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Field Effect Transistors
Published in Bogdan M. Wilamowski, J. David Irwin, Fundamentals of Industrial Electronics, 2018
Bogdan M. Wilamowski, J. David Irwin
VT = kT/q is the thermal potential and η depends on the device geometry and lies between 1.5 and 2.5 (Figure 10.8). The subthreshold conduction is the reason why MOS transistors are actually never completely turned OFF and there is always some current leak through MOS transistors. When the popular CMOS technology was first developed, one of the underlying assumptions was that along the power path, there would never be even one MOS transistor in the OFF state, so power would not be taken from power supply. While a transistor can be in the OFF state, there is a leakage current caused by the subthreshold conduction in CMOS VLSI circuits, which can be very significant in situations where the number of MOS transistors exceeds one billion.
Device Modeling — From Physics to Electrical Parameter Extraction
Published in Louis Scheffer, Luciano Lavagno, Grant Martin, EDA for IC Implementation, Circuit Design, and Process Technology, 2018
Robert W. Dutton, Chang-Hoon Choi, Edwin C. Kan
The expressions given in Figures 25.14a and b represent electrostatically controlled current conditions — the first where the gate is the primary controlling factor and the second expression results from the MLP induced by the drain voltage. As stated above, the subthreshold conduction regime with strong gate control is expected and is useful in low-power design. The condition that results in DIBL-degraded subthreshold leakage is one of several parasitic substrate conduction effects. In order to reduce this effect, the substrate doping, the HALO implant dose, or the body thickness of SOI can be scaled [43]. However, this in turn can result in parasitic conduction at the drain end of the channel. For heavily doped junctions there can be band-to-band (Zener) tunneling as well as trap-assisted tunneling due to damage introduced during the HALO implant. A simulation-based model for extraction of these tunneling currents has been proposed [31] and further refined based on empirical data from experiments using different HALO dose conditions [32]. This again provides evidence that the doping distribution details (and their statistics) are of critical importance in device design and modeling.
MOSFET threshold voltage modelling and parameters optimisation of new ion implant-based subthreshold device
Published in International Journal of Electronics Letters, 2021
Munem Hossain, Masud H. Chowdhury
For super-threshold device, subthreshold current is considered as the undesired leakage, which is one of the most serious problems in current nanoscale IC design. For subthreshold device, this subthreshold current is the useful current. Due to the exponential dependence of the subthreshold conduction on the threshold voltage (see Figure 2) the variation of the threshold voltage is, therefore, a very critical factor in both conventional and subthreshold devices. In Eq. (3), Vbi is the source/drain built-in potential and Nsd is the doping concentration in the lightly doped region.