Lorenzo Lugani

94-GHz Large-Signal Operation of AlInN/GaN High-Electron-Mobility Transistors on Silicon With Regrown Ohmic Contacts

We report the first 94-GHz (W-band) large-signal performance of AlInN/GaN high-electron-mobility transistors (HEMTs) grown on high-resistivity silicon (111) substrates. A maximum output power density of 1.35 W/mm and peak power-added-efficiency of 12% are measured at 94 GHz. The devices exhibit a dc maximum current drain density of 1.6 A/mm and a peak transconductance of 650 mS/mm. In small-signal operation, cutoff frequencies fT/fMAX = 141/232 GHz are achieved. The large-signal performance of our AlInN/GaN HEMTs on silicon at 94 GHz stills lags the best reported results one on SiC substrates but nevertheless confirms the tremendous interest of GaN-on-Si HEMT technology for low-cost millimeterwave electronic applications.

Leakage mechanisms in InAlN based heterostructures

We propose a model for leakage currents in Schottky contacts on InAlN/GaN heterostructures based on two distinct tunneling mechanisms. Our modeling relies on structural parameters, in particular, InAlN dielectric constant, interface polarization charges and Schottky barrier height, which are experimentally determined in the first part of our work. The first leakage mechanism is dominant in heterostructures with very thin (≤7 nm) InAlN barriers and consists in tunneling assisted by a deep level located 1.7 eV below the InAlN conduction band edge. We provide experimental evidence for this level through photocapacitance measurements. The second mechanism is on the other hand dominant in thicker InAlN layers and is linked to the appearance of highly doped regions where direct tunneling through the whole InAlN barrier is significantly enhanced. We also show that the two mechanisms may coexist for InAlN layers of intermediate thickness. Our findings confirm a progressive degradation of the InAlN material qualit...

Ultrathin Body InAlN/GaN HEMTs for High-Temperature (600

Lattice matched 0.25-μm gatelength InAlN/GaN high electron mobility transistors are realized in an ultrathin body mesa technology (50-nm AlN nucleation layer/50-nm GaN buffer) on sapphire. At room temperature, the maximum output current density is I_DS=0.4A/mm, the threshold voltage V_th=-1.4 V with an associated subthreshold voltage swing of 73 mV/dec and a leakage current ≈ 1 pA (for W_G=50 μm) and thus a current on/off ratio of 10¹⁰. At 600°C, the maximum drain current, threshold voltage, and transconductance are nearly unchanged. The current on/off ratio is still approximately 10⁶. First 1-MHz class A measurements with ±2.0 V peak-to-peak signal amplitude have resulted in 109-mW/mm output power at V_DS=8.75 V.

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We report on InAlN/GaN high electron mobility transistors (HEMTs) grown by metal organic vapor phase epitaxy on sapphire with ultrathin buffers. Two dimensional electron gas (2DEG) exhibiting high mobility (1100 cm2/V s) and low sheet resistivity (356 Ω/□) is achieved at room temperature for a buffer thickness as low as ∼0.1 μm. It is shown that despite a huge dislocation density imposed by this thin buffer, surface roughness is the main factor which affects the transport properties. In addition, sapphire surface nitridation is found to drastically affect the properties of the InAlN/GaN 2DEG. Eventually, HEMTs are processed from these heterostructures. Maximum current densities of 0.35 A/mm and current on-off ratios higher than 109 are measured, which make them suitable for high performance GaN based sensing in harsh environments.

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We report the large-signal performance of high electron mobility transistors (HEMTs) fabricated on GaN- and AlN-capped AlInN/GaN epilayers grown on semi-insulating SiC substrates. Large-signal measurements at 10 and 40 GHz are presented with both gate and drain dynamic loadlines to clarify the factors limiting the high-power performance. Devices fabricated with AlN-capped epilayers show a marginal advantage in terms of higher current and reduced dispersion, but GaN-capped epilayers perform better in terms of reduced short-channel effects and better channel control. In large-signal operation at 40 GHz, both device types delivered power densities in excess of 4.5 W/mm. A maximum power density of 5.8 W/mm is achieved on GaN-capped devices which is, to the best of our knowledge, the highest power density reported at 40 GHz in AlInN/GaN-based HEMTs.

