5-6-4-1-1 SiC Schottky Power Rectifiers.
4H-SiC power Schottky diodes (with rated blocking voltages up to 1200 V and rated on-state currents up to 20 A as of this writing) are now commercially available . The basic structure of these unipolar diodes is a patterned metal Schottky anode contact residing on top of a relatively thin (roughly of the order of 10 μm in thickness) lightly n-doped homoepitaxial layer grown on a much thicker (around 200–300 μm) low-resistivity n-type 4H-SiC substrate (8° off axis, as discussed in Section 18.104.22.168) with backside cathode contact metallization . Guard ring structures (usually p-type implants) are usually employed to minimize electric field crowding effects around the edges of the anode contact. Die passivation and packaging help prevent arcing/surface flashover harmful to reliable device operation.
The primary application of these devices to date has been switched-mode power supplies, where (consistent with the discussion in Section 5.3.2) the SiC Schottky rectifier’s faster switching with less power loss has enabled higher frequency operation and shrinking of capacitors, inductors and the overall power supply size and weight . In particular, the effective absence of minority carrier charge storage enables the unipolar SiC Schottky devices to turn off much faster than the silicon rectifiers (which must be pn junction diodes above ~200 V blocking) which must dissipate injected minority carrier charge energy when turned off. Even though the part cost of SiC rectifiers has been higher than competing silicon rectifiers, an overall lower power supply system cost with useful performance benefits is nevertheless achieved. It should be noted, however, that changes in circuit design are sometimes necessary to best enhance circuit capabilities with acceptable reliability when replacing silicon with SiC components.
As discussed in Section 5.4.5, SiC material quality presently limits the current and voltage ratings of SiC Schottky diodes. Under high forward bias, Schottky diode current conduction is primarily limited by the series resistance of the lightly doped blocking layer. The fact that this series resistance increases with temperature (owing to decreased epilayer carrier mobility) drives equilization of high forward currents through each diode when multiple Schottky diodes are paralleled to handle higher on-state current ratings .