Fabrication and device characteristics of bulk GaN-based Schottky diodes
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Fabrication and device characteristics of bulk GaN-based Schottky diodes Yi Zhou, Dake Wang, Claude Ahyi, Chin-Che Tin, John Williams and Minseo Park* Department of Physics, Auburn University, Auburn, AL 36849 N. Mark Williams and Andrew Hanser Kyma Technologies, Inc., 8829 Midway West Road, Raleigh, NC 27617 *Corresponding author: [email protected] Abstract In this investigation, Schottky diodes with different device sizes (150μm, 420μm and 700μm) were fabricated on the Ga-face of a free-standing n--GaN wafer produced by Kyma Technologies, Inc. Full area back side ohmic contact was prepared on the N-face of the bulk GaN using Ti/Al. Without any edge-termination scheme, a relatively high reverse breakdown voltage of 240V was achieved. The reverse breakdown voltage decreases as the device size increases. The forward turn-on voltage was as low as 2.4V at room temperature for the 150μm diameter Schottky diodes. The best on-state resistance was 7.56 mΩcm2 for diodes with VB=240V, producing a figure-of-merit (VB2/RON) of 7.6 MWcm-2. The Schottky diode also showed an extremely short reverse recovery time (< 20 ns) switching from forward bias to reverse bias. Introduction Gallium nitride (GaN) has been considered as an important material for high power electronics because of its wide band gap (3.4eV) [1], high breakdown field (>5 MV/cm) [1], high electron saturation velocity (~3×107 cm/s) [2] and reasonable thermal conductivity (1.3 W/cm K) [3]. Due to the lack of availability of GaN bulk substrate, GaN-based Schottky diodes have been fabricated using GaN films epitaxially grown on foreign substrates, such as sapphire. Even though high reverse breakdown voltage has been achieved in lateral Schottky rectifiers on foreign substrates [4-11], their application is limited due to some inevitable drawbacks such as limited current density, high forward turn on voltage and poor thermal conductivity of the substrate [12]. These drawbacks point to the need for vertical geometry rectifiers, preferably fabricated on low defect density GaN substrate [1]. With bulk GaN substrate, vertically depleting schottky diodes with schottky contacts on the front side and ohmic contacts on the back side can be fabricated, which harnesses the dislocation-blind carrier transport. However, relatively little work has been performed on fabrication of Schottky diode that utilizes bulk GaN wafer [12-14]. In this paper, we report the device characteristics of a vertically depleting GaN schottky rectifier fabricated directly on the bulk GaN substrate with no edge termination scheme. The device showed a relatively high reverse breakdown voltage of 240V for 150μm diameter diodes. The reverse breakdown voltage was found to decrease as the size of device increases. Our devices also showed good characteristics in forward bias current transport, with low turn-on
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voltage and on-state resistance. The reverse recovery time was found to be less than 20 ns, which was the shortest among the values ever reported for GaN schottky re
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