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Dive into the research topics where Jo Das is active.

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Featured researches published by Jo Das.


IEEE Electron Device Letters | 2011

Record Breakdown Voltage (2200 V) of GaN DHFETs on Si With 2-

Puneet Srivastava; Jo Das; Domenica Visalli; Marleen Van Hove; Pawel E. Malinowski; Denis Marcon; Silvia Lenci; Karen Geens; Kai Cheng; Maarten Leys; Stefaan Decoutere; Robert Mertens; Gustaaf Borghs

In this letter, we present a local substrate removal technology (under the source-to-drain region), reminiscent of through-silicon vias and report on the highest ever achieved breakdown voltage (V<sub>BD</sub>) of AlGaN/GaN/AlGaN double heterostructure FETs on a Si (111) substrate with only 2-μm-thick AlGaN buffer. Before local Si removal, V<sub>BD</sub> saturates at ~700 V at a gate-drain distance (L<sub>GD</sub>) ≥ 8 μm. However, after etching away the substrate locally, we measure a record V<sub>BD</sub> of 2200 V for the devices with L<sub>GD</sub> = 20 μm. Moreover, from Hall measurements, we conclude that the local substrate removal integration approach has no impact on the 2-D electron gas channel properties.


IEEE Electron Device Letters | 2010

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Puneet Srivastava; Jo Das; Domenica Visalli; Joff Derluyn; Marleen Van Hove; Pawel E. Malinowski; Denis Marcon; Karen Geens; Kai Cheng; Stefan Degroote; Maarten Leys; Marianne Germain; Stefaan Decoutere; Robert Mertens; Gustaaf Borghs

In this letter, we present a novel approach to enhance the breakdown voltage (<i>V</i><sub>BD</sub>) for AlGaN/GaN/AlGaN double-heterostructure FETs (DHFETs), grown by metal-organic chemical vapor deposition on Si (111) substrates through a silicon-substrate-removal and a layer-transfer process. Before removing the Si substrate, both buffer isolation test structures and DHFET devices showed a saturation of <i>V</i><sub>BD</sub> due to the electrical breakdown through the Si substrate. We observed a <i>V</i><sub>BD</sub> saturation of 500 V for isolation gaps larger than 6 μm . After Si removal, we measured a <i>V</i><sub>BD</sub> enhancement of the AlGaN buffer to 1100 V for buffer isolation structures with an isolation gap of 12 μm. The DHFET devices with a gate-drain (<i>L</i><sub>GD</sub>) distance of 15 μm have a V<sub>BD</sub> > 1100 V compared with ~300 V for devices with Si substrate. Moreover, from Hall measurements, we conclude that the substrate-removal and layer-transfer processes have no impact on the 2-D electron gas channel properties.


IEEE Transactions on Electron Devices | 2006

Buffer Thickness by Local Substrate Removal

Jo Das; Herman Oprins; Hangfeng Ji; Andrei Sarua; Wouter Ruythooren; Joff Derluyn; Martin Kuball; Marianne Germain; Gustaaf Borghs

AlGaN/GaN high electron mobility transistors (HEMT) on sapphire substrates have been studied for their potential application in RF power applications; however, the low thermal conductivity of the sapphire substrate is a major drawback. Aiming at RF system-in-a-package, the authors propose a flip-chip-integration approach, where the generated heat is dissipated to an AlN carrier substrate. Different flip-chip-bump designs are compared, using thermal simulations, electrical measurements, micro-Raman spectroscopy, and infrared thermography. The authors show that a novel bump design, where bumps are placed directly onto both source and drain ohmic contacts, improves the thermal performance of the HEMT


Japanese Journal of Applied Physics | 2003

Silicon Substrate Removal of GaN DHFETs for Enhanced (<1100 V) Breakdown Voltage

Pol Van Dorpe; Vasyl Motsnyi; Mayke Nijboer; Etienne Goovaerts; Viacheslav I. Safarov; Jo Das; Wim Van Roy; Gustaaf Borghs; Jo De Boeck

