MOS field effect transistors formed by gate masked ion implantation

MOS enhancement mode field effect transistors with a circular geometry and with drains offset from the gate by distances from 0.1 mil to 0.9 mil were implanted with boron ions to fill in the offset region and thus achieve perfect alignment (i.e., no overlap) between gate and drain. The energies used were 50 to 100 keV and a 4000 Å-thick aluminum gate acted as a mask to prevent ions from penetrating into the channel region. The best junctions were obtained with 100-keV ions, with the sheet resistances being typically 4000 ω/□ for the implanted region. This additional drain resistance was quite small compared to the channel resistance of the devices and so was not objectionable. Ordinary diffused MOSFET's were included on the same wafers for comparison with the ion implanted MOSFET's. It was found that the differences in noise, leakage, and drain breakdown voltage were not serious. The chief advantage of the ion implanted MOSFET is the extremely low feedback capacitance due to the lack of gate-drain overlap, but this advantage is difficult to exploit in a conventional package because of the package capacitance. However, a significant difference was noted in switching characteristics between diffused and ion implanted MOSFET's mounted on TO-18 headers.