| Silicon carbide (SiC) vertical junction field-effect transistors (VJFET) are investigated as a candidate of high-power unipolar switching devices, which can replace the current Si devices with greatly improved power handling capability. Two types of 4H-SiC VJFETs are fabricated, i.e., one with the embedded blocking junction and internal lateral-JFET gates and the other with the vertical MET gates created with deep trenches. In demonstrating both types of the VJFETs, emphasis is placed on obtaining normally-OFF devices in order to provide fail-safe devices, which can substitute the metal-oxide-semiconductor (MOS) FETs.; Ion-implantation is vigorously used to realize the complicated two- and three-dimensional doping structures required in the 4H-SiC VJFET fabrication. In particular, the embedded blocking junction in the first type of VJFET is created by deep Al implantation at the high energies of 1.8 and 2.8 MeV. A special implantation-masking technique using electron-beam curing of an 8-μm thick photoresist layer is developed to pattern the deep implantation. On the other hand, the vertical MET gates in the second type are created by tilted Al implantation into the sidewalls of the 3.2-μm deep trenches. Furthermore, a self-alignment process based on planarization and etch-back of photoresist is developed to take advantage of the highly corrugated surface of the second-type VJFET. In addition, to obtain normally-OFF devices in each type, the gate-channel thickness is carefully adjusted so that the channel is pinched off by the built-in potential.; The fabricated devices demonstrate normally-OFF operations of switching at 2,510 and 1,720 V in the first and second types of VJFETs, respectively. Very low specific ON-resistances of 3.0–3.6 mΩcm2 are obtained in the second type, which surpass the ultimate level of Si by two orders of magnitude, clearly demonstrating the advantage of SiC as a wide-bandgap semiconductor. On the other hand, the ON-state currents of the first-type VJFET are found low because of the low channel mobility in the internal lateral-JFET gates. Residual damages are suspected in the gate-channel layer over the embedded implantation. |
