Introduction In recent years, novel MOSFET characteristics dominated by a single dopant-atom were reported by several groups [1-4], where carrier transport mechanism is tunneling through individual dopant atoms. In this background, we have proposed and demonstrated a variety of dopant-atom devices, i.e., dopant-atom (DA) transistors [4], DA memories [5], DA turnstiles [6-8] and DA photonic devices [9-11]. In those devices, only one or a few dopants are intentionally used and one dopant works as a quantum well for electron (or hole) tunneling transport. Here, we present a brief overview of such devices from experimental and theoretical point of view. P-donor in nano-Si When we focus on a phosphorous (P) donor atom, it is known that the ionization energy, or the binding energy, with respect to the Si conduction band minimum, is ~45 meV. Therefore, single-electron tunneling P-atom devices can operate only at low temperatures, below ~20 K, since at high temperatures electrons are thermally excited and tunneling transport mechanism does not work. However, when a dopant is embedded in sufficiently small Si structures, its ionization energy is enhanced due to dielectric and quantum size confinement effects [12], leading to high-temperature operation. Recently, we have demonstrated operation of donor-atom SOI-MOSFETs at around 100 K in specifically-designed nano-stub-channels [13]. A good correlation is found between the operation temperature and binding energy (E _a), with the highest SET-operation temperature being achieved for a P donor with E _a≅ 100 meV [13]. A few coupling donors So far, in most investigated dopant-atom devices, the dopants were introduced in random positions. Only a few works addressed directly the control of dopants, either in number, using single ion implantation [14], or in position, with atomic manipulation using scanning tunnelling microscope tips [15]. Most recently, we have challenged a more practical and simpler doping technique using nanoscale doping masks [16]. The channel of SOI-MOSFET was selectively doped within an area of ~30 nm in width by the conventional diffusion process. Even in such a classical doping process, the number of dopants in the doped area may be controlled to around 5. I-V characteristics measured for these selectively-doped FETs are consistent with the model [16]. Nano-pn junctions Individuality of dopants appears not only in nano-FETs, but also in nano-pn diodes. In fact, we have reported dopant-induced random telegraph signals (RTS) in nanoscale pn diodes [17], and attributed the RTS to charging and discharging of a single dopant near the pn junction. Observation of dopant potential In research of dopant-atom devices, it is extremely important to establish an observation tool to monitor dopants’ positions and their individual potentials. For this purpose, we have developed low-temperature Kelvin probe force microscopy (LT-KFM) which allows measurements of devices under regular operation. Using this technique, we succeeded in detection of potential profiles due to individual dopants and potential changes due to single-electron injection effects [18-21]. References [1] H. Sellier et al: Phys. Rev. Lett. Vol. 97 (2006), p. 206805. [2] G.P. Lansbergen et al: Nature Phys. Vol. 4 (2008), p. 656. [3] Y. Ono, et al: Appl. Phys. Lett.Vol. 90 (2007), p. 102106. [4] M. Tabe et al: Phys. Rev. Lett. Vol. 105 (2010), p. 016803. [5] E. Hamid et al: Appl. Phys. Lett. Vol. 97 (2010), p. 262101. [6] D. Moraru et al: Phys. Rev. B Vol. 76 (2007), p. 075332. [7] D. Moraru et al: Appl. Phys. Express Vol. 2 (2009), p. 071201. [8] K. Yokoi et al: J. Appl. Phys. Vol. 108 (2010), p. 053710. [9] M. Tabe et al: Phys. Stat. Sol. A Vol. 208 (2011), p. 646. [10] A. Udhiarto et al: Appl. Phys. Lett. Vol. 99 (2011), p. 113108. [11] A. Udhiarto et al: Appl. Phys. Express Vol. 5 (2012), p. 112201. [12] M. Diarra et al: Phys. Rev. B Vol. 75 (2007), p. 045301. [13] E. Hamid et al: Phys. Rev. B Vol. 87 (2013), p. 085420. [14] E. Prati et al: Nature Nanotechnol. Vol. 7 (2012), p. 443. [15] M. Fuechsle et al: Nature Nanotechnol. Vol. 7 (2012), p.242. [16] D. Moraru et al: Sci. Rep. Vol. 4 (2014), p. 6219. [17] S. Purwiyanti et al: Appl. Phys. Lett. Vol. 103 (2013), p. 243102. [18] M. Ligowski et al: Appl. Phys. Lett. Vol. 93 (2008), p. 142101. [19] M. Anwar et al: Appl. Phys. Lett. Vol. 99 (2011), p. 213101. [20] M. Anwar et al: Jpn. J. Appl. Phys. Vol. 50 (2011),p. 08LB10. [21] R. Nowak et al: Appl. Phys. Lett. Vol. 102 (2013), p. 083109.