Critical discussion of the front-back gate coupling effect on the low-frequency noise in fully depleted SOI MOSFETs

The low-frequency (LF) noise in fully depleted (FD) silicon-on-insulator (SOI) MOSFETs is investigated both theoretically and experimentally, with special emphasis on the front-back gate coupling effect. The impact of different technological parameters on the 1/f noise coupling factor has been investigated, within the frame of a number fluctuations model, ascribing the 1/f noise to traps in the front and buried oxide. It is shown that the coupling effect significantly increases for a smaller film thickness, while the influence of the buried-oxide thickness is less pronounced. The resulting 1/f noise is strongly determined by the ratio of the back and front oxide trap densities and is expected to be higher for the back interface in depletion compared with accumulation. These trends have been verified for FD SOI MOSFETs fabricated in a 0.1-μm CMOS technology and measured in linear operation. Except for long-channel transistors and for a front-gate bias <1 V in absolute value, a higher noise is usually found for the back-gate bias in accumulation. It is shown that in the latter case, additional noise sources become active, which drastically enhance the LF noise. Finally, the origin and back-gate bias dependence of these excess noise sources will be critically discussed.