Multimillion Atom Simulations with Nemo3D

A realistic 40 nm InAs high electron mobility transistor is studied using a two-dimensional, full-band, and atomistic Schrödinger-Poisson solver based on the spds tightbinding model. Bandstructure non-parabolicity effects, strain, alloy disorder in the InGaAs and InAlAs barriers, as well as band-to-band tunneling in the transistor OFF-state are automatically included through the full-band atomistic model. The source and drain contact extensions are taken into account a posteriori by adding two series resistances to the device channel. The simulated current characteristics are compared to measured data and show a good quantitative agreement. Introduction The scaling properties of high electron mobility transistors (HEMTs) with III-V compound semiconductor channels are currently investigated by both industry and academia. The logic performance of InGaAs and InAs based HEMTs with a gate length and a multi-quantum-well channel thickness scaled down to 40 and 10 nm, respectively has been recently reported [1,2]. It is expected that such devices will profit from the very high mobility of InAs, 20,000 cm/Vs, to exhibit highspeed operation, low-power consumption, and to outperform the conventional Si devices. The interest in physics-based computer design to develop novel technology such as InAs HEMTs has considerably increased in the last decade. However standard techniques such as drift-diffusion are not adapted to the nanoscale devices shown in Ref. [1] and [2]. In effect they cannot capture the strong confinement of the electrons and the resulting quantization of the energy levels. In order to remedy to this deficiency quantum mechanical treatments within the effective mass approximation have been recently proposed for III-V devices [3]. However, the non-parabolicity of the InAs lowest conduction band is missing so that the electron states cannot be correctly populated. At a higher level full-band simulations have also been attempted, but they are restricted to very small Si structures [4] (channel thickness of 3 nm, total length of less than 30 nm) or they are based on semi-classical onedimensional approaches [5]. In this paper we present a highly-efficient ballistic, twodimensional, full-band, atomistic, and quantum mechanical simulator [6,7] to study realistic InAs HETMs with InGaAs and InAlAs barriers as proposed in Ref. [2]. The tool is based on the nearest-neighbor spds tight-binding model and a Wave Function approach, equivalent to the Non-Equilibrium Green’s Function formalism, but computationally much more efficient in the case of ballistic transport. Its four levels of parallelism and its optimized numerical algorithms allow the simulation of 140 nm long devices with a channel thickness up to 12 nm [8]. This goes much beyond the capabilities of any other full-band atomistic simulator. Furthermore, quantitative agreement with the experimental data of Ref. [2] is obtained.