  We present the multilevel fabrication and measurement of a Coulomb-blockade
device displaying tunable negative differential resistance (NDR). Applications
for devices displaying NDR include amplification, logic, and memory circuits.
Our device consists of two Al/Al$_{x}$O$_{y}$ islands that are strongly coupled
by an overlap capacitor. Our measurements agree excellently with a model based
on the orthodox theory of single-electron transport.
