FIELD EFFECT AND SPACE-CHARGE-LIMITED THIN FILM TRIODES.
Quarterly rept. no. 3, 1 Jan-31 Mar 64,
RAYTHEON CO WALTHAM MA
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Shelf and operational life tests continued. Vacuum and selenium encapsulated TFT units now show a stability equivalent to that of germanium transistors. A study has been made of various phases of instability which occur in encapsulated thin-film transistors when bias is applied. It has been found that the channel conductivity decreases for positive gate bias and increases for negative gate bias. A steady-state conductivity is reached after about four hours and remains steady for as long as 1000 hours while bias is applied. After removal of the bias, the conductivity returns to its initial value in about 24 hours. At 70 C the time constants are about half of the values at room temperature. The amount of change varies from unit to unit, being larger 25 per cent average for low-conductivity units and negligible for high-conductivity units. Related to these instabilities are changes in channel conductivity which occur when units are handled. These are attributed to static discharges through the insulator. A preliminary comparison one unit operated for 1000 hours of evaporated selenium coating of the finished TFT and vacuum encapsulation indicates that the former process may be as good as the latter. Author