Causal relationships between electric current, field intensity and the internal variables determining the current-voltage characteristic can be topologically represented, paralleling, in a certain sense, the use of Feynman diagrams in quantum field theory. In connection with the diagrams thus formed, quantitative formalism can be developed. The differential equation of the current-voltage characteristic di/du=Σ d (i, u), withΣ d the so-called differential conductivity function, constitutes the main result. Presence of a positive feedback loop in the diagram pattern seems to be an obligatory condition for the occurrence of bulk-originated current-controlled negative differential resistance.