The Response of High Voltage 4H-SiC P-N Junction Diodes to Different Edge Termination Techniques
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intrinsic conduction effects. The high breakdown electric field allows higher avalanche breakdown and much thinner drift regions with lower on low resistances. The high thermal conductivity enables operation at very high power levels with good heat dissipation. The high saturation drift velocity allows operation at RF and microwave frequencies. The advantages of wide bandgap semiconductors like SiC are clearly illustrated by p-n junction diodes [4,5]. In power device circuits, a number of operations such as switching, amplification and rectification involve the use of p-n junction diodes and breakdown characteristics are regarded as some of the most important design parameters for power devices. Among the factors that affect the breakdown voltage are the purity, doping level and thickness of the substrate and epilayers, and the termination at the edge of the device. Crystal growth defects such as micropipes, stacking faults, deep level centers and incompletely ionized dopants are believed to contribute to unstable and premature breakdown of power devices [6,7]. In order to overcome this drawback and provide high quality material, improvements in the crystal growth are needed. Edge termination is crucial to high voltage devices due to electric field enhancement which leads to premature breakdown [8,9]. An optimum edge termination will minimize this field enhancement. The use of mesa isolation for edge termination has been shown to yield nearly ideal breakdown voltage for SiC non-planar p-n junction diodes [10]. A combination of edge termination by mesa isolation and thick n- epilayers has yielded high breakdown voltages [11,12]. Edge termination using 30 keV and 750 keV Ar÷ ion implantation has also been reported [9, 13101 Mat. Res. Soc. Symp. Proc. Vol. 512 01998 Materials Research Society
16]. Implantation is believed to create a highly resistive region around the edges of contacts on a device [9,17]. When reverse bias is applied, this resistive region promotes spreading of the electric potential which reduces the edge electric field and thus improves the breakdown voltage. Results from several studies have shown that there is improvement in the breakdown voltages following implantation edge termination. In most of these cases, the reverse leakage currents have remained high, and the forward voltage characteristics have not differed from those of non-implanted diodes. In this study, we have sought to compare the breakdown characteristics of p'-n diodes with different types of edge termination. In particular, we have looked at the effects of using MeV ion implantation. A limitation on the energy is the provision for a protective layer over the diodes during implantation. Conventional sputter-deposition with lift-off offers only a limited thickness of metal for protection. We will present the method we have used to deposit a thick metal protection for the diodes implanted. EXPERIMENT The substrates used in this study were 5 mm x 5 mm pieces of n+ 4H-SiC (Nd = 5 x 101' cm"3) obtained from Cree Research Inc. with th
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