High-field electrical conduction in polyimide films

Electrical conduction current in polyimide films has been measured at high fields (>1.4 MV cm(-1)) using a Au metal (M)-polyimide film (P)-p-silicon (S)-MIS structure. On the basis of the current-voltage characteristics coupled with the capacitance-voltage characteristics measured under various conditions, it is found that electrical conduction at high fields is mainly due to Fowler-Nordheim type tunneling injection of electrons from the Au gate electrode if it is negatively biased, or from the PI/Si contact if the Au gate electrode is positively biased. The conduction current is strongly dependent on the concentration and the centroid location of the trapped charges. Computer simulation reveals that the trap concentration is of the order of 6X10(17) cm(-3) with its centroid located near the electron injecting contact, and that the trap ledge occurs only when the concentration of the net negatively trapped electron charge is large or the location of its centroid is close to the injecting contact. The decay of the dark charging current after the application of a step-function direct current field is associated mainly with the time-dependent trap-filling process. The photoconduction under an ultraviolet light illumination is due mainly to the photogeneration of free holes. Polyimide is very sensitive to the environmental humidity. Electrical conduction current increases and the breakdown strength decreases rapidly with increasing humidity. (C) 1999 American Institute of Physics. [S0021-8979(99)00810-5].