Lundstrom Model Research Articles

Insight Into Ballisticity of Room-Temperature Carrier Transport in Carbon Nanotube Field-Effect Transistors

Carbon nanotube field-effect transistors (CNT FETs) with gate length down to 10 nm are fabricated and analyzed by fitting the experimental data through the virtual-source (VS) model. Due to the quasi-ballistic carrier transport property, it is shown that carriers in CNTs exhibit the injection velocity of ${3}\times 10^{{7}}$ cm/s, approximately two times of the target velocity in the 10-nm gate length transistors required by International Roadmap for Devices and Systems (IRDS 2017). The transport parameters for the first time, namely, critical length, unidirectional thermal velocity, and backscattering mean free path (MFP), are extracted by the Lundstrom model from the measured data of quasi-ballistic short-channel CNT FETs to diffusive long-channel CNT FETs, which help arriving at a conclusion that, down to 10-nm gate length, the CNT FETs operate close to 73% of the ballistic regime.

Landauer–Datta–Lundstrom model for terahertz transistor amplifier based on graphene

A transistor has been considered in the form of three electrodes connected by graphene ribbons or by metal quantum wires (nanowires) that operate on the principle of the current control by the changing voltage at the central electrode (gate). The analysis has been carried out according to the Landauer–Datta–Lundstrom model in equilibrium approximation for electrodes while fixing their potentials. We have obtained linear models and nonlinear terms in the determining current, and calculated the nonlinear current–voltage performances of graphene nanoribbons.

Analysis of Carrier Transport in Short-Channel MOSFETs

A method for extracting transport parameters in short-channel FETs is presented in the context of the Lundstrom model for quasi-ballistic short-channel FETs. The parameters extracted from measured data are unidirectional thermal velocity, critical length, and mean free path at low and high drain biases. The method is based on an analysis of the channel length dependence of apparent mobility and virtual-source (VS) velocity, which are obtained by fitting the VS model to measured data. Data from (100)-oriented undoped-body extremely thin silicon-on-insulator FETs with neutral stress liners are used to validate the method. Since this method does not assume any theoretical knowledge of band structure parameters, it can be applied to short-channel FETs with any geometry, any channel material, and with unknown levels of channel stress.

A Microscopically Accurate Model of Partially Ballistic NanoMOSFETs in Saturation Based on Channel Backscattering

We propose a model for partially ballistic MOSFETs and for channel backscattering that is alternative to the well known Lundstrom model and is more accurate from the point of view of the actual energy distribution of carriers. The key point is that we do not use the concept of "virtual source". Our model differs from the Lundstrom model in two assumptions: i) the reflection coefficients from the top of the energy barrier to the drain and from top of the barrier to the source are approximately equal (whereas in the Lundstrom model the latter is zero), and ii) inelastic scattering is assumed through a ratio of the average velocity of forward-going carriers to that of backward-going carriers at the top of the barrier kv > 1 (=1 in the Lundstrom model). We support our assumptions with 2D full band Monte Carlo (MC) simulations including quantum corrections in nMOSFETs. We show that our model allows to extract from the electrical characteristics a backscattering coefficient very close to that obtained from the solution of the Boltzmann transport equation, whereas the Lundstrom model overestimates backscattering by up to 40%.

Barrier Lowering and Backscattering Extraction in Short-Channel MOSFETs

In this paper, we propose a fully experimental method to extract the barrier lowering in short-channel saturated MOSFETs using the Lundstrom backscattering transport model in one-subband approximation and carrier degeneracy. The knowledge of barrier lowering at the operative bias point in the inversion regime is of fundamental importance in device scaling. At the same time, we also obtain an estimate of the backscattering ratio and the saturation inversion charge. With respect to previously reported works on extraction of transport parameters based on the Lundstrom model, our extraction method is fully consistent with it, whereas other methods make a number of approximations in the calculation of the saturation inversion charge, which are inconsistent with the model. The proposed experimental extraction method has been validated and applied to results from device simulation and measurements on short-channel poly-Si/SiON gate nMOSFETs with gate lengths down to 70 nm. Moreover, we propose an extension of the backscattering model to the case of 2-D geometries (e.g., bulk MOSFETs). We found that, in this case, backscattering is governed by the carrier transport in a few nanometers close to the silicon/oxide interface and that the value of the backscattering ratio obtained with a 1-D approach can be significantly different from the real 2-D value.

Criticisms on and comparison of experimental channel backscattering extraction methods

In this paper we critically review and compare experimental methods, based on the Lundstrom model, to extract the channel backscattering ratio in nano MOSFETs. Basically two experimental methods are currently used, the most common of them is based on the measurement of the saturation drain current at different temperatures. We show that this method is affected by very poor assumptions and that the extracted backscattering ratio strongly underestimates its actual value posing particular attention to the backscattering actually extracted in nano devices. The second method is based on the direct measurement of the inversion charge by CV characteristics and gets closer to the physics of the backscattering model. We show, through measurements in high mobility p-germanium devices, how the temperature-based method gives the same result of the CV-based method once that its approximations are removed. Moreover we show that the CV-based method uses a number of approximations which are partially inconsistent with the model. In particular we show, with the aid of 2D quantum corrected device simulations, that the value of the barrier lowering obtained through the CV-based method is totally inconsistent with the barrier lowering used to correct the inversion charge and that the extracted saturation inversion charge is underestimated.