Instability of MOSFET´s due to redistribution of Oxide charges

It has been reported that electrical instability of MOSFET´s occurs with the encapsulation of silicon nitride when these devices are stressed. In order to determine the mechanisms responsible, MOSFET´s with and without this capsulation were stressed under combinations of biases which were different from those previously utilized. It is observed that another form of instability is caused by the drain-gate-substrate interaction such that when an abnormally high bias is applied to the drain, oxide charges are redistributed in the direction parallel to the oxide-semiconductor interface. Measurements show that after the stress, net positive charge is accumulated above the channel adjacent to the drain while net negative charge is accumulated in the drain-gate overlap region. The presence of negative charge is apparently associated with hydrogen in the plasma silicon nitride. The positive charge is present with or without the nitride layer and our experiments point out another source of instability other than hot-carrier trapping in devices even without a SiN cap for a drain bias higher than some critical value. It is concluded that this instability is the same as that manifested by the well-known walk-out effect in gated diodes.