Aggressive device scaling has severely degraded the switching characteristics of CMOS transistors. This issue has led to the development of tunneling FETs (TFETs) as an alternative. TFETs, with their asymmetric doping of the source and drain regions, offer improved subthreshold swing (SS) compared to conventional MOSFETs. However, despite this advantage, TFETs still suffer from ambipolar current, which increases off-state current (IOFF). This paper introduces an approach to applying hetero gate dielectrics (HGDs) in nanosheet (NS) TFETs to reduce ambipolar current characteristics. The magnitude of the drain electric field is reduced by selectively forming a high-k dielectric near the source region This configuration allows the TFETs to avoid unintended band-to-band tunneling (BTBT) and suppress ambipolar current during the off-state.

This paper presented an analytical SS model to determine the subthreshold swing (SS) of an elliptic junctionless Gate- All-Around (GAA) FET using ferroelectric. Analyzing a GAA FET with an elliptic cross-section was essential because it is difficult to manufacture a perfectly circular GAA FET. The results of the proposed SS model agreed well with 2D numerical simulation. Using this analytical SS model, SS was analyzed for the eccentricity and the ratio (P_r/E_c) of permanent polarization P_r and coercive electric field E_c in an elliptic junctionless GAA FET with an MFMIS (Metal-Ferroelectric-Metal-Isulator- Semiconductor) structure using ferroelectric. As a result, the changing rate of the average surface potential due to the gate voltage increased and SS decreased as the eccentricity increased. It was found that the inner gate voltage amplified more than the outer gate voltage due to the ferroelectricity, better controlling the carriers in the channel, thereby reducing SS. As the P_r/E_c decreased, the changing rate of the ferroelectric charge for the outer gate voltage increased and SS decreased. As the eccentricity increased, the changing rate of SS for P_r/E_c decreased. There was no significant change in SS until the eccentricity was about 0.5. The SS began to decline above 0.5 due to the changes in ferroelectric charge, inner gate voltage, and average surface potential for the outer gate voltage.

The subthreshold swing (SS) of an asymmetric junctionless double gate (AJLDG) MOSFET is analyzed by the use of Gaussian function. In the asymmetric structure, the thickness of the top/bottom oxide film and the flat-band voltages of top gate (Vfbf) and bottom gate (Vfbb) could be made differently, so the change in the SS for these factors is analyzed with the projected range and standard projected deviation which are parameters for the Gaussian function. An analytical subthreshold swing model is presented from the Poisson’s equation, and it is shown that this model is in a good agreement with the numerical model. As a result, the SS changes linearly according to the geometric mean of the top and bottom oxide film thicknesses, and if the projected range is less than half of the silicon thickness, the SS decreases as the top gate oxide film is smaller. Conversely, if the projected range is bigger than a half of the silicon thickness, the SS decreases as the bottom gate oxide film is smaller. In addition, the SS decreases as Vfbb-Vfbf increases when the projected range is near the top gate, and the SS decreases as Vfbb-Vfbf decreases when the projected range is near the bottom gate. It is necessary that one should pay attention to the selection of the top/bottom oxide thickness and the gate metal in order to reduce the SS when designing an AJLDG MOSFET.

We present a subthreshold swing model for a symmetric junctionless double gate MOSFET. The scale length λ₁ required to obtain the potential distribution using the Poisson's equation is a criterion for analyzing the short channel effect by an analytical model. In general, if the channel length L_g satisfies L_g ＞ 1.5λ₁, it is known that the analytical model can be sufficiently used to analyze short channel effects. The scale length varies depending on the channel and oxide thickness as well as the dielectric constant of the channel and the oxide film. In this paper, we obtain the scale length for a constant permittivity (silicon and silicon dioxide), and derive the relationship between the scale length and the channel length satisfying the error range within 5%, compared with a numerical method. As a result, when the thickness of the oxide film is reduced to 1 nm, even in the case of L_g ＜ λ₁, the analytical subthreshold swing model proposed in this paper is observed to satisfy the error range of 5%. However, if the oxide thickness is increased to 3 nm and the channel thickness decreased to 6 nm, the analytical model can be used only for the channel length of L_g ＞ 1.8 λ₁.

Transparent thin film transistors were fabricated on n+-Si wafers coated by Al2O3/SiO2. Zinctin oxide (ZTO) films deposited by rf magnetron sputtering were employed for active layers. The mobility(μs), threshold voltage (VT), and sub threshold swing (SS) dependances on ZTO thickness were analyzed. The VT decreased with increasing ZTO thickness. The μs raised from 5.1 cm2/Vsec to 27.0 cm2/Vsec byincreasing ZTO thickness from 7 nm to 12 nm, and then decreased with ZTO thickness above 12 nm. The SS was proportional to ZTO thickness.