We demonstrate highly efficient spin injection at low and room temperature in an AlGaAs/GaAs semiconductor heterostructure from a CoFe/AlOX tunnel spin injector. We use a double-step oxide deposition for the fabrication of a pinhole-free AlOX tunnel barrier. The measurements of the circular polarization of the electroluminescence in the Oblique Hanle Effect geometry reveal injected spin polarizations of at least 24% at 80 K and 12% at room temperature.


energy conversion congress and exposition | 2010

Improved Thermal Performance of AlGaN/GaN HEMTs by an Optimized Flip-Chip Design

Jordi Everts; Jo Das; Jeroen Van den Keybus; Jan Genoe; Marianne Germain; Johan Driesen

A boost converter was constructed using a high voltage enhancement mode (E-mode) AlGaN/GaN/AlGaN DHFET transistor grown on Si<111>. The very low dynamic on-resistance (Rdyn ≈ 0.23 Ω) and very low gate-charges (e.g. Qgate ≈ 15 nC at VDS = 200 V) result in minor transistor losses. Together with a proper design of the passive components and the use of SiC diodes, very high overall efficiencies are reached. Measurements show high conversion efficiencies of 96.1% (Pout = 106 W, 76 to 142 V at 512.5 kHz) and 93.9% (Pout = 97.5 W, 78 to 142 V at 845.2 kHz). These are, to our knowledge, the highest efficiencies reported for an enhancement mode GaN DHFET on Si in this frequency range. The transistor switching losses are concentrated in the turn-on interval, and dominate at high frequencies. This is due to a limited positive gate-voltage swing, as the gate-source diode restricts the positive drive voltage.


IEEE Transactions on Electron Devices | 2006

Highly efficient room temperature spin injection in a metal-insulator-semiconductor light-emitting diode

Hangfeng Ji; Martin Kuball; Andrei Sarua; Jo Das; Wouter Ruythooren; Marianne Germain; Gustaaf Borghs

The authors demonstrate the potential of confocal micro-Raman spectroscopy to enable three-dimensional (3-D) thermal analysis of solid state devices. This is illustrated on a flip-chip mounted AlGaN/GaN heterostructure field-effect transistor. To better understand its heat dissipation and for device optimization, it is desirable to know temperature distribution not only in the active device area, but also in the bulk substrate. This cannot be achieved using traditional thermal imaging techniques. 3-D thermal imaging was demonstrated by probing the temperature dependent Raman shift of phonons at different depths within the bulk substrate using confocal micro-Raman spectroscopy. The heatsinking through the metal bumps connecting the active device area to the flip-chip carrier is illustrated. Experimental temperature results are in reasonably good agreement with 3-D finite difference simulations


IEEE Transactions on Electron Devices | 2013

A high-efficiency, high-frequency boost converter using enhancement mode GaN DHFETs on silicon

Puneet Srivastava; Jo Das; Robert Mertens; Gustaaf Borghs

Low-cost GaN-on-Si-based transistors are targeted to function at high ambient temperatures. With this perspective, it is aimed to evaluate the high-temperature (HT) capabilities of GaN-on-Si double-heterostructure field-effect transistors. It is highlighted that HT device operation degrades both ON and OFF states that are directly related to the increase in the on-resistance and the decrease in device breakdown voltage; 2-DEG mobility drops with increasing temperature and is responsible for ON-state degradation. Regarding the OFF-state operation, it is observed that at low-voltage operation and with increasing temperature, there is an increase in the OFF-state leakage current because of thermal-assisted electrical conduction across the III-N layers and various interfaces. The main breakdown limiting mechanism at any temperature is, however, buffer leakage along the AlN/Si interface. Because this parasitic conduction, a negative temperature coefficient of breakdown voltage of approximately -1 V/°C is observed. For devices after Si removal, the leakage across the AlN/Si interface is interrupted and therefore HT OFF-state characteristics show high potential to be used at high operating voltage. A breakdown voltage as high as ~1800V is observed after Si removal compared with ~500 V with Si at 150°C.


