Joachim Wuerfl

Ferdinand-Braun-Institut | FBH · GaN-Electronics

In this study, the impacts of fabrication technology and epitaxial layer design on the transconductance ( g m ) of radio frequency AlGaN/GaN high‐electron‐mobility transistors (HEMTs) are examined. Optimization of the SiN x passivation and AlGaN barrier design of 150 nm gate HEMTs enhances the extrinsic (at V ds = 10 V) and intrinsic (at V ds = 15...

The monolithic integration of the GaN-on-Si HEMTs is challenging since the back-gating effects caused by a common substrate degrade the device performance. In this paper, we introduce novel GaN-on-AlN/SiC power HEMTs. Compared with conventional GaN-on-Si HEMTs, the presented transistors show immunity to back-gating effects, thereby enabling monolit...

In this work, Si implantation and activation for lowering the ohmic contact resistance (Rc) of mm-wave GaN HEMTs has been investigated. Various combinations of annealing temperature/duration and implantation doses were tested. Dopant activation was performed using a modified procedure in an MOCVD tool, involving fast temperature ramping and anneali...

AlN-based GaN-channel HEMTs with MOCVD-grown AlGaN/GaN/AlN epistack were realized on AlN substrates, achieving 400 mA/mm current density at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> = 1 V and 125 V/μm breakdown voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink=...

This work determined the optimum lateral distance (Lr) between recess etching of an AlGaN barrier layer and the ohmic contact metallization overlapping the recess region. It was observed that the optimum Lr, which corresponds to the minimum contact resistance (Rc), is determined by the transfer length (Lt) of the ohmic contact. This is influenced b...

Drain lag is a well‐known phenomenon that leads to radio frequency performance degradation in AlGaN/GaN high‐electron‐mobility transistors. Herein, it is demonstrated that a reduction of the gate‐to‐drain distance ( L gd ) from 2.0 to 0.5 μm results in 7% reduction in the current collapse. This improvement is attributed to a decrease in surface tra...

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH...

This paper presents a novel approach for reducing the gate resistance (Rg) of K and Ka-band GaN HFETs with 150 nm gate length through a new gate metallization technique. The method involves increasing the gate cross-section via galvanic metallization using FBH's Ir-sputter gate technology, which allows an increase in gate metal thickness from the c...

This article presents the application of FBH’s sputtered Iridium (Ir) gate technology for the design and realization of a highly robust power amplifier (PA) intended for satellite communication systems at millimeter-wave frequencies. Implemented in the 0.15- $\mu $ m Gallium nitride (GaN)-on-SiC high-electron mobility transistor (HEMT) technology,...

We present a detailed model of the static and dynamic gate leakage current in lateral β-Ga2O3 MOSFETs with an Al2O3 gate insulator, covering a wide temperature range. We demonstrate that (i) in the DC regime, current originates from Poole–Frenkel conduction (PFC) in forward bias at high-temperature, while (ii) at low temperature the conduction is d...

AlN-based semiconductor devices are considered to outperform lateral AlGaN/GaN HFETs for power-electronic switching applications due to the high AlN-material breakdown field strength. We present an AlGaN/GaN/AlN-HFET transistor without any compensation doping in the AlN-buffer layer. Breakdown voltage scaling as function of the gate-drain sepa...

In this work, we analyze the optimum annealing conditions for the activation of Ge-implanted β-Ga2O3 in order to reach low ohmic contact resistances. The experiments involved the use of a pulsed rapid thermal annealing treatment at temperatures between 900 and 1200°C in nitrogen atmosphere. Our investigations show remarkable changes in the surface...

We optimized gallium nitride drift layers for high voltage and low resistance vertical electronic devices by tuning the doping concentration for a given thickness of 5 μm. The optimization procedure is based on an empirical mobility model in order to maximize the corresponding device´s power figure-of-merit with respect to the drift layers paramete...

In this work, we report on the realization of vertical (100) β-Ga2O3 FinFET devices for the use in power electronics applications. The experiments are carried out on epitaxial (100) β-Ga2O3 wafers consisting of a conductive substrate with a doping concentration ND of 3×1018 cm-3 and an epitaxially grown layer with ND of 5×1016 cm-3 for the drift an...

