D. Theron

InAlAs/InGaAs metamorphic HEMT with high current density and high breakdown voltage

An In/sub 0.3/Al/sub 0.7/As/In/sub 0.3/Ga/sub 0.7/As metamorphic power high electron mobility transistor (HEMT) grown on GaAs has been developed. This structure with 30% indium content presents several advantages over P-HEMT on GaAs and LM-HEMT on InP. A 0.15-/spl mu/m gate length device with a single /spl delta/ doping exhibits a state-of-the-art current gain cut-off frequency F/sub t/ value of 125 GHz at V/sub ds/=1.5 V, an extrinsic transconductance of 650 mS/mm and a current density of 750 mA/mm associated to a high breakdown voltage of -13 V, power measurements performed at 60 GHz demonstrate a maximum output power of 240 mW/mm with 6.4-dB power gain and a power added efficiency (PAE) of 25%. These are the first power results ever reported for any metamorphic HEMT.

AlGaN/GaN high electron mobility transistors as a voltage-tunable room temperature terahertz sources

We report on room temperature terahertz generation by a submicron size AlGaN/GaN-based high electron mobility transistors. The emission peak is found to be tunable by the gate voltage between 0.75 and 2.1 THz. Radiation frequencies correspond to the lowest fundamental plasma mode in the gated region of the transistor channel. Emission appears at a certain drain bias in a thresholdlike manner. Observed emission is interpreted as a result of Dyakonov–Shur plasma wave instability in the gated two-dimensional electron gas.

Gallium Nitride as an Electromechanical Material

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnsons figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

Surface potential of n- and p-type GaN measured by Kelvin force microscopy

n- and p-type GaN epitaxial layers grown by metal-organic chemical vapor deposition with different doping levels have been characterized by Kelvin probe force microscopy (KFM). To investigate the surface states of GaN beyond instrumental and environmental fluctuations, a KFM calibration procedure using a gold-plated Ohmic contact as a reference has been introduced, and the reproducibility of the KFM measurements has been evaluated. Results show that the Fermi level is pinned for n- and p-type GaN over the available doping ranges, and found 1.34±0.15eV below the conduction band and 1.59±0.18eV above the valence band, respectively.

Amplified piezoelectric transduction of nanoscale motion in gallium nitride electromechanical resonators

A fully integrated electromechanical resonator is described that is based on high mobility piezoelectric semiconductors for actuation and detection of nanoscale motion. We employ the two-dimensional electron gas present at an AlGaN/GaN interface and the piezoelectric properties of this heterostructure to demonstrate a resonant high-electron-mobility transistor enabling the detection of strain variation. In this device, we take advantage of the polarization field divergence originated by mechanical flexural modes for generating piezoelectric doping. This enables a modulation of carrier density which results in a large current flow and thus constitutes a motion detector with intrinsic amplification.

An interferometric scanning microwave microscope and calibration method for sub-fF microwave measurements

We report on an adjustable interferometric set-up for Scanning Microwave Microscopy. This interferometer is designed in order to combine simplicity, a relatively flexible choice of the frequency of interference used for measurements as well as the choice of impedances range where the interference occurs. A vectorial calibration method based on a modified 1-port error model is also proposed. Calibrated measurements of capacitors have been obtained around the test frequency of 3.5 GHz down to about 0.1 fF. Comparison with standard vector network analyzer measurements is shown to assess the performance of the proposed system.

AlGaN-GaN HEMTs on Si with power density performance of 1.9 W/mm at 10 GHz

AlGaN-GaN high electron mobility transistors (HEMTs) on silicon substrate are fabricated. The device with a gate length of 0.3-/spl mu/m and a total gate periphery of 300 /spl mu/m, exhibits a maximum drain current density of 925 mA/mm at V/sub GS/=0 V and V/sub DS/=5 V with an extrinsic transconductance (g/sub m/) of about 250 mS/mm. At 10 GHz, an output power density of 1.9 W/mm associated to a power-added efficiency of 18% and a linear gain of 16 dB are achieved at a drain bias of 30 V. To our knowledge, these power results represent the highest output power density ever reported at this frequency on GaN HEMT grown on silicon substrates.

