Geng-Yen Lee

High-Performance AlGaN/GaN Schottky Diodes With an AlGaN/AlN Buffer Layer

High-performance AlGaN/GaN Schottky barrier diodes are fabricated on a composite AlGaN/AlN buffer layer with low screw-type and high edge-type dislocation densities. Without edge termination, the devices with 30-μm anode-to-cathode spacing exhibit a high breakdown voltage (<i>V</i>_B) of 3489 V, a low leakage current (<i>I</i>_R) of less than 0.2 μA at -2000 V, and a low specific on-resistance (<i>R</i>_on) of 7.9 mΩ·cm², resulting in a figure of merit (<i>V</i>_B²/<i>R</i>_on) as high as 1.54 GW/cm². Their switching characteristics as revealed by the reverse-recovery transient waveform exhibit a short reverse-recovery time of 17 ns.

Effects of Lens Shape on GaN Grown on Microlens Patterned Sapphire Substrates by Metallorganic Chemical Vapor Deposition

The epitaxial growth of GaN on patterned c-plane sapphire substrates having microlenses with a flat top, a dull tip, or a sharp tip is carried out. The growth mode, dislocation density, residual strain, and optical properties of GaN are investigated and correlated with the shape of the microlens. Because the growth of GaN does not take place on top of the microlens with a sharp tip, this type of patterned substrate leads to a wider low dislocation density lateral growth region, while it also gives rise to a higher compressive residual strain in GaN. For GaN grown on the microlens with a dull tip, many dislocations appear, resulting from the extra facets on the lens. It, however, has the lowest compressive strain among the samples studied. This work provides a guideline for preparing microlens patterned sapphire substrates for potential applications in high brightness InGaN light emitting diodes as both dislocation density and strain influence their internal quantum efficiency.

AlGaN/GaN high electron mobility transistors for protein–peptide binding affinity study

Abstract Antibody-immobilized AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect a short peptide consisting of 20 amino acids. One-binding-site model and two-binding-site model were used for the analysis of the electrical signals, revealing the number of binding sites on an antibody and the dissociation constants between the antibody and the short peptide. In the binding-site models, the surface coverage ratio of the short peptide on the sensor surface is relevant to the electrical signals resulted from the peptide–antibody binding on the HEMTs. Two binding sites on an antibody were observed and two dissociation constants, 4.404×10−11 M and 1.596×10−9 M, were extracted from the binding-site model through the analysis of the surface coverage ratio of the short peptide on the sensor surface. We have also shown that the conventional method to extract the dissociation constant from the linear regression of curve-fitting with Langmuir isotherm equation may lead to an incorrect information if the receptor has more than one binding site for the ligand. The limit of detection (LOD) of the sensor observed in the experimental result (∼10pM of the short peptide) is very close to the LOD (around 2.7–3.4pM) predicted from the value of the smallest dissociation constants. The sensitivity of the sensor is not only dependent on the transistors, but also highly relies on the affinity of the ligand-receptor pair. The results demonstrate that the AlGaN/GaN HEMTs cannot only be used for biosensors, but also for the biological affinity study.

Low-resistance smooth-surface Ti/Al/Cr/Mo/Au n-type Ohmic contact to AlGaN/GaN heterostructures

A comparative study of the specific contact resistivity and surface morphology of Ti/Al/Ni/Au, Ti/Cr/Mo/Au, and Ti/Al/Cr/Mo/Au metal contact stacks on AlGaN/GaN heterostructures is reported. Compared to the conventional Ti/Al/Ni/Au contact, the Ti/Al/Cr/Mo/Au contact has much smoother surface and achieves minimum specific contact resistivity of 1.1×10−6 Ω cm2. This contact maintains its low contact resistivity after storage at 200 °C for 100 h in N2. The robustness of this contact is attributed to the Cr and Mo layers, which suppress the formation of Al–Au alloys in the contact stack.

Human immunodeficiency virus drug development assisted with AlGaN/GaN high electron mobility transistors and binding-site models

Human immunodeficiency virus (HIV) Reverse Transcriptase (RT)-immobilized AlGaN/GaN high electron mobility transistors (HEMTs) and binding-site models were used to find out the dissociation constants of the HIV RT-inhibitor complex and the number of the binding sites on RT for the inhibitor, Efavirenz. One binding site on the RT for the inhibitor is predicted and the dissociation constant extracted from the binding-site model is 0.212 nM. The AlGaN/GaN HEMTs and the binding-site-models are demonstrated to be good tools to assist drug developments by elucidating the dissociation constants and the number of binding sites, which can largely reduce the cost and time for drug developments.

