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Results of an experimental study are reported of a new instability found in p- and n-channel MOS transistors. This phenomenon is that when a higher voltage in an excess of a brakdown voltage is applied to the drain electrode the breakdown voltage drifts to a higher value and the drain current also increases. The origin of this instability is investigated by extensive measurements and analyses of the electrical characteristics of the transistors. It is concluded that 1) the semiconductor surface near the drain becomes p-like in the p-channel transistors and n-like in the n-channel transistors and thus the active channel length is shortened, 2) this is caused by charging of the gate oxide due to injection of electrons or holes generated during the drain avalanche breakdown, and 3) electron and hole injection is much affected by electric field across the oxide over the drain junction.

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