12 GHz

GaN/AlN/AlGaN/GaN nanowire metal-insulator-semiconductor field-effect transistors (MISFETs) have been fabricated for the first time with submicrometer gate lengths. Their microwave performances were investigated. An intrinsic current-gain cutoff frequency (F T) of 5 GHz as well as an intrinsic maximum available gain (F MAX) cutoff frequency of 12 GHz have been obtained for the first time and associated with a gate length of 0.5 mum. These results show the great potentiality of GaN-based nanowire FETs for microwave applications.

F_{\rm MAX}

The emergence of new materials (nanowires, nanotubes, graphene tapes, and thin films) and devices with nanoscale dimensions give rise to the necessity for developing dedicated techniques that will allow their electrical characterization at high-frequency range. In this article, two possible views have been highlighted to tackle the issue of the measurement of high-impedance nanoscale devices. The first solution is based on the integration of a high-impedance reflectometer and a nanoscale device on the same chip. The microwave impedance of a single CNT has been successfully measured up to 6 GHz using this technique. The second solution consists of inserting an adjustable microwave interferometer between a traditional VNA and the high-impedance device. The interferometer allows adjustment of the impedance to be measured to the highest measurement sensitivity of the measurement system. In particular, capacitances down to 0.35 fF have been measured with an error estimated to be less than 10% using the interferometric technique combined with a scanning microwave microscope. These proofs of concept on one-port nanodevices open the route towards the case of two-port active devices with high impedance. Advances in the manufacturing of next-generation nanodevices will depend on our ability to measure electrical properties and performance characteristics accurately and reproducibly at the nanoscale regime over a broad frequency range.

GaN/AlN/AlGaN Nanowire MISFET

Abstract State-of-the art metamorphic In x Al 1−x As/ In x Ga 1−x As HEMTs (MM-HEMTs) on a GaAs substrate with different indium compositions x=0.33 , 0.4 and 0.5 have been realized and characterized. The gate lengths Lg are 0.1 and 0.25 μm. These devices have been compared with lattice matched HEMTs on an InP substrate. DC-characteristics of 0.1 μm gate length MM-HEMTs show drain-to-source current Ids of the order of 550–650 mA/mm, and extrinsic transconductance of about 800 mS/mm. Schottky characteristics exhibit a gate reverse breakdown voltage varying from −14 to −7 V for x=0.33 –0.5, with an intermediate value of −10.5 V for x=0.4 . A small signal equivalent circuit of our 0.1 μm MM-HEMTs give intrinsic transconductance higher than 1100 mS/mm, with similar values of 1350 and 1450 mS/mm for x=0.5 and the lattice matched HEMT, respectively. The MM-HEMTs with a gate length of 0.25 μm present a cutoff frequency fT close to 100 GHz. To achieve the same result with pseudomorphic HEMTs on GaAs, a smaller gate length has to be realized, which requires the use of an electron beam lithography and therefore increases the device costs. For L g =0.1 μm, fT reaches 160, 195 and 180 GHz for x=0.33 , 0.4 and 0.5, respectively. These values are close to f T =210 GHz obtained for a lattice matched HEMTs on InP realized with the same technological process. The MM-HEMTs are therefore good alternatives to PM-HEMTs on GaAs and LM-HEMTs on InP in the V bands and W bands while maintaining a GaAs substrate. Moreover, metamorphic In0.4Al0.6As/In0.4Ga0.6As HEMTs exhibit a comparable microwave performance with large voltage operation than the MM-HEMT with a 0.5 indium content and the lattice matched HEMTs. These results indicate that a device with indium content x=0.4 is particularly attractive for the realization of low-noise and power circuits on the same wafer.