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We present GaN-based high electron mobility transistors (HEMTs) with a 2-nm-thin InAlN/AlN barrier capped with highly doped n(++) GaN. Selective etching of the cap layer results in a well-controllable ultrathin barrier enhancement-mode device with a threshold voltage of +0.7 V. The n(++) GaN layer provides a 290-Omega/square sheet resistance in the HEMT access region and eliminates current dispersion measured by pulsed IV without requiring additional surface passivation. Devices with a gate length of 0.5-mu m exhibit maximum drain current of 800 mA/mm, maximum transconductance of 400 mS/mm, and current cutoff frequency f(T) of 33.7 GHz. In addition, we demonstrate depletion-mode devices on the same wafer, opening up perspectives for reproducible high-performance InAlN-based digital integrated circuits.

In this letter, successful operation at 10 GHz of T-gate HEMTs on epitaxial structures grown by metal-organic chemical vapor deposition (MOCVD) or MBE on composite substrates is demonstrated. The used device fabrication process is very similar to the process used on monocrystalline SiC substrate. High power density was measured on both epimaterials at 10 GHz. The best value is an output power density of 5.06 W/mm associated to a power-added efficiency (PAE) of 34.7% and a linear gain of 11.8 dB at VDS = 30 V for the components based on MOCVD-grown material. The output power density is 3.58 W/mm with a maximum PAE of 25% and a linear gain around 15 dB at VDS = 40 V for the MBE-grown material.

This paper reports on the dc analysis and radio frequency (RF) characterization of a flexible AlGaN/GaN high-electron mobility transistor with a 120-nm gate length. The device provides a maximum dc current density of 470 mA/mm and a peak extrinsic transconductance of 125 mS/mm under flat condition. When the substrate is bent with 0.88% strain, a rise in the 2-DEG density is experimentally observed through the diminution of the on-resistance. This phenomenon is physically attributed to the modification of the piezoelectric field within the barrier under tensile condition. The device also shows a current gain cutoff frequency (Ft) of 32 GHz and a power gain cutoff frequency (Fmax) of 52 GHz. No major variations of RF performance are observed under bending.

In this letter, for the first time to our knowledge, high dc characteristics of AlGaN/GaN/silicon high-electron-mobility transistors transferred onto a thermally enhanced adhesive flexible tape are reported. Transmission line method (TLM) pattern supported on a flexible tape under 0.5% strain exhibits a high current density of 260 mA/mm. DC measurements performed on a representative gate-TLM device (LG = 2 μm) on a flexible tape are presented. Under 0.16 % strain, the device exhibits a maximum drain current of 300 mA/mm for a gate bias of 0 V and a drain bias of 3 V and withstands VDS = 18 V.

This study reports on a reliability investigation of AlGaN/GaN HEMTs submitted to life tests in High Temperature Operating (HTO) conditions at 150°C, 175°C, 275°C and 320°C. These life tests showed two different degradation steps of the drain current. One is occurring in the first tens of hour of the life test and characterized by a decrease of the drain current. The evolution of the electrical characteristics during ageing does not depend on the bias conditions but rather on the channel temperature. This degradation mode is characterized by a high activation energy of 1.2eV. The small changes of electrical characteristics observed during the life tests results from a combination of trap-related effects before stabilization sets in. The second failure mechanism observed during the HTO tests at 275°C and 320°C results in a higher drain current decrease. Moreover, no stabilization of the parameter drifts was observed before the end of the tests. Pulsed I-V measurements show a large evolution of gate lag and drain lag rates after HTOT275 and HTOT320 life tests. LF 1/f noise after the life tests at high temperature drastically increased by more than two orders of magnitude while it hardly changed after 2000 hours of life test at 150°C and 175°C. It results that the temperature is considered as an acceleration factor of the degradation affecting the conduction channel. TEM observations revealed similar damages in the gate finger cross-section of aged devices after the HTOT275 and HTOT320 life tests. These results could point out a degradation mechanism associated with the inverse piezoelectric effect.

Low microwave noise performance of AlGaN/GaN HEMTs fabricated on MOCVD epitaxial structures grown on composite substrates (SiCopSiC) is reported. They are made of a thin SiC single-crystal layer transferred on top of a thick polycrystalline SiC wafer. The transistor fabrication process is similar to the one developed for AlGaN/GaN devices on SiC substrate. A minimum noise figure of 0.12 dB with an associated gain of 14.8 dB at 3 GHz is found, showing the capability of gallium-nitride based devices for low noise microwave applications in the S-band.

