Modeling and realization of an amorphous silicon device with negative differential resistance

We present the modeling and realization of a two terminal hydrogenated amorphous silicon device with bistable current-voltage (I-V) characteristics, potentially suitable to obtain a new generation of switch and memory in large area application. The structure is basically constituted by three stacked junctions: p⁺-i-n^-, n^--i-p^-, p^- -i-n⁺. A numerical device model allows us to investigate in detail the device behavior pointing out the fundamental role of the two lightly-doped n^- and p^- layers. In the OFF condition the central junction is reverse biased and limits the current in the device. The transition OFF-ON starts when, increasing the applied voltage, one of the two lightly-doped layers becomes completely depleted. In the ON state high injection of both carriers from the external-doped layers completely hides the doping concentrations of the bases and the device behaves like a single forward biased p⁺-i-n⁺ structure. The transition ON-OFF occurs when, decreasing the applied voltage, the free carrier densities in the lightly-doped layers become lower than the dopant concentrations. This transition is thus mainly dependent on the recombination processes occurring in the central-doped layers. Devices with hysteresis around 2 V and turn-on voltages ranging from 12 to 15 V have been obtained