Stability of Thin-Film Transistors

Vacuum evaporated thin-film transistors have been fabricated using cadmium sulfide (n-type), cadmium selenide (n-type), and tellurium (p-type) for the semiconductor material. These devices have been made using both the staggered and coplaner geometries previously described by P.K. Weimer. Life test data has been taken on devices fabricated using these semiconductor materials. Correlation is made between the observed instabilities. Correlation is made between the observed instabilities in each of these materials. At least two distinct instability which takes place in some number of minutes depending on the device under examination. The "long term" instability takes place in some number of hours, again depending on the device under examination. We have observed that thermal processing of a thinfilm transistor subsequent to fabrication can substantially change the instability observed. A unit that exhibited an upward drift in drain current can be altered to reverse this drift and show a downward drift in drain current. The effect of thermal rocessing on thin-film transistors fabricated with each of these materials is considered. A further observation made was that the instability is strongly dependent on the vacuum conditions under which the device is fabricated. Units fabricated in an ion pump system behave quite differently than units fabricated in an oil diffusion system. Instability is also a strong function of gate insulator material. Life test data taken on a substantial number of thinfilm transistors fabricated under various conditions are also presented.