Organic semiconductors with extremely narrow energy gap

This paper describes the discovery and investigation of a group of organic conductors that behave as semiconductors with extremely narrow energy gaps when placed under high hydrostatic pressures. The most prominent character of them is that they have strongly temperature dependent carrier density and mobility and yet the resistance is constant over a wide temperature region. Up to now, we confirmed alpha-type crystals of (BEDT-TTF)(2)I-3, (BEDT-TSeF)(2)I-3, (BEDT-STF)(2)I-3 and theta-type crystal of (BEDT-TTF)(2)I-3 belong to this group. Among them, the typical one is alpha-(BEDT-TTF)(2)I-3. Under high-pressures, it is a semiconductor with the energy gap of about 1 meV. The carrier density decreases by about six orders of magnitude between 300 and I K. The carrier mobility, on the other hand, increases by six orders of magnitude in the same temperature region. These two effects just cancel out and give rise to the temperature independent resistance. At low temperatures, the mobility goes up to a value as high as 10(6) cm(2)/(V S). (C) 2002 Elsevier Science B.V. All rights reserved.