Silicon Single-Electron Pump

Yukinori Ono and Yasuo Takahashi
Device Physics Laboratory

 Single-electron devices (SEDs) are promising for future ultra-large-scale integrated circuits because of their small size and ultra-low power consumption. The single-electron pump is a member of the SED family and can transfer single electrons even when the drain terminal is reversely biased (hence the name "pump"). Among SEDs, the pump dissipates the lowest energy and has the highest transfer accuracy. However, the presently available pumps, which are made of metals, have some drawbacks, including a limited (< 〜100 mK) operational temperature and poor operational stability.
 In order to achieve a high level of transfer accuracy, the single-electron pump commonly employs a chain of nanometer-scaled conducting materials, called Coulomb islands, which is complicated and difficult to fabricate. Technology for making multiple islands in Si is still premature. The single-electron transistor (SET), which is the simplest SED and has only one island, cannot work as a pump because of an inherent leakage current, which fatally lowers the transfer accuracy. However, Si technology has enabled us to fabricate, in a controlled way, SETs operating at high temperatures with good operational stability [1].
 We thus proposed a new structure for Si-based pumps, in which we utilize a high-temperature operating Si SET in combination with two ultra-small metal-oxide-semiconductor field-effect transistors (MOSFETs) sandwiching it. These MOSFETs have extremely high off-resistance, and this makes it possible to almost completely prevent the leakage current from flowing. We have demonstrated pump operation at 25 K [2], which is two to three orders of magnitude higher than that for the metal-based ones.
 Figure 1 shows a scanning electron microscope image of the pump. Figure 2 shows drain current versus drain voltage characteristics measured at 25 K for 1-MHz ac gate biases. The drain currents are quantized in units of ef at around zero drain voltage, where e is the elementary charge and f is the frequency of the voltages to the MOSFETs. In addition, a change in the polarity of the gate-voltage phase shift results in a change in the polarity of the current. These data demonstrate the single-electron transfer.
 The present result is the first step towards producing practical single-electron pumps.

[1] Y. Takahashi, et al., J. Phy.: Condens. Matter 14 (2002) R995.
[2] Y. Ono and Y. Takahashi, Appl. Phys. Lett. 82 (2003) 1221.

Fig. 1. Electron microscope image of the pump.
Fig. 2. Current characteristics.

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