A Study on Electronic Transport in Nanoscale Field-Effect Transistors with Non-Equilibrium Green's Function

Abstract

As the number of transistors in the integrated circuit has increased, power management has become the main concern. Reducing the leakage current in devices is one of the solutions in power management. The high-k dielectric materials, such as HfO2 and ZrO2, and compound semiconductors are used for reducing the leakage current. The use of these high-k dielectric materials for the gate oxide reduces the gate leakage. Further, the power dissipation can be reduced by decreasing the operating voltage, which is realized by substituting silicon with the compound semiconductors. This thesis investigates the electronic transport in high-k dielectric materials and in compound semiconductors using non-equilibrium Green's function (NEGF) formalism. First, the electronic properties of the materials are obtained by the calculation of the band structure for each material. The band structures of materials are calculated by the sp3d5s* nearest neighbor tight-binding method. The band parameters for the electronic transport simulation are obtained from the calculated band structures of Si, III-V compound semiconductors, and high-k dielectric materials. Second, the electronic transport property is calculated with the NEGF formalism. The Hamiltonian in NEGF formalism is developed with the parabolic dispersion relation using the conduction band edge and conduction band effective mass. The calculation is performed assuming a ballistic transport using ideal contact conditions. The simulation results with a gate length of 20 nm are compared with the experimental data from a reported paper. Based on this comparison, the I-V characteristics of the device with the gate length of sub-10 nm are predicted. The I–V characteristics show the current curves of conventional FETs, and the saturation levels differ for each gate voltage. Further, the tunneling device is simulated to analyze the leakage current characteristics using the developed simulator. Si and III-V compound semiconductors with dielectric material are simulated. The results show that higher leakage current appears in the device using the compound semiconductors. Finally, a rigorous approach is performed to investigate the leakage current. The full band calculation with the self-energy obtained from the surface Green's function is performed for the accurate description of surface condition in hetero-structure. The electronic transport properties are simulated in the semiconductor/dielectric material/semiconductor structures. Si and various kinds of III-V compound semiconductors are implemented as channel materials. I-V characteristics of the simulation results with Si/HfO2/Si structure show a reasonable tendency in the comparison made with the experimental data of another reported paper. It is observed that the simulation results show a better coincidence with the experimental data than the simulation results of the tunneling device. On the basis of this comparison, the leakage current in the compound semiconductor/HfO2/compound semiconductor structure is predicted. The tunneling currents of the compound semiconductor/HfO2/compound semiconductor structures appear at low oxide voltages, while tunneling currents of Si/HfO2/Si structures show very low levels at low oxide voltages. The oxide voltages for the saturation point depend on the differences between the conduction band edge and the Fermi level of the semiconductor materials. The Fermi level of channel material and conduction band edge of gate material should be adjusted according to the devices in use. As the devise size goes into the atomic scale regime, the simulation helps greatly in analyzing and predicting the device properties. The incoherent transport simulation should be considered for predicting the real device properties. This study may be viewed as the first step for incoherent transport calculation.