Showing papers by "Michael Mikulla published in 2010"

Influence of the surface potential on electrical properties of AlxGa1-xN/GaN heterostructures with different Al-content: Effect of growth method

Abstract: The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped AlxGa1−xN/GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase (MOVPE) and plasma-assisted molecular beam epitaxial (PA-MBE) grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6×1012 to 1.2×1013 cm−2. A difference of the concentrations for a fixed Al-content was found between the different epitaxial techniques. This result indicates unambiguously different surface potentials determined quantitatively from the carrier concentration, and is verified in addition by the results of photoreflectance spectroscopy. The GaN surface potentials of MOVPE and PA-MBE grown samples amounts to (0.26±0.04) and (0.61±0.10) eV irrespective of the Al-content of the barrier layer. After device fabrication, we find that due to the identical surface potential defined by the Ni Schottky gate, the threshold voltage for a given Al-cont...

AlGaN/GaN epitaxy and technology

Abstract: We present an overview on epitaxial growth, processing technology, device performance, and reliability of our GaN high electron mobility transistors (HEMTs) manufactured on 3- and 4-in. SiC substrates. Epitaxy and processing are optimized for both performance and reliability. We use three different gate lengths, namely 500 nm for 1–6 GHz applications, 250 nm for devices between 6 and 18 GHz, and 150 nm for higher frequencies. The developed HEMTs demonstrate excellent high-voltage stability, high power performance, and large DC to RF conversion efficiencies for all gate lengths. On large gate width devices for base station applications, an output power beyond 125 W is achieved with a power added efficiency around 60% and a linear gain around 16 dB. Reliability is tested both under DC and RF conditions with supply voltage of 50 and 30 V for 500 and 250 nm gates, respectively. DC tests on HEMT devices return a drain current change of just about 10% under IDQ conditions. Under RF stress the observed change in output power density is below 0.2 dB after more than 1000 h for both gate length technologies.

Metamorphic HEMT technology for submillimeter-wave MMIC applications

Abstract: Metamorphic high electron mobility transistor (mHEMT) technologies with 50 and 35 nm gate length were developed for the fabrication of submillimeter-wave monolithic integrated circuits (S-MMICs) operating at 300 GHz and beyond. Heterostructures with very high electron sheet density of 6.1×1012 cm−2 and 9800 cm2/Vs electron mobility were grown on 4” GaAs substrates using a graded quaternary InAlGaAs buffer layer. For proper device scaling channel-gate distance and source resistance were reduced. Maximum transconductance of 2500 mS/mm and a transit frequency of 515 GHz were achieved for the 35 nm mHEMT with 2 × 10 µm gate-width. Already the 50 nm technology allows the realization of S-MMIC operation frequencies up to 320 GHz, the current limit of on-wafer probe availability. A compact four-stage H-band amplifier circuit based on a grounded coplanar waveguide (GCPW) layout is presented in 50 and 35 nm technology, respectively. The 50 nm mHEMT amplifier has a linear gain of 19.5 dB at 320 GHz and more than 15 dB between 240 and 320 GHz. The same amplifier utilizing 35 nm gate-length transistors achieves more than 20 dB gain within the entire H-band from 220 to 320 GHz.

Reliability status of GaN transistors and MMICs in Europe

Abstract: Recent DC- and RF-reliability results of European GaN HEMTs for high frequency power and MMIC applications between 2 and 18 GHz will be presented. The DC-stress test experiments have been performed at high current and high voltage settings in order to test the devices in the different regimes during large signal operation. GaN HEMTs and one stage MMICs have also been tested under RF-operation conditions and the correlation to DC-stress tests has been investigated.

Investigation of Leakage Current of AlGaN/GaN HEMTs Under Pinch-Off Condition by Electroluminescence Microscopy

Abstract: Results from on-wafer electroluminescence (EL) microscopy on AlGaN/GaN high-electron-mobility transistors with leakage currents varying over four orders of magnitude are presented. In the off-state region the integrated EL intensity is proportional to the leakage current and independent of gate width for the devices under study. The slope of the integrated EL–leakage current dependence is determined by the electrical field in the source–drain direction. The influence of the GaN cap thickness is small or even negligible for higher drain bias. Stress during accelerated aging results in enhanced degradation for areas of enhanced leakage current and/or electric field values.

Design and realization of GaN RF-devices and circuits from 1 to 30 GHz

Abstract: The design, realization, and characterization of highly efficient powerbars and monolithic microwave integrated circuit (MMIC) high-power amplifiers (HPAs) with AlGaN/GaN high electronic mobility transistors (HEMTs) are presented for the frequency range between 1 and 30 GHz. Packaged powerbars for the frequency range between 1 and 6 GHz have been developed based on a process called GaN50 with a gate length of 0.5 μm. Based on a GaN25 process with a gate length of 0.25 μm, high-power MMIC amplifiers are presented starting from 6 GHz up to advanced X-band amplifiers and robust LNAs in microstrip transmission line technology.

Development of rugged 2 GHz power bars delivering more than 100 W and 60% power added efficiency

Abstract: In this work we systematically study the structural, optical and electrical properties of AlGaN/GaN heterostructures grown by MOCVD 3-inch on SiC substrates The finally developed HEMTs demonstrate excellent high-voltage stability, high power performance and large power added efficiencies. For a drain bias of 100 V an output-power-density around 26 W/mm with 25 dB linear gain is obtained. On 36 mm gate width devices an output power beyond 100 W is achieved with a power added efficiency above 60% and a linear gain around 17 dB. Ruggedness on these large devices is proven by successfully passing harsh intentional device mismatch tests during operation at 50 V. Reliability is tested at a drain bias of 50 V. Under DC conditions a drain-current degradation below 20% after 20 years is extrapolated. Under RF stress the observed change in output power is well below 0.1 dB after a test duration of more than 500 h. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN power FETs for next generation mobile communication systems

Abstract: This paper presents the RF-performance of Al-GaN/GaN high electron mobility transistors (HEMT) for applications at L-/S-band frequencies. The powerbars provide a linear gain of 20.3 dB with 90 W output power operated at 50 V drain voltage in cw-operation with a maximum drain efficency of 59% at 2.14 GHz. This gain level is competititve to state-of-the-art LDMOS.

Gallium Nitride RF-devices: An overview on the development activities in Europe

Abstract: Gallium Nitride is a key technology with high system impact for European industries for communication-, military-, and space-applications. This paper reviews the recent technical development activities of III-Nitride devices for RF-applications in Europe. After the completion of several significant research projects, the technical status is reviewed based on recent examples relevant to the state-of-the-art in Europe. Further, a European perspective for the industrialization and exploitation of this technology will be discussed.