IEEE Electron Device Letters | 2009

Three-dimensional thermal analysis of a flip-chip mounted AlGaN/GaN HFET using confocal micro-Raman spectroscopy

Pawel E. Malinowski; Joachim John; Jean Yves Duboz; Geert Hellings; Anne Lorenz; J.G.R. Madrid; C. Sturdevant; Kai Cheng; Maarten Leys; Joff Derluyn; Jo Das; Marianne Germain; Kyriaki Minoglou; P. De Moor; E. Frayssinet; F. Semond; J.-F. Hochedez; B. Giordanengo; Robert Mertens

In this letter, we report on the fabrication of near-ultraviolet photodetectors based on gallium nitride (GaN) layers grown on a Si(111) substrate. Optoelectronic characterization was performed using front-side and backside illumination, the latter possible by locally etching the Si substrate under the detectors using reactive ion etching. The dark current after removal of the Si substrate decreased by two orders of magnitude to around 20 fA at -1 V for a 300-mum-diameter Schottky photodiode. Responsivity at the cutoff wavelength (370 nm) was equal to 35 mA/W for the backside illumination. Detection at smaller wavelengths was not possible due to a nonoptimized layer stack. These first results do however illustrate the potential of backside-illuminated GaN-on-Si Schottky photodiodes in 2-D UV imagers.


Japanese Journal of Applied Physics | 2010

Silicon Substrate Engineered High-Voltage High-Temperature GaN-DHFETs

Denis Marcon; Marleen Van Hove; Domenica Visalli; Joff Derluyn; Jo Das; Farid Medjdoub; Stefan Degroote; Maarten Leys; Kai Cheng; Robert Mertens; Marianne Germain; Gustaaf Borghs

AlGaN/GaN high electron mobility transistors (HEMTs) were electrically stressed in pinch-off condition at a drain voltage up to 200 V for 200 h at an ambient temperature of 200 °C. The tested transistors which were grown and processed on 4-in. silicon substrate showed negligible degradation. This proves that a combination of a high quality AlGaN/GaN/AlGaN double heterostructure, the in-situ Si3N4 deposition technique and an accurately optimized gate technology result in excellent device stability under harsh conditions.


international reliability physics symposium | 2010

Backside-Illuminated GaN-on-Si Schottky Photodiodes for UV Radiation Detection

Denis Marcon; Farid Medjdoub; Domenica Visalli; Marleen Van Hove; Joff Derluyn; Jo Das; Stefan Degroote; Maarten Leys; Kai Cheng; Stefaan Decoutere; Robert Mertens; Marianne Germain; Gustaaf Borghs

In this work the stability of Gallium Nitride based high electron mobility transistors grown on 4-in Si substrate (GaN-on-Si HEMTs) were tested both in off-state at high drain voltage (200 V) and in on-state at large gate voltage (+2 V) with low drain bias (5 V). In each stress experiment the ambient temperature was fixed at 200°C. Remarkably, despite the considerably large drain voltage used in the off-state stress on only 5 μm gate-drain spaced transistors, negligible signs of degradation were observed after more than 350 hours of testing. Similar results were obtained after the on-state stress. In fact, only small degradation signs were reported in spite of the large gate current and high junction temperature the devices have to withstand during the on-state stress. These results show the robustness of these devices to operate under high electric field conditions, high temperature and to withstand also a large gate current for considerable time.

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Marianne Germain

Katholieke Universiteit Leuven

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Gustaaf Borghs

Katholieke Universiteit Leuven

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Kai Cheng

Katholieke Universiteit Leuven

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Robert Mertens

Katholieke Universiteit Leuven

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Joff Derluyn

Katholieke Universiteit Leuven

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Denis Marcon

Katholieke Universiteit Leuven

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Stefan Degroote

Katholieke Universiteit Leuven

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Marleen Van Hove

Katholieke Universiteit Leuven

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Domenica Visalli

Katholieke Universiteit Leuven

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