar systems.

High power, 0.81-μm-emitting, semiconductor diode lasers are used as pump sources for Nd:YAG solid-state lasers. Devices (1-mm-long) consisting of a InGaAsP/In0.5(Ga0.9Al0.1)0.5P/In0.5(Ga0.5Al0.5)0.5P laser structure provide a threshold-current density, Jth, of 290 A/cm2 and a relatively high threshold-current characteristic temperature, T0 (140 K). Uncoated diode lasers (1.2-mm-long) have a maximum continuous wave output power of 5 W (both facets) at 20 °C. The internal power density at catastrophic optical mirror damage (COMD), P̄COMD, is determined to be 9.1 MW/cm2; that is, 1.8 times that for GaAs-active layer, Al-free, uncoated devices. Coated, InGaAsP-active devices are expected to have P̄COMD=18 MW/cm2, more than twice the P̄COMD of AlGaAs-active, 0.81-μm-emitting devices with the same emitting aperture. Therefore, 0.81-μm-emitting, InGaAsP-active diode lasers should operate reliably at powers at least twice those of AlGaAs-based devices with the same contact-stripe geometry.

The gate length and gate contour of a GaAs metal–semiconductor field effect transistor (MESFET) device play important roles in determining the small signal circuit parameters and large signal breakdown voltage behavior. GaAs MESFETs with different gate lengths and gate contours were studied by the two-dimensional (2-D) semiconductor device simulations to investigate the dependence of small signal circuit parameters and breakdown voltage on gate length and gate contour. The results show that gate length affects small-signal circuit parameter Cgs while gate contour affects Cgd. The breakdown voltage has strong dependence on gate contour and little dependence on gate length.

The research described in this thesis has been carried out within a joint project between the Radboud Universiteit Nijmegen (RU) and the Technische Universiteit Eindhoven (TU/e) with the title: Performance enhancement of GaN-based microwave power amplifiers: material, device and design issues. This project has been granted by the Dutch Technology Foundation STW under project number NAF 5040. The aims of this project have been to develop the technology required to grow state-of-the-art AlGaN/GaN epilayers on sapphire and semi-insulating (s.i.) SiC substrates using metal organic chemical vapor deposition (MOCVD) and to fabricate microwave (f > 1 GHz) high-power (Pout > 10 W) heterostructure field-effect transistors (HFETs) on these epitaxial films. MOCVD growth of AlGaN/GaN epilayers and material characterization has been done within the group Applied Materials Science (AMS) of RU. Research at the Opto-Electronic Devices group (OED) of TU/e has focused on both electrical characterization of AlGaN/GaN epilayers and design, process technology development, and characterization of GaN-based HFETs and CPW passive components. Although a considerable amount of work has been done during this research with respect to processing of CPW passive components on s.i. SiC substrates, this thesis focused on active AlGaN/GaN devices only. GaN is an excellent option for high-power/high-temperature microwave applications because of its high electric breakdown field (3 MV/cm) and high electron saturation velocity (1.5 x 107 cm/s). The former is a result of the wide bandgap (3.44 eV at RT) and enables the application of high supply voltages (> 50 V), which is one of the two requirements for highpower device performance. In addition, the wide bandgap allows the material to withstand much higher operating temperatures (300oC - 500oC) than can the conventional semiconductor materials such as Si, GaAs, and InP. A big advantage of GaN over SiC is the possibility to grow heterostructures, e.g. AlGaN/GaN. The resulting two-dimensional electron gas (2DEG) at the AlGaN/GaN heterojunction serves as the conductive channel. Large drain currents (> 1 A/mm), which are the second requirement for a power device, can be achieved because of the high electron sheet densities (> 1 x 1013 cm-2) and high electron saturation velocity. These material properties clearly indicate why GaN is a very suitable candidate for next-generation microwave high-power/high-temperature applications such as high-power amplifiers (HPAs) for GSM base stations, and microwave monolithic integrated circuits (MMICs) for radar systems. In this thesis we have presented the design, technology, and measurement results of n.i.d. AlGaN/GaN:Fe HFETs grown on s.i. 4H-SiC substrates by MOCVD. These devices have submicrometer T- or FP-gates with a gate length (Lg) of 0.7 µm and total gate widths (Wg) of 0.25 mm, 0.5 mm, and 1.0 mm, respectively. The 1.0 mm devices are capable of producing a maximum microwave output power (Pout) of 11.9 W at S-band (2 GHz - 4 GHz) using class AB bias conditions of VDS = 50 V and VGS = -4.65 V. It has to be noted that excellent scaling of Pout with Wg has been demonstrated. In addition, the associated power gain (Gp) ranges between 15 dB and 20 dB, and for the power added efficiency (PAE) values from 54 % up to 70 % have been obtained. These results clearly illustrate both the successful development of the MOCVD growth process, and the successful development and integration of process modules such as ohmic and Schottky contact technology, device isolation, electron beam lithography, surface passivation, and air bridge technology, into a process flow that enables the fabrication of state-of-the-art large periphery n.i.d. AlGaN/GaN:Fe HFETs on s.i. SiC substrates, which are perfectly suitable for application in e.g. HPAs at S-band.

In this paper, we present a high-efficiency S-band gallium nitride (GaN) power amplifier (PA). This amplifier is fabricated based on a self-developed GaN high-electron-mobility transistor (HEMT) with 10 mm gate width on SiC substrate. Harmonic manipulation circuits are presented in the amplifier. The matching networks consist of microstrip lines and discrete components. Open-circuited stub lines in both input and output are used to tune the 2nd harmonic wave and match the GaN HEMT to the highest efficiency condition. The developed amplifier delivers an output power of 48.5 dBm (∼70 W) with a power-added efficiency (PAE) of 72.2% at 2 GHz in pulse condition. When operating at 1.8–2.2 GHz (20% relative bandwidth), the amplifier provides an output power higher than 48 dBm (∼ 65 W), with a PAE over 70% and a power gain above 15 dB. When operating in continuous-wave (CW) operating conditions, the amplifier gives an output power over 46 dBm (40 W) with PAE beyond 60% over the whole operation frequency range.

Continuous wave and rhenium disulphide-based Nd:GdTaO4 laser under direct pumping

With the amplification of 380 fs, 1 mJ ultrashort pulses with a nearly diffraction limited beam quality, Yb:YAG single crystal fibers (SCF) have recently proven their strong potential as power amplifier. Furthermore, during a first power scaling experiment, up to 250 W continuous wave (cw) power were obtained from an Yb:YAG SCF oscillator at 570 W of pump power. In this paper, we present a power scaling experiment of Yb:YAG SCF amplifiers.

Epitaxial growth of zinc oxide thin films on silicon