Design considerations and performance of n(++) GaN/InAlN/AlN/GaN normally off high-electron mobility transistors (HEMTs) are analyzed. Selective and damage-free dry etching of the gate recess through the GaN cap down to a 1-nm-thick InAlN barrier secures positive threshold voltage, while the thickness and the doping of the GaN cap influence the HEMT direct current and microwave performance. The cap doping density was suggested to be 2 x 10(20) cm(-3). To screen the channel from the surface traps, the needed cap thickness was estimated to be only 6 nm. Design is proved by an experiment showing a constant value of the HEMT dynamical access resistance, while a single-pulse experiment indicated almost collapse-free performance. On the other hand, it is found that the n(++) GaN cap does not contribute to the HEMT drain current conduction, nor does it provide a path for the OFF-state breakdown. HEMTs with a gate length of 0.25 mu m and a 4-mu m source-to-drain distance show a drain-to-source current of 0.8 A/mm, a transconductance of 440 mS/mm, a threshold voltage of similar to 0.4 V, and a cutoff frequency of 50 GHz. A thin and highly doped GaN cap is also found to be suitable for the processing of normally on HEMTs by adopting the nonrecessed gate separated from the cap by insulation.

The fabrication of high transconductance AlGaN/GaN high electron mobility transistors (HEMTs) grown on high-resistivity silicon substrate is reported with an AlGaN barrier thickness of only 12.5 nm. A maximum DC current density of 655 mA/mm, a current gain cutoff frequency (F T ) of 75 GHz and a power-gain cutoff frequency (F MAX ) of 125 GHz are obtained for a 0.125 µm gate length transistor. The device provides a record peak extrinsic transconductance of 332 mS/mm and an intrinsic value of 509 mS/mm. To the authors' knowledge, the obtained transconductances are the highest reported values from AlGaN/GaN devices grown on a Si(111) substrate. This performance demonstrates the potential of GaN transistors on silicon for low-cost microwave power applications.

We correlate dc maximal drain current I DSmax , pulsed output characteristics, rf small-signal and breakdown performance of normally-off InAlN/GaN HEMTs with varied gate-to-drain distance d GD . It is found that parasitic lag effects which are related to the possible surface states are not appearing at longer d GD . On the other hand compromise need to be found between the improved gate performance and impaired I DSmax and f T as the d GD is increased. The leakage current of the 0.5 µm-long gate may be reduced by up-to three orders of magnitude, down to µA/mm at −20 V bias if d GD increases from 3 to 15 µm. On the other hand I DSmax and f T drop by about one third of the original value (from about 0.6 A/mm and 34 GHz down to 0.4 A/mm and 21 GHz, respectively) if d GD changes.

International audience; This work is a study of the degradations of AlGaN/GaN HEMTs induced by 2000 h of ageing tests. The methodology is based on cross-characterisation analysis.The life tests (HTO 150 °C, HTO 175 °C and HTRB 175 °C and Idq 90 °C) have mainly induced a decrease of the saturation drain current, occurring during the first 50 h, followed by a stabilisation. There is a shift of the pinch-off voltage in the range of 0.1–0.2 V while the Schottky contact is rather stable after ageing. The evolution of the electrical characteristics after ageing does not depend on the bias conditions but rather more on the channel temperature. It seems to be neither field nor current driven. Low frequency drain current noise demonstrates that there is no trap creation and the weak evolution of the 1/f noise confirms that there is no degradation in the channel. Moreover, pulsed I–V measurements show a weak evolution of gate lag and drain lag rates after ageing. The same degradation mode is demonstrated for all life tests with rather high activation energy of 1.6 eV. The weak evolution of electrical characteristics observed during the life tests cannot be obviously explained by a single physical mechanism and results from a combination of trap-related effects before stabilisation.

We study the formation of the InP on GaAsSb interface grown by molecular beam epitaxy at 450 °C. Using angle resolved x-ray photoemission spectroscopy (XPS), we show that Sb strongly segregates whereas As does not, leading to a Sb-rich InP surface. Similarly, XPS spectra recorded on air-exposed samples reveal oxidized Sb but no oxidized As. We perform a quantitative analysis and determine a Sb segregation coefficient very near to 1. This result is in good agreement with previous reflectance anisotropy spectroscopy observations on the same interface.

In this reported work, the thermal resistance of AlGaN/GaN HEMTs processed on SopSiC composite substrate is determined by electrical I-V pulsed measurement. SopSiC substrate is based on an innovative structure with a thin Si single crystal layer transferred on top of a thick polycrystalline SiC wafer. For the first time, it is demonstrated that the thermal resistivity of such devices reaches 18.9 K·mm/W when 7.5 W/mm power is dissipated, while 23.5 K·mm/W are measured on silicon in the same conditions. This result shows the capabilities of composite substrates to compete with silicon for microwave power applications.