Showing papers by "Michael Mikulla published in 2012"

Generation of traps in AlGaN/GaN HEMTs during RF-and DC-stress test

Abstract: The effect of RF stress at 10 GHz and DC stress on AlGaN/GaN HEMTs have been investigated by comparing static and transient characteristics before and after stress. It was found that the threshold voltage shifts in both tests significantly to the negative. A defect level of 0.44 eV was detected during Id-trapping analysis. Using the experimental trap data and simulating different locations of traps in the device it was established that the defective region is extended throughout the gate region. Quantitative approximations of the trap density suggest an extension of the traps into the barrier layer with a concentration of approximately 1018 cm−3.

Development of 100 nm gate AlGaN/GaN HEMT and MMIC technology suitable for mm-wave applications

Abstract: In this paper a Gallium Nitride MMIC technology for RF- and microwave and high power amplifiers based on 100 nm gate technology is presented. The MMIC technology includes alloyed ohmic contacts, T-shaped 100 nm gates, air-bridges, MIM capacitors, inductors, and a full through wafer viahole backside process. As a key element for a high MMIC yield, we'll present low ohmic contact resistances with a high yield on several wafers in different processing batches and low leakage currents for a reliable device performance, respectively. Single HEMT performance exhibits PAE values of up to 60% and power densities of 1.3 W/mm. For a single stage power amplifier with a WG = 4 × 45 µm dual-gate HEMT configuration, a gain beyond 10 dB at 60 GHz is presented. By using only one transistor for the amplifier stage, a maximum output power of more than 21 dBm and a PAE of 9.3% is achieved. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiative inter-valley transitions as a dominant emission mechanism in AlGaN/GaN high electron mobility transistors

Abstract: We report on electroluminescence (EL) emission from AlGaN/GaN high electron mobility transistors (HEMTs). Intensity maxima at the drain-side edge of the gate foot and at the drain-side edge of the gate field plate are observed. To relate the EL intensity profile to the electric field along the channel, 2D device simulations have been performed at different drain biases. The dependences of both EL maxima on the electric field reveal a threshold which closely correlates with the electric field strength at which a transfer of conduction band electrons from the zone centre minimum to satellite valleys sets in. We further analyze the dependence of the EL spectra on the drain voltage. The obtained results strongly suggest that the EL emission observed in AlGaN/GaN HEMTs is dominated by radiative inter-valley electron transitions.

High efficiency X-band AlGaN/GaN MMICs for space applications with lifetimes above 10 5 hours

Abstract: We report on technology, performance and reliability of state-of-the-art AlGaN/GaN MMICs for space applications. Our quarter-micron gate length HEMTs have breakdown voltages beyond 150 V and deliver 5 W/mm output power density at 30 V drain bias with 50% PAE at 10 GHz operating frequency. Packaged two-stage MMICs with 8 W output power for telemetry applications have a PAE above 40% with a lifetime above 105 h at a channel temperature of 200°C. Initial space evaluation tests indicate a suitable stability of our technology in space.

GaN‐based high‐frequency devices and circuits: A Fraunhofer perspective

Abstract: We present the current status of our technology for GaN-based HEMTs and MMICs as well as results ranging from the L-band up to the W-band. Epitaxial growth is carried out on 4H-SiC(0001) substrates by both MOCVD and MBE. Processing is done using standard III–V equipment including both frontside and backside processing. For L-band power bars we arrive at output powers, efficiencies and gains beyond 100 W, 60% and 17 dB, all measured under cw conditions at 50 V drain bias. The X-band MMICs are characterized by a high efficiency above 40% for two-stage amplifiers. Towards mm-wave applications we have fabricated HEMTs with transit frequencies above 100 GHz and W-band MMICs delivering 0.5 W/mm at 94 GHz with 7% PAE. First quaternary InAlGaN barriers show promising results for this new materials system. Reliability tests return a very good long-term stability of our devices even at an elevated channel temperature of 200 °C with an extrapolated lifetime of 5 × 105. Initial space capability tests including total ion dose radiation insensitivity, radiation displacement damage, hydrogen poisoning and single event effect are successfully passed.

Influence of AlGaN barrier thickness on electrical and device properties in Al0.14Ga0.86N/GaN high electron mobility transistor structures

Abstract: We investigate the influence of the AlGaN barrier thickness in Al0.14Ga0.86N/GaN heterostructures on both the electrical properties of the heterostructure itself as well as on high electron mobility transistors fabricated on these structures. With increasing barrier thickness, we observe decreasing sheet resistances, transconductances, and threshold voltages. The observed changes are well-described by modelling. We demonstrate that an increase in the barrier thickness in AlGaN/GaN heterostructures results in an increase in available high-frequency input power swing before turn-on of the Schottky gate. The device long-term stability under direct current stress is not affected by the increase in barrier thickness as shown by on-wafer reliability tests at 150 °C base plate temperature. These results pave the way towards AlGaN/GaN transistors offering high robustness under extreme mismatch conditions as well as excellent high-frequency power performance.

