Narrow-channel accumulated-body bulk Si MOSFETs with wide-range dynamic threshold voltage tuning

Side-gated bulk Si nMOSFETs with Si₃N₄ shallow trench isolation (STI) have been previously demonstrated to have significantly reduced off-currents and improved subthreshold characteristics [1, 2]. The improvement is shown to be due to accumulation of the Si body with the holes as the polysilicon side-gate surrounding the body as a guard ring is negatively biased (Fig 1). The threshold voltage (V_T) of the narrow channel devices can be dynamically controlled by the side-gate (Fig 2) voltage (V_side) in a wide range [2, 3], mainly due to the increase in the channel energy barrier (Fig. 3) [4]. Here, we report experimental results on narrow bulk Si accumulated body n-channel FETs with SiO₂ side-gate dielectric and STI and p-type side-gates (Fig 2). The fabrication is compatible with established front and back end-of-line processes with only an added side-gate formation and side-gate contact step over conventional FET fabrication. 9 nm thermal SiO₂ serves as the side-gate dielectric and 3.6 nm thermal SiO₂ is used as gate dielectric. Final body doping is estimated to be at 1 x 10¹⁷ cm^-3 (Boron). Gate, side-gate, source and drain have high n+ doping (~1 x 10²⁰ cm^-3).