High-frequency current oscillations in solid dielectrics

Abstract

Experimental evidence is provided that common industrial organic insulators (phenol-impregnated pressboard, bakelite) may exhibit negative differential resistance. Insulators have been subjected to high non-uniform electric fields with maximum values which varied between 0.2 MV cm-1 and 1.2 MV cm-1 and, according to experimental data, the time-averaged I-V characteristics showed a distinct N-type NDR region, followed by an S-type NDR region, for electric fields near the pre-breakdown region. Various impulse voltages have been applied to solid dielectrics and periodic current pulses were received by the anode electrode. The aging region, at about 0.8 MV cm-1, was associated with successive current pulse formation with typical values of the order of 2 A and frequencies of the order of 1 MHz. During the breakdown (which occurred for applied electric fields of the order of 1.2 MV cm-1) only two or three current pulses of 20-40 A were observed. An attempt has been made to interpret and correlate the above results with similar phenomena which occur in semiconductors with a negative differential resistance.