Reverse bias stress test of GaN HEMTs for high-voltage switching applications

Abstract: The degradation of packaged GaN HEMTs for high power applications has been studied under long term reverse bias step stress tests. Increases of leakage current and dynamic R on resistance have been found. This degradation is possibly caused by the formation of localized defects which have been observed by backside electroluminescence imaging. In addition the effect of device layout and substrate material on the dynamic R on as well as its temperature, recovery behavior, and drain voltage dependence have been investigated on wafer-level. The recovery behavior and the temperature dependence indicate that the dynamic R on resistance increase is caused by surface or buffer carrier trapping. By reducing the buffer trap density the dynamic R on resistance was reduced. A slightly higher dynamic R on of GaN HEMTs on silicon compared to transistors on SiC substrate has been observed.

Trade‐offs between performance and reliability in AlGaN/GaN transistors

Abstract: We report on the trade-off between performance and reliability for AlGaN/GaN transistors. It is shown that changes in epitaxial growth, transistor design and process may lead to an improvement in performance but are, at the same time, accompanied by a degradation of device reliability. As a result we show strategies in order to balance performance and reliability as both are linked. Based on these findings we have realized state-of-the-art power bars for mobile communication systems and X-band MMICs for radar applications. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A high-gain high-power amplifier MMIC for V-band applications using 100 nm AlGaN/GaN dual-gate HEMTs

Abstract: In this paper we present the design and realization of a high-power amplifier in grounded coplanar transmission line technology using AlGaN/GaN dual-gate High electron mobility transistors (HEMTs) with a gate-length of 100 nm to achieve a high gain per stage and high output power. A large-signal model was extracted for the dual-gate HEMT based on the state-space approach. For the fabricated dual-stage amplifier a continuous-wave saturatedoutput power of up to 24.8 dBm (0.84 W/mm) was measured at 63 GHz for 20 V drain bias. A small-signal gain of more than 20 dB was achieved between 56 and 65 GHz.

Microscopic Degradation Analysis of RF-Stressed AlGaN/GaN HEMTs

Abstract: An AlGaN/GaN high electron mobility transistor (HEMT) stressed at 10 GHz and increased channel temperatures of T ≈ 260 °C has been analyzed by electroluminescence microscopy (ELM) and infrared thermography (IRT). After stress a negative threshold shift is seen in the electrical characteristics. Based on the current dependence of the electroluminescence (EL) intensity image and a local increase of T this shift can be assigned to the degradation of one of its gate fingers. Transmission electron microscopy (TEM) images of this gate finger revealed structural changes along the drain-side edge of the gate.

Hafnium oxide passivation of InGaAs/InP heterostructure bipolar transistors by electron beam evaporation

Abstract: In this contribution, we investigate the passivation of InGaAs/InP heterostructures using plasma assisted e-beam evaporated hafnium oxide (HfO2). The microstructure and optical properties of the HfO2 layers are first examined by X-ray reflectivity and spectroscopic ellipsometry on Si substrates. The current gain and breakdown voltage of InGaAs/InP heterostructure bipolar transistors (HBTs) have subsequently been used to evaluate the impact and efficiency of the e-beam evaporated HfO2 passivation layers. The results from these structures have been contrasted with data from similar samples encapsulated with SiO2 using conventional plasma enhanced chemical vapor deposition. The HfO2 passivated InGaAs/InP HBTs show comparable current gains as compared to unpassivated structures. More importantly, in contrast to SiO2-PECVD devices, the common emitter characteristics of HfO2 passivated HBTs show no degradation in device breakdown voltage. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A U-band broadband power amplifier MMIC in 100 nm AlGaN/GaN HEMT technology

Abstract: The design and manufacture of a three-stage broadband power amplifier is presented in this paper. The amplifier MMIC is realized using AlGaN/GaN HEMTs with a gate-length of 100 nm in a grounded coplanar transmission line technology. A small-signal gain of over 20 dB was measured in the frequency range from 38 to 62 GHz (U-band) for the designed amplifier. This corresponds to a very high small-signal bandwidth of over 48 %. Related to the current-gain cutoff frequency, the bandwidth of the amplifier is 30 % of f T and the gain-bandwidth-product is approximately five times f T . Furthermore, a high continuous-wave saturated output power of 25.8 dBm (380 mW) and a high saturated power density of over 1 W/mm in the output stage are provided by the MMIC.