Ultrathin InAlN/GaN heterostructures on sapphire for high on/off current ratio high electron mobility transistors

Lattice-matched InAlN/GaN high electron mobility transistors (HEMTs) have been prepared in a silicon-on-insulator (SOI)-like configuration. Here, this implies an ultrathin body 50 nm GaN channel/50 nm AlN nucleation layer material structure on sapphire with the active areas confined by mesa etching, resulting in semi-enhancement mode device characteristics. In contrast to conventional technologies, the device characteristics (maximum drain current, threshold voltage and 1 MHz large signal operation) change only within less than approx. 10% up to 600 degrees C compared to room temperature (RT). The current on/off ratio decreases from 10(10) at RT to 10(6) at 600 degrees C, due to residual defect activation. These first results of ultrathin body GaN-on-sapphire-based materials and device technology may indicate that essential improvements in the temperature-handling capability of electronic device structures beyond what is common at present may be possible with only limited sacrifice of device performance.

AlInN-Based HEMTs for Large-Signal Operation at 40 GHz

We report on the low-temperature growth of heavily Si-doped (>1020 cm−3) n+-type GaN by N-rich ammonia molecular beam epitaxy (MBE) with very low bulk resistivity (<4 × 10−4 Ω·cm). This is applied to the realization of regrown ohmic contacts on InAlN/GaN high electron mobility transistors. A low n+-GaN/2 dimensional electron gas contact resistivity of 0.11 Ω·mm is measured, provided an optimized surface preparation procedure, which is shown to be critical. This proves the great potentials of ammonia MBE for the realization of high performance electronic devices.

GaN-on-insulator technology for high-temperature electronics beyond 400 °C

The capacitance-voltage-temperature characteristics of nonintentionally doped In0.16Al0.84 N/n+-GaN Schottky diodes were measured at 1 MHz and in the 90–400 K range. They are discussed in the framework of existing theories, which properly treat the Poissons equation, especially near the edge of the space-charge region, the so-called transition region. The concentration of a shallow donor and of a deep DX-like center, previously reported, is properly determined. The key parameter to discuss the temperature dependence of the capacitance is the ratio between the frequency of the small ac modulating signal and the temperature-dependent emission rate associated to each level. The capacitance-voltage C-Va curves were successfully fitted using a three parameters expression over the full range of temperatures. The concentration of both shallow and deep levels exceeds a few 1018 cm−3. Based on secondary ion mass spectrometry profiling, we assign both levels to the dominant oxygen impurity. This result supports ou...

n(+)-GaN grown by ammonia molecular beam epitaxy: Application to regrown contacts

We investigate the thermal stability of nearly lattice-matched InAlN layers under metal organic vapor phase epitaxy conditions for temperatures >800 °C and show that they are not fully stable. In particular, InAlN top layers undergo degradation during high temperature annealing due to a surface related process, which causes the loss of crystal quality. This strongly impacts the transport properties of InAlN/GaN HEMT heterostructures; in particular, the mobility is significantly reduced. However, we demonstrate that high thermal stability can be achieved by capping with a GaN layer as thin as 0.5 nm. Those findings enabled us to realize in situ passivated HEMT heterostructures with state of the art transport properties.

Capacitance behavior of InAlN Schottky diodes in presence of large concentrations of shallow and deep states related to oxygen

We report the characterization of GaN high electron mobility transistors (HEMTs) using a new AlN-capped AlInN/GaN epilayer structure developed to achieve high current densities and reduced gate leakage currents. Devices with gate lengths of 75 and 200 nm and various and source-drain separations were fabricated simultaneously, allowing the selection of the most favorable configuration for power performance. We show that, as anticipated, aggressive scaling of source-drain spacing and gatelength does not benefit power performance because of early breakdown and more pronounced short-channel effects. For a non-field-plated 0.2 mu m gate length in a 4 mu m source-drain gap, the new epitaxial structure achieved a power density of 4.5W/mm at 40 GHz. To the best of our knowledge, this is the highest power reported at 40 GHz for AlInN/GaN-based transistors, and the first report of the large-signal performance of an AlN-capped AlInN/GaN-based HEMT. (c) 2013 The Japan Society of Applied Physics