AlN/GaN MOS-HEMTs technology

The ever increasing demand for higher power devices at higher frequencies has prompted much research recently into the aluminium nitride/gallium nitride high electron mobility transistors (AlN/GaN HEMTs) in response to theoretical predictions of higher performance devices. Despite having superior material properties such as higher two-dimensional electron gas (2DEG) densities and larger breakdown field as compared to the conventional aluminium gallium nitride (AlGaN)/GaN HEMTs, the AlN/GaN devices suffer from surface sensitivity, high leakage currents and high Ohmic contact resistances. Having very thin AlN barrier layer of ∼ 3 nm makes the epilayers very sensitive to liquids coming in contact with the surface. Exposure to any chemical solutions during device processing degrades the surface properties, resulting in poor device performance. To overcome the problems, a protective layer is employed during fabrication of AlN/GaN-based devices. However, in the presence of the protective/passivation layers, formation of low Ohmic resistance source and drain contact becomes even more difficult. In this work, thermally grown aluminium oxide (Al2O3) was used as a gate di- electric and surface passivation for AlN/GaN metal-oxide-semiconductor (MOS)-HEMTs. Most importantly, the Al2O3 acts as a protection layer during device processing. The developed technique allows for a simple and effective wet etching optimisation using 16H3PO4:HNO3:2H2O solution to remove Al from the Ohmic contact regions prior to the formation of Al2O3 and Ohmic metallisation. Low Ohmic contact resistance (0.76Ω.mm) as well as low sheet resistance (318Ω/square) were obtained after optimisation. Significant reduction in the gate leakage currents was observed when employing an additional layer of thermally grown Al2O3 on the mesa sidewalls, particularly in the region where the gate metallisation overlaps with the exposed channel edge. A high peak current ∼1.5 A/mm at VGS=+3 V and a current-gain cutoff frequency, fT , and maximum oscillation frequency, fMAX , of 50 GHz and 40 GHz, respectively, were obtained for a device with 0.2 μm gate length and 100 μm gate width. The measured breakdown voltage, VBR, of a two-finger MOS-HEMT with 0.5μm gate length and 100 μm gate width was 58 V. Additionally, an approach based on an accurate estimate of all the small-signal equivalent circuit elements followed by optimisation of these to get the actual element values was also developed for AlN/GaN MOS-HEMTs. The extracted element values provide feedback for further device process optimisation. The achieved results indicate the suitability of thermally grown Al2O3 for AlN/GaN-based MOS-HEMT technology for future high frequency power applications

Self-switching diodes as RF rectifiers: evaluation methods and current progress

In the advancement of the Internet of Things (IoT) applications, widespread uses and applications of devices require higher frequency connectivity to be explored and exploited. Furthermore, the size, weight, power and cost demands for the IoT ecosystems also creates a new paradigm for the hardware where improved power efficiency and efficient wireless transmission needed to be investigated and made feasible. As such, functional microwave detectors to detect and rectify the signals transmitted in higher frequency regions are crucial. This paper reviewed the practicability of self switching diodes as Radio Frequency (RF) rectifiers. The existing methods used in the evaluation of the rectification performance and cut-off frequency are reviewed, and current achievements are then concluded. The works reviewed in this paper highlights the functionality of SSD as a RF rectifier with design simplicity, which may offer cheaper alternatives in current high frequency rectifying devices for application in low-power devices

AlN/GaN HEMTs with SiN Passivation and Recessed Ohmics

We report the fabrication of good performing SiN/AlN/GaN MIS-HEMT devices with maximum drain current density, IDS, of 1.9A/mm. Compared to the same devices on SiN/AlGaN/GaN MIS-HEMT structure, these have superior performance. This is achieved by recessing the Ohmic contacts to lower the contact resistance. These devices employ a 10nm PECVD deposited SiN dielectric layer under the gate to reduce the leakage current. This paper will compare the DC performance of AlGaN/GaN and AlN/GaN HEMTs employing SiN passivation and recessed Ohmic contacts. We will also comment on the performance of RF devices on AlGaN/GaN material

AlN/GaN HEMTs with SiN Passivation and Recessed Ohmics

We report the fabrication of good performing SiN/AlN/GaN MIS-HEMT devices with maximum drain current density, IDS, of 1.9A/mm. Compared to the same devices on SiN/AlGaN/GaN MIS-HEMT structure, these have superior performance. This is achieved by recessing the Ohmic contacts to lower the contact resistance. These devices employ a 10nm PECVD deposited SiN dielectric layer under the gate to reduce the leakage current. This paper will compare the DC performance of AlGaN/GaN and AlN/GaN HEMTs employing SiN passivation and recessed Ohmic contacts. We will also comment on the performance of RF devices on AlGaN/GaN material