Efficiency Enhancement of InGaN LEDs With an n-Type AlGaN/GaN/InGaN Current Spreading Layer

This letter reports an InGaN light-emitting diode (LED) structure that has an n-type Al0.1Ga0.9N/GaN/In0.06Ga0.94N current spreading layer under its multiple-quantum-well active region. As indicated by simulation, the Al0.1Ga0.9N/GaN/In0.06Ga0.94 N heterostructure induces a higher electron concentration than an n-AlGaN/GaN cladding layer and an n-GaN/InGaN current spreading layer that are used in conventional LEDs. As a result, the proposed n-type spreading layer is expected to alleviate current crowding and improve external quantum efficiency. Experimentally, the light output uniformity across the chips is greatly improved. The output power and wall-plug efficiency are enhanced by about 18.2% and 22.2% at an injection current of 350 mA for the LEDs employing the new double-heterostructure current spreading layer.

Improved Surface Morphology and Edge Definition for Ohmic Contacts to AlGaN/GaN Heterostructures

We demonstrate that Ti/Al/Cr/Mo/Au ohmic contact has an extremely smooth surface morphology of 29.0 nm and a low specific contact resistivity (ρc) of 1.1×10-6 ohm-cm2 on n-type AlGaN/GaN heterostructures. The use of Cr interlayer in Ti/Al/Cr/Mo/Au contacts leads to significantly improved contact morphology without any degradation on the contact resistance. This is attributed to the reduced inter-diffusion and reaction between the layers in the contact stack.

Viscosity-dependent drain current noise of AlGaN/GaN high electron mobility transistor in polar liquids

The drain current fluctuation of ungated AlGaN/GaN high electron mobility transistors (HEMTs) measured in different fluids at a drain-source voltage of 0.5 V was investigated. The HEMTs with metal on the gate region showed good current stability in deionized water, while a large fluctuation in drain current was observed for HEMTs without gate metal. The fluctuation in drain current for the HEMTs without gate metal was observed and calculated as standard deviation from a real-time measurement in air, deionized water, ethanol, dimethyl sulfoxide, ethylene glycol, 1,2-butanediol, and glycerol. At room temperature, the fluctuation in drain current for the HEMTs without gate metal was found to be relevant to the dipole moment and the viscosity of the liquids. A liquid with a larger viscosity showed a smaller fluctuation in drain current. The viscosity-dependent fluctuation of the drain current was ascribed to the Brownian motions of the liquid molecules, which induced a variation in the surface dipole of the gate region. This study uncovers the causes of the fluctuation in drain current of HEMTs in fluids. The results show that the AlGaN/GaN HEMTs may be used as sensors to measure the viscosity of liquids within a certain range of viscosity.

Growth of high quality AlN on sapphire by using a low-temperature AlN interlayer

Aluminum nitride is a material of great potential for high power electronic devices, UV photonic devices as well as acoustic devices. However, the lack of a good crystal growth technology for bulk material and substrate hinders the development of these AlN-based devices. While AlN has been successfully grown on sapphire substrate for some time, the presence of a large number of dislocations in the material is still a major barrier to overcome [1]. In this work, we demonstrate a low-dislocation-density AlN template on sapphire by inserting an AlN interlayer by metal-organic chemical vapor deposition. The main idea of our approach is to change the growth mode in the course of the epitaxial growth by decreasing growth temperature and changing V/III ratio. As the growth mode changes, dislocations tend to be redirected and/or form dipole half loops via annihilation processes [2]. The etch-pit-density of the AlN templates is reduced from 3.6×109 cm-2 to 1.7×109 cm-2. Accordingly, the full width at half maximum of the (0002) x-ray rocking curve is reduced from 37 arcsec to 12 arcsec. The result indicates that the AlN template has low screw and mixed type dislocations. AlGaN/GaN Schottky diodes fabricated on this high quality AlN template exhibit very high breakdown voltage (> 2000 V), which sets a record-high figure of merit of 1.15 GW/cm2.

Improving the off-state characteristics and dynamic on-resistance of AlInN/AlN/GaN HEMTs with a GaN cap layer

The electrical characteristics of a series of AlInN high-electron-mobility transistors (HEMTs) with a GaN cap layer ranging from 0 to 26 nm are investigated for power switching applications. The breakdown voltage (VB), mobility of two-dimensional electron gas, on-state resistance (Ron), and dynamic Ron of the HEMTs are improved by increasing the cap layer thickness. The improved electrical characteristics are attributed to the GaN cap layer, which not only reduces the surface E-field but also raises the conduction band of the barrier layer and effectively prevents electrons from being trapped in the AlInN barrier and above.