Junctionless transistors made of silicon have previously been demonstrated experimentally and by simulations. Junctionless devices do not require fabricating an abrupt source-drain junction and thus can be easier to implement in aggressive geometries
. In this paper, we explore a similar architecture for aggressively scaled devices with the channel consisting of doped carbon nanotubes (CNTs). Gate all around (GAA) field effect transistor (FET) structures are investigated for n- and p-type doping. Current-voltage characteristics and sub-threshold characteristics for a CNTbased junctionless FET is compared with a junctionless silicon nanowire (SiNW) FET with comparable dimensions. Despite the higher on-current of the CNT channels, the device characteristics are poorer compared to the silicon devices due to the smaller CNT band gap.
Inspired by recent experimental realizations and theoretical simulations of thin silicon nanowire-based devices, we perform predictive first-principles simulations of junctionless gated Si nanowire transistors. Our primary predictions are that Si-bas
ed transistors are physically possible without major changes in design philosophy at scales of ~1 nm wire diameter and ~3 nm gate length, and that the junctionless transistor may be the only physically sensible design at these length scales. We also present investigations into atomic-level design factors such as dopant positioning and concentration.
We present a simple model to describe the lowest-subbands surface scattering in locally oxidized silicon nanowires grown in the [110] direction. To this end, we employ an atomistically scaled effective mass model projected from a three-dimensional ef
fective mass equation and apply a quantum transport formalism to calculate the conductance for typical potential profiles. Comparison of our results with hole-transport calculations using atomistic models in conjunction with density functional theory (DFT) points to an intra-subband scattering mechanism from a potential well.
The influence of local oxidation in silicon nanowires on hole transport, and hence the effect of varying the oxidation state of silicon atoms at the wire surface, is studied using density functional theory in conjunction with a Greens function scatte
ring method. For silicon nanowires with growth direction along [110] and diameters of a few nanometers, it is found that the introduction of oxygen bridging and back bonds does not significantly degrade hole transport for voltages up to several hundred millivolts relative to the valence band edge. As a result, the mean free paths are comparable to or longer than the wire lengths envisioned for transistor and other nanoelectronics applications. Transport along [100]-oriented nanowires is less favorable, thus providing an advantage in terms of hole mobilities for [110] nanowire orientations, as preferentially produced in some growth methods.
Band gap modification for small-diameter (1 nm) silicon nanowires resulting from the use of different species for surface termination is investigated by density functional theory calculations. Because of quantum confinement, small-diameter wires exhi
bit a direct band gap that increases as the wire diameter narrows, irrespective of surface termination. This effect has been observed in previous experimental and theoretical studies for hydrogenated wires. For a fixed cross-section, the functional group used to saturate the silicon surface significantly modifies the band gap, resulting in relative energy shifts of up to an electronvolt. The band gap shifts are traced to details of the hybridization between the silicon valence band and the frontier orbitals of the terminating group, which is in competition with quantum confinement.
For investigation of electron transport on the nanoscale, a system possessing a simple to interpret electronic structure is composed of alkane chains bridging two electrodes via end groups; to date the majority of experiments and theoretical investig
ations on such structures have considered thiols bonding to gold electrodes. Recently experiments show that well defined molecular conductances may be resolved if the thiol end groups are replaced by amines. In this theoretical study, we investigate the bonding of amine groups to gold clusters and calculate electron transport across the resulting tunnel junctions. We find very good agreement with recent experiments for alkane diamines and discuss differences with respect to the alkane dithiol system.