DESCRIPTION This chapter reviews recent advances in the fabrication of lateral β-Ga2O3 power transistors for high-voltage switching applications. In this regard, certain aspects in the design principles and strategies are being discussed that are needed for realizing high-performing transistor devices. In addition, essential process modules includi...

In this work, we have investigated the influence of mechanical strain induced by passivation layers on the electrical performance of AlGaN/GaN HFETs. We studied the physical mechanism of a threshold voltage (Vth) shift for the monolithically fabricated on/off devices reported by our group in [1]. For that, theoretical calculations, simulation‐based...

The chemical, physical and electrical properties and the robustness of post metallization annealed Al2O3 atomic layers deposited on n-type GaN are investigated in this work. Planar metal insulator capacitors are used to demonstrate a gate-first with following ohmic contacts formation at elevated temperature up to 600°C process flow. X-ray photoelec...

Power electronic systems employing wide-bandgap GaN transistors promise high efficiency operation and power density but require minimized parasitic circuit elements and an effective cooling concept. This article presents a half-bridge module integrating two 600 V/170 mΩ gallium nitride (GaN) high-electron mobility transistors with their gate drive...

This paper reports on the isolation properties and failure mechanism of n-type vertical Pt / n-GaN Schottky barrier diodes and the dependence on the drift layer doping concentration. The results indicate that (i) independently on doping density, the Schottky barrier height is lower than the theoretical expectation, and this limits the blocking prop...

In this paper, we present the results of a 6-year experiment in space that studied the effects of radiation in Gallium Nitride (GaN) electronics in geostationary orbit. Four GaN transistors in a Colpitts oscillator configuration were flown in the Component Technology Test-Bed aboard the Alphasat telecommunication satellite. A heuristic analysis was...

The present work investigates the use of the refractory metal alloy TiW as a possible candidate for the realization of ohmic contacts to the ultrawide bandgap semiconductor β-Ga2O3. Ohmic contact properties were analyzed by transfer length measurements of TiW contacts annealed at temperatures between 400 and 900 °C. Optimum contact properties with...

In this paper, we extensively characterize and model the threshold voltage instability in lateral β-Ga 2 O 3 MOSFETs with Al 2 O 3 gate dielectric. Specifically, (i) the results indicate that the threshold voltage instability originates from electron trapping at gate dielectric border traps close to the Al 2 O 3 /β-Ga 2 O 3 interface. (ii) Logarith...

In this work, we report on the realization of SnO/β-Ga2O3 heterojunction vertical diodes and lateral field-effect transistors for power electronic applications. The p-type semiconductor SnO is grown by plasma-assisted molecular beam epitaxy on n-type (100) β-Ga2O3 with donor concentrations of 3×10 17 cm-3 for the diode devices and 8.1×10 17 cm-3 fo...

This work clarifies the special characteristics of GaN transistors with non-insulating gate and challenges to drive them. Different existing driver circuits are compared, and the issues occurring in high -frequency or a large duty cycle operations are illustrated. An improved driver circuit with an active discharge path is proposed and verified exp...

Herein, the fabrication of Au‐free ohmic contacts for mm‐wave GaN heterojunction field‐effect transistors is investigated. To find an optimum metal stack and annealing recipe, different metallization and rapid thermal annealing temperature/duration combinations are tested. They are compared with the well‐known Ti/Al/Ni/Au ohmic contact scheme optim...

This paper presents the design of a dual-band power amplifier (PA) featuring similar performance at 28 GHz and 38 GHz. In the new radio (NR) of the fifth generation (5G) communication system, the inter-band carrier aggregation technique is commonly adopted for data rate enhancement. In such configuration, operation at both bands of the 5G frequency...

In this paper, we present the results of a 6-year experiment in space that studied the effects of radiation in GaN electronics in geostationary orbit. Four GaN transistors in a Colpitts oscillator configuration were flown in the Component Technology Test-Bed aboard the Alphasat telecommunication satellite. A heuristic analysis was performed by obse...

In this paper, we investigate changes of DC characteristics in GaN HFETs linked to the intrinsic stress of the 2 nd passivation layer (gate encapsulation). In the earlier work of our group, it was shown [1] that the leakage current of transistors is highly sensitive to the gate encapsulation with SiNx. In this study, the device characteristics befo...

Mechanical stress/strain is altering the charging properties of piezoelectric materials. For AlGaN and GaN heterostructures, this phenomenon has been described theoretically (Ambacher et al 2000 J. Appl. Phys. 87 334–44). Recently it was shown that a mechanically stressed SiN x passivation layer may produce local strain in the AlGaN/GaN layers unde...

