Changhua Tan

Numerical analysis for root-mean-square roughness of SiO2/Si interface on direct tunneling current in ultrathin MOSFETs

Abstract Interface roughness effects on direct tunneling current in ultrathin MOSFETs are investigated by numerical analysis. The interface roughness is described in terms of Gauss distribution. It is shown that the effects of rough surface on tunneling current cannot be neglected while tunneling occurs in the regime of direct tunneling. The effects increase exponentially with oxide thickness or applied voltage across oxide decreasing. This means that the effects become more and more important while the direct tunneling current takes up the main contribution to the gate leakage current.

Study of Fowler-Nordheim tunneling current oscillations of thin insulator MOS structure by wave interference method

Abstract In this paper, a novel method – electron waves interference method – is presented to study Fowler–Nordheim tunneling current oscillations. Using this new analysis, the applied voltage V at the extrema of Fowler–Nordheim tunneling oscillations can be accurately and simply determined. A phase shift of π/4 when electron traverses the forbidden band of perfect medium barrier once is observed by using interference method. Some phenomena observed in experiment can be easy to explain by this method. The extrema of oscillations can be used to determine the barrier thickness in metal-oxide-semiconductor device and the average effective electron mass in the conduction band of the barrier. An important feature of this method is that it is applicable to various shapes of potential barriers and wells.

Thickness measurements for ultrathin-film insulator metal–oxide–semiconductor structures using Fowler–Nordheim tunneling current oscillations

An interference method is introduced to analyze tunneling current oscillations, and a fresh way to extrapolate the oxide thickness in ultrathin-film insulator metal–oxide–semiconductor structures by using the oscillations in the Fowler–Nordheim tunneling currents is presented. A comparison between this extrapolation algorithm and a previous algorithm using tunneling current oscillations shows that the new extrapolation algorithm provides a more accurate and convenient solution to a first principles calculation especially for ultrathin oxide. Another important feature of the proposed method is that it can be applied to various shapes of potential barriers and wells.

The double level calculation of oxygen related donor states in Si and SiO2

Abstract A calculation of the levels related to oxygen donors in Si and SiO 2 has been performed based on a helium-like model. These calculations agree well with the experimental data while the energies of these levels are in the ratio of about 2:1 and the effective radii of these levels are in the ratio of about 2:1.

Proportional difference operator method and its application in studying subthreshold behavior of MOSFETs

Abstract The proportional difference operator (PDO) method is presented to study the subthreshold behavior of MOSFETs. By applying the PDO, acting on an expression for the drain current in the subthreshold region, a new proportional difference characteristic of MOSFET can be obtained. The analytical results show that the proportional difference of the subthreshold equation is a peak function. Its peak position, being dependent on the measurement voltage, is related to the characteristic parameters, so the most important parameters (such as thermal and threshold voltages) can be easily obtained. A surface potential approximation has been developed and implemented in studying a MOSFET’s proportional difference subthreshold behaviors. The relationships between the classical threshold voltage and the Lindner threshold voltage are also discussed. Values of voltage constants are shown to agree well with those obtained by standard methods.

Analysis of the rate of change of inversion charge in thin insulator p-type Metal-Oxide-Semiconductor structures

A model of the rate of change of inversion charge has been used to investigate the capacitance relaxation associated with Fowler-Nordheim tunneling current in thin insulator p-type Metal-Oxide-Semiconductor (MOS) structures. In this model, the rate of change of inversion charge is taken to be equal to the difference between the generation current in the space charge region of Si and the Fowler-Nordheim tunneling current through the oxide. Using a step voltage, analytical expression for high frequency capacitance relaxation is obtained, the experimental results are consistent with model calculations.

The effect of image potential on electron transmission and electric current in the direct tunneling regime of ultra-thin MOS structures

Abstract The direct tunneling current through ultra-thin gate dielectrics is modeled by calculating the transmission coefficient of an idealized potential barrier that is modified by the image force. A numerical solution to the Schrodinger equation shows that the barrier lowering induced by image-potential affects the tunneling current largely. An analytical expression for the current is obtained within the Wentel–Kramers–Brillouin approximation. The effects of image force on the direct tunneling current are found to increase with the applied voltage across oxide (Vox) and to decrease with the oxide thickness (Tox).

Effect of SiO2/Si interface roughness on gate current

Abstract In many theoretical investigations of the electric-tunnel effect through an ultrathin oxide in metal-oxide-semiconductor (MOS) structure, it is commonly assumed that the oxide is of uniform thickness. One example of nonuniformity in oxides is interface roughness. Interface roughness effects on direct tunneling current in ultrathin MOS structures are investigated theoretically in this article. The roughness at SiO 2 /Si interface is described in terms of Gauss distribution. It is shown that the transmission coefficient increases with root-mean-square (rms) roughness increasing, and the effect of rms roughness on the direct tunneling current decreases with the applied voltage increasing and increases with rms roughness increasing.

Direct tunneling relaxation spectroscopy in ultra-thin gate oxide MOS structures

Abstract For the first time, a difference analysis method has been applied to separate and characterize interface and oxide traps generated in an ultra-thin direct tunneling (DT) gate oxide. It provides a useful tool to study the mechanism of degradation in ultra-thin MOSFET and extract parameters of interface trap and oxide trap, such as the generation/capture cross-section, centroid and density when a large DT is injected through the gate oxide. This method is an extension of oxide trap relaxation spectroscopy (OTRS) technique. It has the advantage of being direct, fast and convenient in the study of MOSFET reliability.

A new lifetime prediction method for hot-carrier degradation in n-MOSFETs with ultrathin gate oxides under Vg=Vd

Abstract Hot-carrier degradation of n-MOSFETs at high gate voltages ( V g = V d ) is examined. A new lifetime prediction method is developed based on the universal power law between the degradation of saturated drain current (d I dsat ) and the product of the injected charge fluence times the gate current, which is independent of gate or drain voltages. This method is applied to 4 and 5 nm n-MOSFETs and lifetimes are estimated under their operation conditions. It is applicable to n-MOSFETs with ultrathin gate oxides.