Claude Ahyi

Temperature-dependent electrical characteristics of bulk GaN Schottky rectifier

The temperature-dependent electrical characteristics of Schottky rectifiers fabricated with a SiO2 field plate on a freestanding n− gallium nitride (GaN) substrate were reported in the temperature range of 298–473K. The Schottky barrier heights evaluated from forward current-voltage measurement revealed an increase of Schottky barrier height and series resistance but a decrease of ideality factor (n) with increasing temperature. However, the Schottky barrier heights evaluated from capacitance-voltage measurement remained almost the same throughout the temperature range measured. The Richardson constant extrapolated from ln(J0∕T2) vs 1∕T plot was found to be 0.029Acm−2K−2. A modified Richardson plot with ln(J0∕T2) vs 1∕nT showed better linearity, and the corresponding effective Richardson constant was 35Acm−2K−2. The device showed a high reverse breakdown voltage of 560V at room temperature. The negative temperature coefficients were found for reverse breakdown voltage, which is indicative of a defect-assi...

Electrical characteristics of bulk GaN-based Schottky rectifiers with ultrafast reverse recovery

A vertical Schottky diode rectifier was fabricated using a bulk n−GaN wafer. Pt Schottky contacts were prepared on the Ga face and full backside ohmic contact was prepared on the N face by using Ti∕Al. The root mean square surface roughnesses of the Ga and N faces are 0.61 and 4.7nm, respectively. A relatively high breakdown field of 5.46kV∕cm was achieved with no additional edge termination. The breakdown field decreases as the size of the device increases. The background electron concentration of the bulk GaN wafer was low (5×1015cm−3), which may lead to a relatively high breakdown field even with no special edge termination. The forward turn-on voltage was as low as 2.4V at the current density of 100A∕cm2. The device exhibited an ultrafast reverse recovery characteristics (reverse recovery time <20ns).

Ultra-low leakage and high breakdown Schottky diodes fabricated on free-standing GaN substrate

Vertical Schottky diodes were fabricated on the bulk GaN substrate with decreasing impurity concentration from N-face to Ga-face. An array of circular Pt Schottky contacts and a full backside Ti/Al/Ni/Au ohmic contact were prepared on the Ga-face and the N-face of the n-GaN substrate, respectively. The Schottky diode exhibits a minimum specific on-state

Systematic structural and chemical characterization of the transition layer at the interface of NO-annealed 4H-SiC/SiO2 metal-oxide-semiconductor field-effect transistors

We present a systematic characterization of the transition layer at the 4H-SiC/SiO2 interface as a function of nitric oxide (NO) post-annealing time, using high-resolution transmission electron microscopy for structural characterization and spatially resolved electron energy-loss spectroscopy for chemical analysis. We propose a systematic method for determining transition layer width by measuring the monotonic chemical shift of the Si-L2,3 edge across the interface, and compare its efficacy to traditional measures from the literature, revealing the proposed method to be most reliable. A gradual shift in the Si-L2,3 edge onset energy suggests mixed Si-C/Si-O bonding in the transition layer. We confirm an inverse relationship between NO-anneal time and transition layer width, which correlates with improved channel mobility, enhanced N density at the interface, and decreased interface trap density. No excess C was noted in the interfacial region.

Biofunctionalized AlGaN/GaN high electron mobility transistor for DNA hybridization detection

Label-free electrical detection of deoxyribonucleic acid (DNA) hybridization was demonstrated using an AlGaN/GaN high electron mobility transistor (HEMT) based transducer with a biofunctionalized gate. The HEMT DNA sensor employed the immobilization of amine-modified single strand DNA on the self-assembled monolayers of 11-mercaptoundecanoic acid. The sensor exhibited a substantial current drop upon introduction of complimentary DNA to the gate well, which is a clear indication of the hybridization. The application of 3 base-pair mismatched target DNA showed little change in output current characteristics of the transistor. Therefore, it can be concluded that our DNA sensor is highly specific to DNA sequences.

Fabrication and device characteristics of Schottky-type bulk GaN-based “visible-blind” ultraviolet photodetectors

The authors present the fabrication and characterization of vertical-geometry Schottky-type ultraviolet (UV) photodetectors based on a bulk n-GaN substrate. By using low temperature rapid thermal annealing of the semitransparent Schottky contacts (nickel with 7% vanadium), they obtained an ultralow dark current of 0.56pA at −10V reverse bias. A responsivity of ∼0.09A∕W at zero bias was measured for wavelength shorter than the absorption edge of GaN, and it was found to be independent of the incident power in the range measured (50mW∕m2–2.2kW∕m2). The devices are visible blind, with an UV/visible contrast of over six orders of magnitude. An open-circuit voltage of 0.3V was also obtained under a broadband UV illumination.

A comparison of 63 MeV proton and 10 keV X-ray radiation effects in 4H-SiC depletion-mode vertical trench JFETs

In this paper we compare 10 keV X-ray and 63 MeV proton radiation effects on 4H-SiC vertical trench JFET power transistors. The schematic cross-section of the 4H-SiC depletion-mode vertical trench JFET was investigated. For a direct comparison, total dose levels were held constant between the two radiation sources. The significant differences in device response for the two different radiation sources, and the isochronal annealing effects on proton-irradiated devices, are presented in this paper.

Passivation of Oxide Layers on 4H-SiC Using Sequential Anneals in Nitric Oxide and Hydrogen

The interface passivation process based on post-oxidation, high temperature anneals in nitric oxide (NO) is well established for SiO{sub 2} on (0001) 4H-SiC. The NO process results in an order of magnitude or more reduction in the interface state density near the 4H conduction band edge. However, trap densities are still high compared to those measured for Si/SiO{sub 2} passivated with post-oxidation anneals in hydrogen. Herein, we report the results of studies for 4H-SiC/SiO{sub 2} undertaken to determine the effects of additional passivation anneals in hydrogen when these anneals are carried out following a standard NO anneal. After NO passivation and Pt deposition to form gate contacts, post-metallization anneals in hydrogen further reduced the trap density from approximately 1.5 x 10{sup 12} cm{sup -2}eV{sup -1} to about 6 x 10{sup 11} cm{sup -2}eV{sup -1} at a trap energy of 0.1 eV below the band edge for dry thermal oxides on both (0001) and (11-20) 4H-SiC.

Bulk GaN-Based Schottky rectifier and UV photodetector

High power switches are indispensable components in electronic subsystems for applications such as advanced hybrid electric vehicles. Due to its unipolar nature, a Schottky diode does not exhibit the minority carrier storage effect, and the device has a negligible reverse current transient. Therefore, faster switching can be achieved with Schottky diodes compared to p-n junction diodes. The successful application of Schottky diodes for power rectification also requires efficient thermal management. In this regard, GaN offers an additional advantage due to its relatively high thermal conductivity (compared to Si). A major disadvantage of sapphire substrates that are widely used for epitaxial growth is the poor thermal conductivity (0.5W/cm-K) which limits high current conduction. Recent studies have shown that high quality, low dislocation density, bulk GaN substrates have a relatively high thermal conductivity of 2.3W/cm-K. Furthermore, it is expected that the bulk GaN substrate allows vertical device geometries with a full backside ohmic contact for much higher current conduction compared to lateral rectifiers fabricated on insulating substrates.