Data collecting from extremely high temperature environment such as space, earth core, and manufacturing places will provide fruitful new information. This will contribute to further development of the wide variety of research fields ranging from basic sciences to cutting-edge technologies. For that, new type of logic circuit which can work at extremely high temperature is necessary. As a candidate of such high temperature logic circuits, wide band gap materials such as SiC, GaN, AlN, and diamond have been proposed and widely studied. Although these devices work properly at higher temperature than the conventional Si-based devices, much higher temperature, for instance 1273 K, is desired for the data collecting from an extreme environmental condition. However, such high temperature is undesirable for the electronic devices, because thermally excited electron and hole severely disturb proper operation. For the realization of the ultra-high temperature logic circuits, it is necessary to develop a new type of logic circuit device based on a new working mechanism. In this study, we propose an ionic diode as a promising candidate for the ultra-high temperature logic circuits.

Basic idea of the ionic diode is the analogy to the p-n junction of the typical electronic devices. Similar to the electronic depletion layer at the interface of p-type and n-type semiconductors, the ionic depletion layer can be formed at the interface of a vacancy conductor and an interstitial ion conductor. Under the forward bias condition, interstitial ions and vacancies are supplied to the ion depletion layer which makes the ion depletion layer narrower. In this case, the interface is not resistive. Under the reverse bias condition, on the other hand, interstitial ions and vacancies are extracted from the ion depletion layer which makes the ion depletion layer wider. In this case, the interface become resistive. Then, one can expect an asymmetric current-voltage characteristics at this interface, so-called a rectifying effect. Since the thermal excitation of ionic defects are smaller than that of the electronic species, this ionic diode is considered to work properly even at high temperatures.

In this study, ionic diodes were fabricated from oxygen vacancy conductors, yttria stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC), and interstitial oxide ion conductors, La-doped PbWO₄ (PLW) and La-Si apatite oxides (LSO). A dense pellet of these oxides were prepared and one side of the pellet was mirror polished. Pt electrodes were set on the opposite surface as a current electrode and on the side face of the pellet as a potential electrode. Bulk type cell was constructed by contacting polished surface of an oxygen vacancy conductor pellet and an interstitial oxygen conductor pellet. To make good contact, these pellets were mechanically pressed by a spring. The current-voltage characteristics of the bulk cells were evaluated at 873-1373 K in an artificial air condition.

Since mobile ionic defects are interstitial oxygen and oxygen vacancy, the forward bias is polarizing the oxygen vacancy conductor positively and the interstitial oxygen conductor negatively. For the YSZ/PLW cell and the YSZ/LSO cell, a gradual current increase was observed under the forward bias condition while a gradual current decrease was observed under the reverse bias condition. The bulk cells of the YSZ/PLW and the YSZ/LSO showed weak but clear rectifying behavior, while the YSZ/YSZ and the PLW/PLW cells showed a linear current-voltage characteristics. The rectifying ratio (the forward current over the reverse current) of the YSZ/PLW and the YSZ/LSO reaches about 1.4 at 973 K and 1.3 at 1273 K, respectively. The possible cause of the low rectification ratio of the bulk cells are imperfect contacts of the pellets, large ohmic resistance of the thick pellets, and the formation of an interphase at the interface.

Although the rectifying ratio is small at the present stage, we have succeeded to demonstrate an ultra-high temperature ionic diode. In the presentation, we are going to talk about our latest progress and some theoretical discussions.