Wrap around field plate technique for GaN Schottky barrier diodes
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Wrap around field plate technique for GaN Schottky barrier diodes Sowmya Kolli1, Robert Hickman1, 2 and Bruce W Alphenaar1 1. Department of Electrical and Computer Engineering, University of Louisville, Louisville, KY 40292 2. APIQ Semiconductor LLC, Louisville, KY 40217-7719 Abstract In this paper, we propose a wrap around field termination technique for a vertical Schottky barrier diode fabricated on a free standing GaN substrate. Unlike conventional field plate designs, in the wrap around structure the field plate surrounds the active device area. This allows for better control of the electric field distribution, and reduces field-crowding. 2D finite element simulations using ATLAS show a uniform field distribution across the device. Calculations show that the Ron increases relative to the conventional field plate. A break down voltage of 1300V was predicted for a 5um thick epilayer. Introduction Gallium nitride (GaN) has enormous potential for use in devices operating at high power, frequency and temperature. Its wide band gap (WBG), high critical electric field and favorable carrier properties lead to lower switching losses and conduction losses in power electronic devices. This makes GaN a potential substitute for Si transistor switches and diodes. However most GaN devices reported to date exhibit an ON-resistance (RON) versus breakdown voltage (VBR) much below theoretical predictions. Heteroepitaxial growth of GaN on substrates such as SiC, Si[1], and sapphire[2] suffer from a high density of threading dislocations defects due to the mismatch in lattice constants and thermal expansion coefficients. This introduces mid-gap states that effectively trap the charges and decrease the device efficiency. Vertical devices [3, 4] in which a bulk GaN substrate is used have much lower defect densities. However, field crowding at the periphery of the rectifying contact remains a problem and results in avalanche break down at much lower voltages than the theoretical maximum. In ideal diodes structure, the electric field is uniformly distributed along the lateral direction and there are no fringing field issues. However, for a real diode with finite size Schottky and ohmic contacts as shown in figure 1, the electric field is non-uniform. It is concentrated at the schottky contact edges. The goal of edge termination techniques is to achieve an electric field distribution as similar to the ideal case.
(ii)
Si3N4
Schottky
Si3N4
N GaN epi layer N+ GaN buffer layer N+ GaN bulk substrate Ohmic contact
Figure 1: Practical diode with finite length contacts A number of edge modification techniques[5] have been used, including field plates[2], damage implants[6] , guard rings[7], and different combinations of these strategies[8]. (As shown in Figure 2). In a field plate structure the Schottky contact is extended over the dielectric. The extended contact acts like a free standing metal-insulator semiconductor (MIS) electrode and distributes the field away from the contact periphery along the epi layer surface. Damage implant t
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