Vincent Desmaris

A Swedish heterodyne facility instrument for the APEX telescope

Aims. In March 2008, the APEX facility instrument was installed on the telescope at the site of Lliano Chajnantor in northern Chile. The main objective of the paper is to introduce the new instrument to the radio astronomical community. It describes the hardware configuration and presents some initial results from the on-sky commissioning. Methods. The heterodyne instrument covers frequencies between 211 GHz and 1390 GHz divided into four bands. The first three bands are sideband-separating mixers operating in a single sideband mode and based on superconductor-insulator-superconductor (SIS) tunnel junctions. The fourth band is a hot-electron bolometer, waveguide balanced mixer. All bands are integrated in a closedcycle temperature-stabilized cryostat and are cooled to 4 K. Results. We present results from noise temperature, sideband separation ratios, beam, and stability measurements performed on the telescope as a part of the receiver technical commissioning. Examples of broad extragalactic lines are also included.

A 1.3-THz Balanced Waveguide HEB Mixer for the APEX Telescope

In this paper, we report about the development, fabrication, and characterization of a balanced waveguide hot electron bolometer (HEB) receiver for the Atacama Pathfinder EXperiment telescope covering the frequency band of 1.25-1.39 THz. The receiver uses a quadrature balanced scheme and two HEB mixers, fabricated from 4- to 5-nm-thick NbN film deposited on crystalline quartz substrate with an MgO buffer layer in between. We employed a novel micromachining method to produce all-metal waveguide parts at submicrometer accuracy (the main-mode waveguide dimensions are 90 times 180 mum ). We present details on the mixer design and measurement results, including receiver noise performance, stability and ldquofirst-lightrdquo at the telescope site. The receiver yields a double-sideband noise temperature averaged over the RF band below 1200 K, and outstanding stability with a spectroscopic Allan time more than 200 s.

Large-signal modelling and comparison of AlGaN/GaN HEMTs and SiC MESFETs

The Large Signal (LS) model for GaN and SiC FET devices was developed and evaluated with DC, S, and LS measurements. Special attention was paid to improve the management of harmonics and to provide a more physical treatment of the dispersion. The model was implemented in a commercial CAD tool and exhibits good overall accuracy.

Comparison of the DC and Microwave Performance of AlGaN/GaN HEMTs Grown on SiC by MOCVD With Fe-Doped or Unintentionally Doped GaN Buffer Layers

In this brief, the authors present a comparative and comprehensive investigation of the effect of the type of resistive GaN buffers on the dc, dynamic, microwave, and power performance of AlGaN/GaN high electron mobility transistors (HEMTs). Two types of buffer layers were investigated: 1) a nonintentionally doped resistive GaN buffer and 2) an Fe-compensated buffer. The Fe modulation-doped buffer is shown to be favorable for better dc isolation. The RF small-signal performance of the HEMTs does not exhibit any significant dependence on the type of resistive GaN buffer. However, the type of GaN buffer influences considerably the dynamic large-signal characteristics of the processed AlGaN/GaN HEMTs. The continuous-wave output power density of the AlGaN/GaN HEMTs at 3 GHz was increased from 3.4 to 9.7 W/mm by using a nonintentionally doped buffer instead of an Fe-doped one. Based on this observation combined with pulsed current-voltage measurements, we ascribe this difference to the deep trapping of electrons by defects in the GaN buffer introduced by the incorporation of Fe

Performance of the First ALMA Band 5 Production Cartridge

We present performance of the first ALMA Band 5 production cartridge, covering frequencies from 163 to 211 GHz. Atacama Large Millimeter/sub-millimeter Array (ALMA) Band 5 is a dual polarization, sideband separation (2SB) receiver based on all Niobium (Nb) superconductor-insulator-superconductor (SIS) tunnel junction mixers, providing 16 GHz of instantaneous RF bandwidth for astronomy observations. The 2SB mixer for each polarization employs a quadrature configuration. The sideband separation occurs at the output of the IF hybrid that has integrated bias-T for biasing the mixers, and is produced using superconducting thin-film technology. Experimental verification of the Band 5 cold cartridge performed together with warm cartridge assembly, confirms that the system noise temperature is below 45 K over most of the RF band, which is less than 5 photon noise (5 hf/k). This is to our knowledge, the best results reported at these frequencies. The measurement of the sideband rejection indicates that the sideband rejection is better than 10 dB over 90% of the observational band.

