A vertically integrated resonant tunneling device with multiple negative differential resistances

It was recently shown that connecting two resonant tunneling diodes (RTDs) in parallel so that the bias across them was offset resulted in a current-voltage (I-V) characteristic with two separate negative differential resistance (NDR) regions. More recently, two discrete InAlAs/InGaAs RTDs were combined in tandem to obtain near-ideal room-temperature I-V characteristics that had two well-defined NDR regions. Using molecular-beam epitaxy, the authors have extended this idea by vertically integrating five InAlAs/InGaAs double-barrier RTDs in sequence to obtain a vertically integrated diode (VID) that had five NDR regions. The five tunneling structures were separated from each other by 500-Å n⁺ InGaAs layers that which destroyed electron coherence between the tunneling regions, so that each resonant-tunneling structure switched sequentially with increasing bias. The VID has been used to demonstrate a multilevel memory element that has five distinct voltage states that can be set by using small current pulses. The VID was also used in a circuit to generate the parity of an 11-bit word