Electronic structures and device applications of transparent oxide semiconductors: What is the real merit of oxide semiconductors?

The main objective of this review is to provide an idea how to create new functions in oxides and how to find suitable applications only oxides can realize. Oxides have crystal and electronic structures largely different from those of conventional semiconductors such as Si and GaAs. Therefore, we should design suitable applications according to the inherent properties of oxide semiconductors if we intend to develop practical optoelectronic devices using oxides. In this review, we first briefly describe the characteristic features of oxide semiconductors from the viewpoints of crystal and electronic structures. Then three materials and related device applications are shown as examples. N-type amorphous oxide semiconductors (AOSs) can have electron transport properties superior even to silicon if they are in amorphous states. We propose that AOSs are favorable materials for active layers in low-temperature thin film device technology and demonstrate high-performance thin film transistors fabricated at room temperature on flexible plastic sheets. The second example is transparent p-uype semiconductors. Employing chalcogen orbitals and layered crystal structures, large hole mobilities, degenerate p-type conduction, and room-temperature stable excitions are rendered in wide bangap materials. Room-temperature operation of excitonic blue light-emitting diodes was thereby demonstrated. The last is 12CaO center dot 7Al(2)O(3) in which the use of subnanometer-sized cages and anions clathrated in the cages creates many chemical, optical, and electronic functions.