12CaO·Al2O3 (C12A7) has a unique crystal structure composed of positively charged cages ∼0.4 nm in inner diameter and a free oxygen ion (O2-) clathrated in one-sixth of the cages. C12A7 can be converted to inorganic electride by replacing the clathrated oxygen ions with electrons, and the electride exhibits degenerate-type conduction with room-temperature conductivities >100 S cm-1. In intermediate states, semiconductive C12A7 can be obtained by controlling electron density. In this study, we examined effects of electric field on carrier transport properties of the semiconductive C12A7 using a field-effect transistor (FET) structure targeted for future mesoscopic devices and electrochemical devices that will utilize the quantum-dot-like cage structures and chemically active clathrated anions in C12A7. FETs were fabricated using two types of samples, (i) single-crystalline bulk and (ii) polycrystalline thin films, for channels. First, conditions to form good contacts for source and drain electrodes were examined because the semiconductive C12A7 has a small work function, and it was difficult to form good electrical contacts with metals. It was found that Pt was the best metal with the lowest contact resistance to C12A7, and thermal annealing at ≥300 °C improved its non-ohmic characteristics. Electrical conductivity was modulated by 1-2 orders of magnitude by applying gate voltage. Apparent field-effect mobilities were 0.02-0.08 cm2 (V s) -1, which were comparable with the drift mobilities of the semiconductive C12A7. This is the first demonstration of a semiconductor device using an electride.
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