The electrostatic potential of p+-n+ junctions, as in Esaki (tunnel) diodes, originates from the Coulomb potentials of ionized dopants in the depletion-layer, but it has been modeled so far based on uniform space-charge regions, ignoring the discrete and random dopant distribution. This model can explain well the band-to-band tunneling (BTBT) between the opposite bands of the quasineutral regions (conduction band in the n+-region and valence band in the p+-region). In this letter, we show that a BTBT transport model should contain the mechanism of tunneling via “inherent” localized bandgap-states, created by dopant-induced potential fluctuation, which becomes detectable as a parallel transport mechanism in nanoscale Esaki diodes. This is manifested by the observation of single-charge (SC) BTBT at 5.5 K in nanoscale Si Esaki diodes. Numerical analysis of nanoscale p+-n+ junctions with random dopant-atom distributions suggests that SC-BTBT is mediated by a potential dip created by a number of dopants “clustered” near each other, i.e., by a multiple-dopant cluster.

From the viewpoint of high- (room-) temperature operation of donor-based single-electron transistors, we make a comparative study of nano-scale silicon-on-insulator transistors with phosphorus-doped channels for two dopant concentrations: ND ≈ 1.0×10^18 and 2.0×10^20 cm-3. We experimentally show that the high-ND devices can provide room-temperature single-electron tunneling operation owing to a large tunnel-barrier height, while operation temperature is limited to about 100 K for the low-ND devices. Numerical simulations of random donor-atom distributions indicate that donor clustering plays a dominant role in the formation of quantum dots, and suggests that clusters comprising of more-than-three donors are responsible for room-temperature operation.

Non-degenerately doped lateral nanoscale p-n and p-i-n silicon-on-insulator devices have been fabricated and characterized at room temperature (297 K). In both types of devices, p-type Si substrate is used as a backgate to modify the potential in the top Si layer in both forward- and reverse-bias regimes. In the forward-bias regime, both types of devices exhibit negative differential transconductance (NDT), with the current peak position and level controlled by the backgate and anode voltage. In the reverse-bias regime, the devices exhibit a sharp current increase as a function of the backgate voltage, which is a signature of the band-to-band tunneling (BTBT) mechanism. These findings suggest that NDT and the sharp increase of current, induced by the contribution of the BTBT mechanism, can be achieved even in non-degenerately doped backgated diodes, which opens new possibilities for BTBT-based functionalities, benefiting from a simple design and CMOS compatibility.

Electron transport through a few-donor cluster flanked by acceptors is studied by first-principles and semi-empirical simulations in gated Si-nanowire transistors with n + electrostatically-doped source/drain. Local density-of-states spectra are probed by electrical characteristics at room temperature for clarifying modifications induced by acceptor-atoms on the energy states of the few-donor cluster. It is found that acceptor-atoms located between the few-donor cluster and the leads mainly shift the cluster potential, introducing a minor distortion to its energy spectrum. The results change only weakly as the acceptor-atoms are moved towards the Si nanowire surface, and systematically depend on the number of acceptors.

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