Threshold voltage and transconductance of fully depleted thin-film SOI MOSFETs

Summary form only given. In lightly doped or near-intrinsic thin-film SOI MOSFETs, the Fermi potential φ_B is close to zero. If threshold is defined by extrapolation to zero current from the linear region, then at the threshold condition the concentration of free carriers close to the surface is not zero, and may be greater than that of impurity charges in these films. In this case simulation shows that the front-surface potential Ψ_sf, which is the band bending from a hypothetical neutral film body to the front surface, is substantially greater than 2 φ_B. Therefore, the definition of threshold condition Ψ_sf=2 φ_B as the onset of strong inversion is not consistent with the experimental technique of determining threshold by extrapolation. The simulations indicate that critical surface-potential bending of φ_B+0.35 V is required to reach the threshold condition at zero back-gate bias when φ_B is less than 0.35 V. This critical surface-potential bending is found to be a weak function of back-gate bias. The threshold voltage and surface potentials of fully depleted SOI MOSFETs at the threshold condition are not significantly affected by the presence of inversion-layer charge even if it is much greater than the impurity charge as in the case of lightly doped Si films. The front-gate linear transconductance is relatively insensitive to the back-gate device parameters even though the front-gate threshold voltage is dependent on them. Simulations show that the transconductance remains nearly constant up to about 10¹⁵ cm^-3 and then falls off rapidly with increasing doping concentration as a result of mobility degradation. The transconductance is independent of Si film thickness if the mobility effect is not significant and the source/drain resistance is not high enough to become the limiting factor