Semiconductor, Amorphous

a substance in the amorphous solid state that has the properties of a semiconductor. Amorphous semiconductors are divided into three groups: covalent amorphous semiconductors, such as amorphous Ge and Si, InSb, and GaAs; chalcogenide glasses, such as As₃₁Ge₃₀Se₂₁-Te₁₈;; and oxide glasses, such as V₂O₅-p₂O₅, and dielectric films, such as SiO_x, Al₂O₃, and Si₃N₄.

The energy spectrum of amorphous semiconductors differs from that of crystal semiconductors in the presence of density “tails” of electronic states that penetrate the energy gap. According to one theory, an amorphous semiconductor should be considered as a heavily doped, heavily compensated semiconductor such that the bottom of its conduction band and the top of its valence band fluctuate. These fluctuations are large-scale and of the order of the width of the energy gap. Electrons in the conduction band and holes in the valence band are divided into a system of “droplets” located in potential wells and separated by high barriers. Electrical conduction in amorphous semiconductors is accomplished at very low temperatures by means of electron tunneling through the barriers between wells in a manner analogous to hopping conduction. At higher temperatures, electrical conduction is due to the thermal excitation of carriers into higher energy levels.

Amorphous semiconductors have various practical applications. Chalcogenide glasses are used in television camera tubes and for hologram recording because of their transparency to infrared radiation, high resistance, and high photosensitivity. Dielectric films are also used in metal-dielectric-semiconductor structures.

In systems consisting of an amorphous semiconductor film between two metals, the rapid (— 10^-l0 sec) transition (switching) of the amorphous semiconductor from a highly resistive state to a conductive state is possible when the applied voltage exceeds a threshold voltage. In particular, there exists memory switching wherein the highly conductive state is retained even after the voltage is removed; the memory is usually erased by a short intense current pulse. The conductive state in memory systems is due to the partial crystallization of amorphous semiconductors.