Revision of interface coupling in ultra-thin body SOI MOSFETs
(2012) International Conference “Micro- and Nanoelectronics-2012” (ICMNE-2012) — Location: Moscow-Zvenigorod (Russia) (1.October.2012)
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- The fully depleted (FD) ultra-thin-body (UTB) SOI MOSFET with low-doped or undoped channel is presently considered as one of the best candidates for nano-scaled CMOS technologies [1]. The effect of the charge coupling between the front and back SOI interfaces is a fundamental property of any FD SOI MOSFET, which determines its characteristics [2]. The interface coupling is also widely used for characterization purposes. Besides, the interface coupling lies at the basis of the back-gate controlled schemes. Thus an adequate understanding of the behavior of interface coupling in UTB SOI MOSFETs is of crucial importance. The main manifestation of the interface coupling in a FD SOI MOSFET is the modulation of the threshold voltage at one gate by the opposite gate bias, which is usually described by the classical Lim-Fossum model [2]. According to this model, the threshold voltage in a FD SOI MOSFET varies linearly on the back-gate gate as long as the back silicon film interface is depleted and saturates with strong accumulation or inversion at the back interface. In accordance to the Lim-Fossum model, the slope of the linear region of the coupling curves is determined by the structure capacitances, and thus it is frequently used for the evaluation of the silicon film thickness. Nonetheless, several publications reported deviations from the Lim-Fossum model in the case of UTB SOI MOSFETs, in particular, the absence of saturation in coupling curves [3]. In this work, we revise the classical interface coupling model [2], using 1-D numerical simulations in both classical and quantum-mechanical modes and their comparison with classical model and experimental data. It is demonstrated that the behavior of interface coupling in UTB SOI MOSFETs has two distinctions from the classical Lim-Fossum model. The first is an increased threshold voltage shift with opposite gate bias (i.e., an increased slope of the coupling curves), which originates mainly from quantum-mechanical (QM) effects. This QM effect is caused by the variation of the electric field in the Si film, and thus variation of the electrical confinement at threshold. It is well-marked even in rather thick-film (>10 nm) SOI devices, whose threshold voltage is usually considered to be unaffected by quantization effects [1]-[3], though it is stronger for thinner films. The second is significantly wider linear region of the coupling curve compared to the Lim-Fossum model. It is demonstrated that there are three factors responsible for extending of the linear region of the coupling curves. The first consists in the fact that the surface potential at threshold conditions in thin-film moderately or low-doped SOI MOSFETs significantly exceeds 2ϕF (where ϕF is the Fermi potential in the semiconductor body), and, in addition, it increases with opposite gate bias. The second (coupled to the first) is that the accumulation surface potential needed for stabilization of the potential and carrier distributions in the silicon film (which means interface de-coupling and saturation of the coupling curve) significantly differs from the conventionally assumed 0 V. The third factor contributing to the extended range of the linear region of coupling curves in UTB SOI MOS is related to QM effects. These findings should be taken into account in the analysis, characterization and modeling of advanced UTB SOI MOSFETs. Taking into consideration the foregoing findings, a revised analytical model of interface coupling valid for UTB SOI MOSFETs is derived. The presented model is validated by numerical simulations and experimental results.
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Rudenko, T., Nazarov, A., Kilchytska, V., & Flandre, D. (2012). Revision of interface coupling in ultra-thin body SOI MOSFETs. Book of Abstracts of the International Conference “Micro- and Nanoelectronics-2012” (ICMNE-2012), 1. https://hdl.handle.net/2078.5/252926