Optimum Design of a SiC Schottky Barrier Diode Considering Reverse Leakage Current due to a Tunneling Process

The optimization of the Schottky barrier height (SBH) and the maximum electric fields at the interface of a 4H-SiC Schottky barrier diode (SBD) is discussed, considering the reverse leakage current due to tunneling process. We first show that the reverse characteristics of a Ti/4H-SiC SBD are well described by the tunneling theory. Based on the tunneling theory, we show that the maximum electric field decreases as the Schottky barrier height decreases, and becomes smaller than the avalanche breakdown field of SiC. The on-state voltage as a function of the SBH is calculated, considering the specification of a leakage current. The calculated results show that the optimum SBH for a 4H-SiC SBD of 600V class is 0.9eV and that of 2000V class is 1.2eV. Introduction An SBD on 4H-SiC has outstanding features, namely higher speed and lower loss than a conventional SBD on silicon. A SiC SBD is designed so that the electric field at the Schottky interface is about ten times as large as that of a Si SBD, when the reverse bias is applied, because the breakdown electric field of SiC is about ten times as large as that of Si. This design principle of a SiC SBD is the origin of the high performance. However, the high electric fields at the Schottky interface of a SiC SBD bring about the increase of the reverse leakage current. In fact, the measured reverse leakage current of a SiC SBD is much higher than the reverse leakage current calculated by the classical Schottky theory. Thus, the effects of high electric fields at the Schottky interface on the reverse leakage current of SBDs have attracted much attention. Crofton and Sriram pointed out that the reverse leakage current of a SiC SBD is mainly due to tunneling process induced by the high electric field at the Schottky interface [1]. Treu and his co-workers investigated the reverse characteristics of 4H-SiC SBDs, and concluded that they are described by the thermionic field emission model [2], which is the approximation of tunneling current derived by Padvani and Stratton [3]. The present authors identified dominant mechanisms of the reverse leakage current as a function of the electric field and the SBH. We derived the compact and closed analytical expression of the reverse leakage current of a SiC SBD [4]. As the reverse leakage current due to tunneling process depends mainly on the SBH and the electric fields at the interface, it is important to optimize the SBH and the maximum electric fields at the interface for the design of a SiC SBD. In this work, we first present the reverse characteristics of Ti/4H-SiC SBDs and show that they are well described by the tunneling theory. Then we discuss the optimization the SBH and the maximum electric fields at the interface, considering the reverse leakage current due to tunneling process. Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 831-834 doi:10.4028/www.scientific. net/MSF.433-436.831 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-12/03/20,10:57:04) 2 Title of Publication (to be inserted by the publisher) Reverse Characteristics of a Ti/SiC Schottky Barrier Diode Figure 1 shows the typical forward and reverse characteristics of a Ti/4H-SiC SBD. The breakdown voltage is 1200V and the typical on-state resistance is 2. 5mΩcm. The SBDs used in this study were fabricated on 4H-SiC epitaxial wafers, in the case that an n-type 10um drift layer was grown on an n-type substrate supplied by Cree Research. The net donor concentration of the drift layer is about 15 10 5× cm. Junction termination extension by aluminum implantation was used as the edge termination of the diodes. Figure 2 shows the reverse characteristics of some SBDs at room temperature, where the diameter of SBDs is 300μm. The reverse characteristics of SBDs coincide well except for the accidental sudden increase of leakage current, which may be due to a crystal defect. The good reproducibility of the reverse characteristics of SBDs implies that the reverse leakage current originates from the intrinsic nature of Schottky interface. Figure 3 shows a comparison of the measured reverse leakage current and the theoretical model considering the tunneling process at the Schottky interface with the temperature as a parameter. It can be seen that the theoretical calculations well describe the exponential increase of the reverse leakage current characteristic of the SiC SBD. The reverse leakage current model used in the fitting of the experimental results in Fig. 3 is the compact expression of thermionic field emission current TFE J :