We developed a flexible-logic-gate single-electron device (SED) in which logic functions can be selected by changing the voltage applied to the control gate. It consists of an array of nanodots with multiple inputs and multiple outputs. Since the gate electrodes couple capacitively to the many dots underneath, complicated characteristics depending on the combination of the gate voltages yield a selectable logic gate when some of the gate electrodes are used as control gates. One of the important issues is how to design the arrangement of nanodots and gate electrodes. In this study, we fabricated a Si nanodot array with two simple input gates and two output terminals, in which each gate was coupled to half of the nanodot array. Even though the device had a very simple input-gate arrangement and just one control gate, we could create a half-adder function through the use of current maps as functions of the input gate voltages. We found that the nanodots evenly coupled capacitively to both input gates played an important role in getting a basic set of logic functions. Moreover, these results guarantee that a more complicated input-gate structure, in which each gate evenly couples many nanodots, will yield more complicated functions.

Si single-electron transistors with a high voltage gain at a considerably high temperature have been fabricated by vertical pattern-dependent oxidation. The method enables the automatic formation of very small tunnel junctions having capacitances of less than 1 aF. In addition, the use of a thin (a few ten nanometers thick) gate oxide allows a strong coupling of the island to the gate, which results in a gate capacitance larger than the junction capacitances. It is demonstrated at 27 K that an inverting voltage gain, which is governed by the ratio of the gate capacitance to the drain tunnel capacitance, exceeds 3 under constant drain current conditions.

Accuracy of a method for analyzing the interface defect properties; time-domain charge pumping method, is evaluated. The method monitors the charge pumping (CP) current in time domain, and thus we expect that it gives us a noble way to investigate the interface state properties. In this study, for the purpose of evaluating the accuracy of the method, the interface state density extracted from the time-domain data is compared with that measured using the conventional CP method. The results show that they are equal to each other for all measured devices with various defect densities, demonstrating that the time-domain CP method is sufficiently accurate for the defect density evaluation.

A multifunctional Boolean logic circuit composed of single-electron transistors (SETs) was fabricated and its operation demonstrated. The functions of Boolean logic can be changed by the half-period phase shift of the Coulomb-blockade (CB) oscillation of some SETs in the circuit, and an automatic control based on a feedback process is used to attain an exact shift. The amount of charges in the memory node (MN), which is capacitively coupled to the SET, controls the phase of the CB oscillation, and the output signal of the SET controls the amount of charge in the MN during the feedback process. This feedback process automatically adjusts SET output characteristics in such a way that it is used for the multifunctional Boolean logic. We experimentally demonstrated the automatic phase control and examined the speed of the feedback process by SPICE circuit simulation combined with a compact analytical SET model. The simulation revealed that programming time could be of the order of a few ten nanoseconds, thereby promising high-speed switching of the functions of the multifunctional Boolean logic circuit.

A single-electron turnstile and electrometer circuit was fabricated on a silicon-on-insulator substrate. The turnstile, which is operated by opening and closing two metal-oxide-semiconductor field-effect transistors (MOSFETs) alternately, allows current quantization at 20 K due to single-electron transfer. Another MOSFET is placed at the drain side of the turnstile to form an electron storage island. Therefore, one-by-one electron entrance into the storage island from the turnstile can be detected as an abrupt change in the current of the electrometer, which is placed near the storage island and electrically coupled to it. The correspondence between the quantized current and the single-electron counting was confirmed.