Designing bulk-driven MOSFETs for ultra-low-voltage analogue applications

Abstract

This paper presents the design of an n-type bulk-driven MOSFET which is intended to facilitate the scaling of analogue circuitry down to 0.7 V, the minimum supply voltage predicted for the end of bulk CMOS. The bulk-driven MOSFET design will be carried out in two parts using the design rules of a standard 90 nm bulk CMOS technology. First, the impact of gate oxide scaling will be investigated to see how the bulk transconductance behaves as the oxide thickness is varied. Results will indicate that the oxide scaling requirements of a MOSFET can be relaxed by 0.4 nm when the bulk is used as the input terminal rather than the gate. Second, by taking advantage of a larger oxide thickness, it will be shown that a delta-doped profile is capable of improving the intrinsic gain and cut-off frequency of a bulk-driven MOSFET by as much as 429% and 71%, respectively, when compared to a uniformly doped bulk-driven device consistent with the specifications of a 90 nm bulk CMOS process. Overall, the delta-doped bulk-driven MOSFET will exhibit a long-channel bulk-to-gate transconductance ratio equal to 0.51, compared to 0.27 in the uniformly doped device (the gate transconductance was the same in both devices). The new delta-doped design will also increase the bulk-driven-to-gate-driven cut-off frequency ratio to 0.095–0.492 for channel lengths ranging from 80–800 nm, which is a 16–34% improvement over the uniformly doped case. When used in a differential amplifier circuit, the delta-doped bulk-driven MOSFET was found to have a dc voltage gain 185% higher than that of a similar amplifier utilizing uniformly doped devices.