Oxygen Ion Implantation Isolation Planar Process for AlGaN/GaN HEMTs

A multienergy oxygen ion implantation process was demonstrated to be compatible with the processing of high- power microwave AlGaN/GaN high electron mobility transistors (HEMTs). HEMTs that are isolated by this process exhibited gate-lag- and drain-lag-free operation. A maximum output power density of 5.3 W/mm at Vgs = -4 V and Vds = 50 V and a maximum power added efficiency of 51.5% at Vgs = -4 V and Vds = 30 V at 3 GHz were demonstrated on HEMTs without field plates on sapphire substrate. This isolation process results in planar HEMTs, circumventing potential problems with enhanced gate leakage due to the gate contacting the 2-D electron gas at the mesa sidewall.

C-band linear resistive wide bandgap FET mixers

In this paper the performance of two C-band resistive FET mixers are presented and compared. The first mixer uses a SiC-MESFET as a mixing element and the second uses an AlGaN/GaN-HEMT. The mixers have a minimum conversion loss of 7.8 dB and 7.3 dB respectively. The maximum third-order input intercept points are 30 dBm and 36 dBm respectively; for LO drives of 23 dBm and 30 dBm.

All-metal micromachining for the fabrication of sub-millimetre and THz waveguide components and circuits

A novel technology for the manufacturing of micromachined all-metal waveguide circuits and structures for frequency bands ranging from 200 up to 7000 GHz (sub-millimetre and THz) is presented. The waveguide circuits are formed by using metal electroplating with preceding sputtering of a thin metal film seed layer over a photo-lithographically patterned thick SU-8 photoresist. The process provides the possibility of making three-dimensional structures via facilitating multi-level (layered) designs. The surface roughness of the THz waveguide structure was demonstrated to be as low as 30 nm. This technology was used to build a state-of-the-art waveguide balanced 1.3 THz hot electron bolometer mixer and other applications for radio astronomy instrumentation.

Low-Resistance Si/Ti/Al/Ni/Au Multilayer Ohmic Contact to Undoped AlGaN/GaN Heterostructures

A Si/Ti/Al/Ni/Au metallization scheme was investigated for the formation of very low ohmic contacts on undoped AlGaN/GaN heterostructures. The influences of the silicon layer thickness and the annealing temperature on thecontact resistance were studied using linear transmission line model (TLM) patterns. A contact resistance of 0.23 Ω mm, corresponding to a specific contact resistance of 1.06 × 10 - 6 Ω cm 2 , was achieved with a 30 A thick silicon layer annealed at 800°C for 30 s.

DC and microwave performance of AlGaN/GaN HEMTs passivated with sputtered SiNx

The effects of sputtered and room temperature plasma enhanced chemical vapour deposition (RT-PECVD) SiNx passivation on the dc and microwave performance of AlGaN/GaN high electron mobility transistors (HEMTs)are studied. The pulsed I–V characteristics from a class B quiescent bias point and transient measurements indicate that the sputtered SiNx passivation is more efficient in suppressing lag effects in AlGaN/GaN HEMTs. Dispersion-free sputtered SiNx passivated AlGaN/GaN HEMTs were obtained using this technique. Continuous-wave (CW) measurements without active cooling give a maximum output power density of 6.6 W mm−1 at Vgs=−4 V, Vds = 50 V and a maximum power added efficiency of 51.3% at Vgs=−4 V, Vds = 30 V at 3 GHz on 2 × 50 μm AlGaN/GaN HEMTs on the sapphire substrate, with a gate length of 2 μm and without field-plated gates. To the best of our knowledge, this is the highest level power density reported on the sapphire substrate without field-plate design. The extrinsic cut-off frequency ( ft) and maximum oscillation frequency ( fmax) are 51 GHz and 100 GHz, respectively, on 2 × 50 × 0.15 μm HEMTs. To our knowledge, the sputtered SiNx passivation for AlGaN/GaN HEMTs is a unique technique, which has never been published before.