SiC planar MOS-Schottky diode: a high voltage Schottky diode with low leakage current

doi:10.1016/S0038-1101(01)00145-9

Solid-State Electronics

Volume 45, Issue 7, July 2001, Pages 1085-1089

https://doi.org/10.1016/S0038-1101(01)00145-9 Get rights and content

Abstract

A new diode structure, called planar MOS-Schottky Diode (MOSSD), is proposed to reduce the reverse leakage current in SiC Schottky diode. The reverse leakage current has been reduced by one order of magnitude by using the MOSSD structure while the reverse breakdown characteristic is improved from soft breakdown to abrupt breakdown. Further, measurements on MOSSDs with thick and thin oxide MOS regions indicate that the forward current through a MOSSD with a thin oxide can be up to 90% that through a conventional Schottky diode without buried MOS structures. From high temperature measurements it is evident that the reverse characteristics of MOSSDs have less temperature dependence compared to pure Schottky diodes.

Introduction

Improved power Schottky diodes are of interest for applications in power systems due to their superior switching characteristics. The main problem for Schottky diodes is their fairly high reverse leakage current compared to pin diodes [1]. Schottky diodes used in high voltage applications tend to exhibit an increase in the leakage current with increasing the reverse bias voltage, resulting in soft breakdown characteristics; moreover, the leakage current increases significantly at high temperatures compared to that at room temperature. In 1995, Manoj and Baliga presented a novel silicon Schottky diode structure named Trench MOS Barrier Schottky (TMBS) diode with a reverse breakdown voltage up to three times higher than that of a conventional (non punch-through) Schottky diode at the same doping concentration [2]. They had demonstrated that due to the coupling between the charges in the drift region and the trench electrodes, the electric field at the Schottky interface was reduced, and hence there was an exponential decrease in the reverse leakage current through the reduction of Schottky barrier height lowering. While a modification to the TMBS in the form of a graded doped TMBS was later proposed to overcome the limitations of the TMBS due to increase in field crowding at the trench corners with increase in trench depth, the process complexity increased [3]. In this paper, we present a variation of the TMBS diode structure consisting of a large number of MOS and Schottky regions integrated together on the same plane without the need for complicated trench etching steps. This new diode structure named planar MOS Schottky Diode (MOSSD) exhibits significantly lower leakage currents and a high breakdown voltage compared to a conventional Schottky Diode with the same footprint area, without seriously sacrificing the forward current characteristics.

Section snippets

Experimental

The basic structure of a MOSSD is shown in Fig. 1. As seen from Fig. 1, the MOSSD is fabricated by oxidizing the SiC wafer and opening several hexagonal shaped windows (with a side length of 20 μm) in the oxide prior to sputter depositing and patterning Ni contacts of 600 μm diameter. While the Ni contact over the oxide regions formed the metal-oxide-semiconductor (MOS) structures, the Schottky regions were formed by the metal contacts on the semiconductor via the windows in the oxide. A blanket

Reverse bias leakage current

The presence of MOS regions interspersed with Schottky regions in a MOSSD results in significant reduction in the diode leakage current. From Fig. 2 it is clear that the reverse leakage current through the MOSSD is approximately an order of magnitude less than that through a conventional Schottky diode with the same footprint area fabricated on the same SiC wafer. The presence of MOS structure results in an effective reduction in the total area available for leakage current conduction or in

Conclusions

Planar MOS Schottky diodes have been demonstrated on SiC wafers with an order of magnitude lower reverse leakage currents than through a conventional Schottky diode. The reverse breakdown voltage of the MOSSDs has been observed to be more than two times than that of the conventional Schottky diode and high temperature measurements indicate that while the reverse hold off capability of conventional Schottky diodes suffers drastically with increase in temperature, the MOSSDs exhibit only slight

Acknowledgements

This research was supported from ONR, grant no. N00014-99-1-1099. We are grateful to Drs. C. Wood and M. Yoder for their interest and support of this research.

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