Nanoampere Charge Pump by Single-electron Ratchet Using Silicon Nanowire MOSFET
Akira Fujiwara, Katsuhiko Nishiguchi, and Yukinori Ono
Physical Science Laboratory
Clocked transfer of single electrons is attracting a great deal of interest in its application to current standards in metrology and integrated single-electron (SE) circuits. While the conventional metal-based SE pumps and turnstiles employ fixed tunnel junctions of metal oxide, the use of gate-induced electrostatic tunable barriers in semiconductors has a clear advantage for higher-frequency operation because the tunnel resistance can be tuned to reduce the RC time. Although we showed 100-MHz operation of an SE turnstile using Si nanowire MOSFETs with two barriers modulated by two phase-shifted AC signals , higher-frequency operation is desired because nanoampere current is necessary for the so called metrological triangle experiment.
We recently developed a nanoampere SE pump operated based on a simple scheme using only one AC signal applied to one gate . We named the proposed device an SE ratchet since it utilizes the asymmetry potential to capture electrons from the source and eject them to the drain, thereby causing directional SE transport without any source-drain bias. Figures 1(a) and 1(b) show a top-view scanning-electron-microscope (SEM) image of the device and a schematic diagram of the ratchet potential, respectively. We apply AC pulse voltage VG1 at frequency f to the center gate and constant lower voltage VG2 to the drain-side gate. SEs are captured from the source into the island, and then ejected to the drain. The number of transferred electrons per cycle is controlled by the upper gate voltage, VUG. As shown in Fig. 1 (c), current plateaus (I = mef, m: integer) are clearly observed at 20 K in a wide range of the drain bias (VD), which indicates that the SE transfer is directional regardless of the bias polarity. The VD range can be much larger than the charging energy of the island (〜10 meV) thanks to the potential control of the island as well as the barrier. Nanoampere charge pumping is obtained at f =2.3 GHz as shown in Fig. 2. The transfer accuracy is roughly estimated to be in the order of 10-2. It is found that the nonadiabacity of the electron capture affects the shape of the current steps. This is an important finding for further study towards higher accuracy. The SE ratchet is a promising device in the application to large-current source for metrological standards. This work was partially supported by KAKENHI (16206038, 19310093).
 A. Fujiwara, et al., Appl. Phys. Lett. 84 (2004) 1323.
 A. Fujiwara, et al., Appl. Phys. Lett. 92 (2008) 042102.
Fig. 1. (a) SEM image before upper gate formation. (b) Schematic diagram of SE ratchet. (c) Contour plots of current for VD and VUG.
Fig. 2. Pump current at f = 2.3 GHz.
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