DC and RF performance of AlN/GaN MOS-HEMTs

This paper reports the DC and RF characteristics of AlN/GaN MOS-HEMTs passivated with thin Al2O3 formed by thermal oxidation of evaporated aluminium. Extraction of the small-signal equivalent circuit is also described. Device fabrication involved wet etching of evaporated Al from the Ohmic contact regions prior to metal deposition. This approach yielded an average contact resistance of ∼0.76 Ω.mm extracted from transmission line method (TLM) characterisation. Fabricated two-finger AlN/GaN MOS-HEMTs with 0.2 µm gate length and 100 µm gate width showed good gate control of drain currents up to a gate bias of 3 V and achieved a maximum drain current, IDSmax of ∼1460 mA/mm. The peak extrinsic transconductance, Gmax, of the device was ∼303 mS/mm at VDS = 4 V. Current-gain cut-off frequency, fT, and maximum oscillation frequency, fMAX, of 50 GHz and 40 GHz, respectively, were extracted from S-parameter measurements. For longer gate length, LG = 0.5 µm, fT and fMAX were 20 GHz and 30 GHz, respectively. These results demonstrate the potential of AlN/GaN MOS-HEMTs for high power and high frequency applications

The impact of procurement and contracting practices on health and safety A literature review

SIGLEAvailable from British Library Document Supply Centre-DSC:4274.85375(99/02) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

High Performance of AlGaN/GaN HEMTs using Buffer-Free GaN on SiC Structure

This paper reports on the processing and device characteristics of AlGaN/GaN high electron mobility transistors using buffer-free GaN grown on SiC substrate. This new concept of thin AlGaN/GaN heterostructure (<330nm of epitaxial layers) as compared to a conventional structure with a thick GaN buffer layer (>2-3μm). As-grown epitaxial structure provides a twodimensional electron gas (2DEG), ns, of 1 x 1013 cm-2 , an electron mobility, µ, of > 2000 cm2 /V.s, and sheet resistance, Rsh, of 330 Ω/□. The fabricated AlGaN/GaN HEMT buffer-free GaN with a 3-μm gate long, two-finger 2 × 50 µm wide device demonstrates a maximum drain current density of 801 mA/mm at VGS = 2 V and maximum peak transconductance of 189 mS/mm at VDS = 5 V. This device also produces low gate leakage currents of IGS = 2.1x10-4 A/mm at VGS = -10 V and a breakdown voltage, VBR, of over 200 V. The maximum cut-off frequency, fT, and maximum oscillation frequency, fmax, of 4.7 GHz and 9.4 GHz were obtained respectively. These results indicate the potential of using buffer-free GaN heterostructure for future high power high frequency applications

Dielectric and microstructural properties of BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics

BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics were investigated regarding their dielectric and microstructure properties via conventional solid state reaction method. The phase pure samples were obtained when heated at 1400°C for overnight. The effect of Er3+ doped into BaTiO3 on dielectric properties and microstructural properties was investigated for composition of BaTiO3 and Ba0.9925Er0.0075TiO3. The analysis was made by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Impedance Analyzer. The XRD patterns of BaTiO3 and Ba0.9925Er0.0075TiO3 are phase pure and identical with tetragonal perovskite structure with space group of P4mm. The lattice parameters and unit cell volume of BaTiO3 increased by doping with Erbium as the crystallite size decreased. Measurements of dielectric properties were carried out as a function of temperature up to 200°C at different frequencies. Ba0.9925Er0.0075TiO3 exhibit the high value of dielectric constant (ε=6179) at Curie temperature (TC) of 120°C. SEM analysis of BaTiO3 and Ba0.9925Er0.0075TiO3 ceramics showed that the grain sizes of BaTiO3 and Ba0.9925Er0.0075TiO3 were ranged from 3.3µm-7.8µm and 2.2µm-4.7µm respectively

The role of selective pattern etching to improve the Ohmic contact resistance and device performance of AlGaN/GaN HEMTs

In this work, we report the processing and DC performance of fabricated AlGaN/GaN HEMT devices using 3 different patterned Ohmic contact structures. The types of Ohmic contact patterns used are horizontal, vertical and chess. A device with a conventional Ohmic contact was also fabricated for comparison. Two different etch depths were investigated, a ~ 9 nm and ~ 30 nm for a shallow and deep Ohmic recess etching, respectively. The lowest contact resistance of 0.32 Ω.mm was observed for a deep horizontal patterned structure. The fabricated device with this structure also demonstrated the highest maximum saturation drain current of 1285 mA/mm and maximum transconductance of 296 mS/mm compared to other devices. The horizontal patterned structure utilizes the uneven AlGaN layer thickness underneath the Ohmic metal contacts. The formation of sidewall areas on AlGaN surface during the patterned etching process provides better contact of Ohmic metal resulting in more tunnelling current between the Ohmic metal and AlGaN barrier thus reducing the contact resistance. This approach also provides the lowest contact resistance due to removal of AlGaN barrier layer (patterned etching) and it is in parallel with the lateral current of the 2DEG resulting in better tunnelling current compared to the vertical and chess patterned structures. The contact resistance can be further improved by optimization the etching depth prior to Ohmic metal deposition. The results indicate the potential of the Ohmic patterned etching structure to further improving the performance of GaN devices