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A modified method of the signal flow graphs is used to visualize the causal relationships in the thermionic model of current flow in the metal-semiconductor contact. A bias-dependent barrier and internal resistance are assumed. The representative diagram contains two branches connecting the input voltage vertex V with the output current vertex J and a feedback loop attached to the output vertex and connected with the vertex R of the internal resistance. The mathematical apparatus coming with the diagrammatic method is used to evaluate current-voltage characteristics and the so-called transmission functions of diagram lines, in particular of the ones assigned to the VJ edge and the loop, under various conditions. The transmission function of the loop is always negative (which excludes possibility of sigmoidal J-V characteristics) and its influence is significant only at the upper end of the physically admissible voltage region.