ON-state conductance properties of vertical GaN n-channel trench MISFETs manufactured on different GaN substrates and having different gate trench orientations are studied up to 200 ∘C ambient temperature. The best performing devices, with a maximum output current above 4 kA / cm2 and an area specific ON-state resistance of 1.1 mΩ.cm2, are manufact...

In this work, the fabrication of high-performance metal-oxide-semiconductor field-effect transistors (MOSFETs) on Si-doped homoepitaxial layers on (100) Mg-doped semi-insulating β-Ga2O3 substrates is demonstrated. Furthermore, the influence of additional gate and source field-plates is analyzed in regards to the breakdown properties of the MOSFET d...

In this work, the authors developed vertical gallium nitride metal‐insulator‐semiconductor field‐effect transistors (GaN MISFETs) for direct chip‐on‐chip assembly with gallium arsenide‐based broad area distributed Bragg reflector diode laser. The intention of this work had been to provide a high‐speed driver solution having extremely small parasiti...

Static characterization and fast switching processes of lateral β-Ga2O3 MOSFETs are presented. The investigated transistors with 10 mm gate width and 6 μm gate drain distance achieve on-state resistances of 5 Ω and saturation currents above 2.4 A. Hard switching in a double pulse test setup with an inductive load results in voltage slopes up to 65...

The paper presents a highly efficient GaN-based synchronous buck converter suitable for switching in the lower GHz range. The module includes a very compact 2-stage GaN half-bridge converter MMIC (monolithic microwave integrated circuit) for low parasitic inductances between switches and drivers and a hybrid output network with core-less inductors...

Vertical SiC transistors and lateral GaN transistors for power-electronic applications currently target applications with different voltage and power ratings. Meanwhile, research and development activities continue on vertical GaN transistors and new gallium oxide transistors. What are their perspectives in application and how do they compete aga...

-Lateral β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> MOSFET for power switching applications with a 1.8 kV breakdown voltage and a record power figure of merit of 155 MW/cm <sup xml...

Switching of GaN-based vertical-trench gate MISFET on ammonothermal n-type GaN substrate are studied as a function of temperature. Pulsed ON-state IV characterization revealed three unexpected independent effects. First, drain current increases with temperature, although electron mobility decreases with temperature. Second, drain current decreases...

In this study we report on the application of multiple energy nitrogen ion implantation for the electrical isolation of electronic devices on monoclinic β-Ga2O3. By the introduction of uniformly distributed midgap damage-related levels in the Ga2O3 crystal lattice we are able to increase the sheet resistances by more than 9 orders of magnitude to ≥...

The purpose of this work is to introduce a novel concept for integrating enhancement mode (E-mode) and depletion mode (D-mode) AlGaN/GaN HEMTs in the same fabrication process. Unlike existing E/D integration technologies (gate recess etching, fluorine implantation etc.), the proposed technology does not involve process steps that damage the semicon...

The design and gate-control management of monolithically integrated bidirectional normally OFF GaN-HFET-based switches are presented. Unidirectional 600-V p-GaN-gate HFETs are converted into bidirectional devices by adding a second gate to the transistor drift zone. As demonstrated in direct comparison, the double-gate structure exhibits a similar...

This paper investigates the local control of two dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures by applying external mechanical stress. The proposed 2DEG tuning mechanism relies on a local modification of the piezoelectric contribution to the total 2DEG concentration at an AlGaN/GaN heterostructure interface. Discontinuities...

In this work, a vertical n-channel MISFET homoepitaxially grown on ammonothermal n-type GaN substrates by MOVPE is demonstrated. The MIS gate module consists of plasma enhanced atomic layer deposition of Al2O3 combined with in-situ NH3 plasma surface pre-treatment. An annealing step performed at 350 °C in N2 ambient drastically improves device perf...

The proposal, processing and performance of n+GaN/InGaN/AlGaN/GaN normally-off metal-oxide-semiconductor (MOS) HEMTs with a recessed 2-μm long gate/8-μm source-drain distance are presented. It is shown that by using a negative polarization charge at the InGaN/AlGaN interface together with 10-nm thick Al2O3 gate insulation, a threshold voltage VT in...

