As the gate length of metal-oxide-semiconductor field-effect-transistors (MOSFETs) has been scaled down to the deca-nanometer range, the device characteristics are expected to be further enhanced. However, the operation mechanism of such kind of nanoscale transistors will be quite different from the conventional drift-diffusion based charge transport. Ballistic transport, where the electron velocity significantly exceeds the saturation values as given by the drift-diffusion mode of charge transport, has been observed in the nanoscale tr ansistors. Meanwhile, band-to-band tunneling (BTBT) for carrier injection has been adopted as an important measure for maintaining the continuous supply voltage scaling in the nanoscale transistors. Due to these fundamental changes in device operation mechanisms, the existing current-voltage models need to be rebuilt. Several attempts have been made in this respect. However, the models constructed so far are still far from complete. The present research aims to further improve, by incorporating a number of important underlying physical effects, the modeling of two main types of new devices: the ballistic mode transistors and the tunnel field-effect transistors. The specific contributions of this work are highlighted below:
• First, a quasi-analytical model for precise prediction of the current-voltage characteristics of a cylindrical gate-all-around ballistic MOSFET is developed. The quantum effects are incorporated into the Poisson's equation in a self-consistent way and by taking into account the contributions of the subband energy levels. Better agreements with numerical simulations are obtained as compared to several models previously reported in the literature.
• Second, an analytical model is developed for the sub-threshold operation of the ballistic transistors. By using the potential distribution obtained from a two-dimensional Poisson's equation and by performing some perturbations to the subband energy levels, a subthreshold drain current model is obtained. This model is further used for predicting the subthreshold swings and the threshold voltages of the ballistic transistors.
• Third, an analytical model for predicting the drain current characteristics of the gate-all-around source pocket tunnel field-effect transistor (TFET) is developed. The model is derived by dividing the source, drain and channel regions into several portions so that some simple approximations for the surface potential across the tunneling junction and the channel can be applied. The tunneling current is obtained analytically by integrating the generation rate and using the developed surface potential model. The approach is further evaluated for the devices with very short channel lengths.
These results do not only provide practical models with better accuracies as demonstrated with the good agreements with numerical simulation under various conditions, they also provide a better insight in the device physics. The developed models and the proposed can serve as useful tools for further exploring the promises and the limitations of these emerging devices.