Zhijian Li

Effective density-of-states approach to QM correction in MOS structures

Abstract MOS structure threshold voltage shift due to quantum mechanical effects (QMEs) has a substantial influence on deep-submicron MOSFET characteristics. However, its physical nature has not been thoroughly investigated and an analytical model is absent. In this paper, a numerical solution of the Schrodinger equation with parabolic potential well and an analytical solution with triangular well are compared, and the validity of the triangular well approximation is verified. Based on the calculation of the subband structure in the quantized region in a weak inversion regime, the concepts of surface layer effective density-of-states (SLEDOS) is proposed. Carrier distribution in subbands is then analyzed and physical base of MOSFETs Vth shift due to QMEs are discussed. The single subband occupation approximation used in earlier works is proved to be invalid and a new analytical threshold voltage (Vth) shift model due to QMEs including multisubband occupation is derived based on the concept of SLEDOS. The model reveals the physical nature of QMEs on Vth shift and gives consistent results with experiments and self-consistent calculation.

Patterned buried oxide layers under a single MOSFET to improve the device performance

A novel quasi-silicon-on-insulator (Q-SOI) metal-oxide-semiconductor field-effect transistor (MOSFET) has been successfully fabricated, where the drain and source regions were positioned on the patterned buried oxide (BOX) layers while the channel region was left connected to the substrate. The high-quality BOX layers patterned under a single MOSFET were formed by the masked separation by implantation of oxygen technique. The electrical and thermal properties of such a novel Q-SOI device were investigated and the results demonstrated that this Q-SOI device has improved performances over the SOI counterpart because of suppression of the floating-body and self-heating effects.

Modified Airy function method for modeling of direct tunneling current in metal–oxide–semiconductor structures

Using a modified Airy function (MAF) to solve the Schrodinger equation in the whole metal–oxide–semiconductor structure, a fully quantum-mechanical model of direct tunneling current from an inverted p-Si substrate through ultrathin oxides is presented. The effects of tunneling on the electrostatic potential and the distribution of electrons are also included when self-consistently solving the Schrodinger and Poisson equations in silicon. Due to the semianalytical nature of the MAF method, the model has high efficiency. Model results are compared with experimental data and show excellent agreement. Moreover, an approximately linear relationship between the logarithm of the direct tunneling current and oxide thickness is found out.

A new charge model including quantum mechanical effects in MOS structure inversion layer

Abstract Based on the analysis of the distribution of inversion layer carriers in MOS structure, the concept of surface layer effective density-of-states (SLEDOS) is proposed. Then a new charge control model suitable for both semi-classical and quantum mechanical theory is established in which the effects of inversion layer carrier distribution on surface potential are included. In this model, a newly developed efficient iteration method is introduced, which has high efficiency and satisfied stability. Based on the model, the effects of quantum mechanical effects (QMEs) on inversion layer charge density both in weak and strong inversion regions and the surface potential are studied. Model results are compared with the self-consistent solutions of Schrodinger and Poisson equations, which proves the high accuracy of the new model.

Characterization and modeling of threshold voltage shift due to quantum mechanical effects in pMOSFET

Abstract The threshold voltage shift due to quantum mechanical effects (QMEs) has substantial influences on modern pMOSFET characteristics. Investigations of QMEs in a pMOSFET are classified into two approaches: full-band calculation and effective mass approximation. In this paper, formulation of carrier distribution in the pMOSFET inversion layer in the threshold region based on the effective mass approximation is presented, and a new method to calculate the threshold voltage shift due to QMEs is developed. The results with the effective mass approximation are compared with the full-band calculation and show satisfactory coincidence. Based on the present model, the subband structure of the hole inversion layer and the carrier distribution characteristics are investigated. The quantum mechanical and semi-classical two-dimensional density of states (2D DOS) are calculated and compared. The dependence of the threshold voltage shift due to QMEs on substrate doping concentration is then analyzed from the DOS point of view.

Comprehensive analytical physical model of quantized inversion layer in MOS structure

Abstract A new analytical charge model in quantized inversion layer in MOS structure is developed by comprehensively considering the quantum mechanical effects. The model is based on the newly proposed concept of space charge capacitance. Explicit expressions of surface potential and inversion layer carrier density are given which have smooth transition characteristics from depletion to weak to strong inversion regions. Threshold voltage shift, finite inversion layer thickness and the increment of depletion charge after strong inversion are properly modeled. Model results are compared with numerical data by self-consistent solution of Schrodinger and Poisson equation and the high accuracy of the model is demonstrated.

Thorough analysis of quantum mechanical effects on MOS structure characteristics in threshold region

Threshold voltage shift due to quantum mechanical effects (QMEs) are studied for both n- and p-MOS structure in the paper. Subband structure and carrier distribution are formulated for both type of MOS structure in effective mass approximation. QMEs on threshold voltage shift are thoroughly analyzed based on the model. Carrier distribution in subbands for both n-MOS and p-MOS are calculated and analyzed from density-of-states point of view. Model results for n- and p-MOS structure are compared with experimental and full-band self-consistent calculation results and show good coincidence. It is proved that at least in threshold region, effective mass approximation has similar accuracy as the full-band self-consistent method to predict the influence of QMEs MOS structure characteristics.

Statistical analysis of quantized inversion layer in MOS devices with ultra-thin gate oxide and high substrate doping levels

The degree of degeneracy of a quantized inversion layer in an MOS structure is investigated by a fully quantum mechanical approach via self-consistent solution of Schrodinger and Poisson equations. The relative error of carrier sheet density induced by Boltzmann statistics is used as a measurement of the degeneracy. It is shown that the degree of degeneracy of the inversion layer is much weaker due to the quantization of carrier energy compared with the semi-classical case.

On the degeneracy of quantized inversion layer in MOS structures

Abstract Degree of degeneracy of quantized inversion layer in metal-oxide semiconductor structure are investigated by a fully quantum mechanical approach via self-consistent solution of Schrodinger and Poisson equations. The relative error of carrier sheet density induced by Boltzmann statistics is used as a measurement of the degeneracy. It is shown that the degree of degeneracy of inversion layer is much weaker due to the quantization of carrier energy compared with the semi-classical one.

Synthesis and thermal conductivity measurement of high-integrity ultrathin oxygen-implanted buried oxide layers

Ultrathin buried oxide (BOX) layers have been synthesized by low-dose and low-energy separation by implantation of oxygen (SIMOX) technique. The formed BOX layers were examined by transmission electron microscopy (TEM) and the results demonstrated that the BOX layers are of high integrity without any detectable silicon islands therein. A modified method was introduced to measure the thermal conductivity of the synthesized high-integrity BOX layers with different thicknesses. It is found that ultrathin SIMOX BOX layers exhibit a thermal conductivity of ~0.92 Wm-1K-1, which is approximately 34% lower than that of bulk SiO2, 1.4 Wm-1K-1. In addition, the boundary thermal resistance of the Si/BOX interfaces was also measured.