The purpose of this work is to introduce a novel concept for the AlGaN/GaN HEMT embedded gate fabrication. The novelty of the proposed gate fabrication method is to obtain a stress relief volume inside short embedded gates (150 nm and less). This volume compensates strain generated during device processing after gate module definitionor durin...

Cross conduction in GaN HFETs is particularly critical due to the high d v DS /d t and small gate- source capacitances in conjunction with moderate drain-gate feed-back capacitances. Therefore, the switching speed is limited in current devices, which leads to increased losses. In this work, the turn-on peak current and increased turn-on losses...

Cross conduction in GaN HFETs is particularly critical due to the high dv DS /dt and small gate- source capacitances in conjunction with moderate drain-gate feed-back capacitances. Therefore, the switching speed is limited in current devices, which leads to increased losses. In this work, the turn-on peak current and increased turn-on losses d...

GaN-based transistors with p-GaN gate are commonly accepted as promising devices for application in power converters, thanks to the positive and stable threshold voltage, the low on-resistance and the high breakdown field. This paper reviews the most recent results on the technology and reliability of these devices by presenting original data. The...

Drift effects in semiconductors are changing electrical device properties in dependence on their electrical, thermal and other treatments. In a similar manner as degradation effects they are adversely influencing device performance. However, in contrast to degradation, drift effects are fully recoverable. This means that device performance can be b...

In this paper, current collapse phenomena and thermal effects in normally-off and normally-on GaN HEMTs are investigated. The experimental results show that a high-off-state voltage and high-switched drain current at high junction temperature cause an increase of the on-state resistance RDSON. Varying the switching frequency from 50 to 400 kHz, an...

Enhancement-mode GaN-on-Si HFETs for power switching application are investigated under fast switching and elevated ambient temperatures conditions. The switching characteristics are used to evaluate the dynamic on-state resistance at temperatures up to 175 °C, while switching drain voltage up to 400 V. The devices show a low increase of < 25% in d...

The piezoelectric polarization of the AlGaN/GaN high-electron-mobility transistors (HEMTs) is strongly related to the strain state in the active area. Therefore, understanding the strain behavior inside the channel is crucial to the device electrical performance improvement of devices. This paper, for the first time, reveals the potential of optimi...

We perform a comprehensive electrical transport and physical characterization of metal oxide semiconductor heterostructure field effect transistors with ZrO2 gate dielectrics, having partially (referred here as MOS-HFET) and fully (here called true-MOS-FET) recessed GaN/AlGaN/GaN barrier, giving normally-on and normally-off behavior, respectively....

In the present work, we compare two different embedded-gate technologies used for the fabrication of 150 nm AlGaN/GaN HEMTs intended for K- and Ka-band satellite communication applications. DC performance of fabricated transistors has been similar for both technologies. Load-pull measurements at Vds= 28 V, Ids= 30 % Idsmax and tuned for the maximum...

This paper reports an experimental demonstration of the time-dependent failure of GaN-on-Si power high-electron-mobility transistors with p-GaN gate, submitted to a forward gate stress. By means of combined dc, optical analysis, and 2-D simulations, we demonstrate the following original results: 1) when submitted to a positive voltage stress (in th...

Gate reliability of normally-off p-type- GaN/AlGaN/GaN high-electron mobility transistors grown on Si substrate subjected to forward bias stress at different gate voltages and temperatures was analyzed. Stress-induced gate current degradation was found to be consistent with the percolation process. Obtained time-to-breakdown data were interpreted u...

GaN technology has shown improved characteristics at high power and high frequency conditions, playing a key opportunity in the space industry. In this way, this paper presents a GaN-based MMIC power amplifier (PA), designed for X-band space applications. The PA design consists of a single stage class-B and achieves 10.5 dB gain at 9 GHz and an out...

This letter reports on the influence of illumination conditions on the detector response of three-terminal devices. Antenna-coupled field-effect transistors for the plasmonic detection of THz radiation (TeraFETs) were realized using a 0.25- μm AlGaN/GaN process. Integrated bow-tie antennas are connected to the source, drain and gate terminals of el...

This paper reports an analysis of the degradation mechanisms of GaN-based normally-off transistors submitted to off-state stress, forward-gate operation and electrostatic discharges. The analysis was carried out on transistors with p-type gate, rated for 600. V operation, developed within the European Project HIPOSWITCH. DC measurements, thermal an...