An adaptive thin-film transistor

A thin-film transistor (TFT) is described whose transfer characteristic can be reversibly adapted by a short duration voltage pulse applied to a high impedance gate electrode. The device is a four-terminal two-gate structure. A source, drain, and insulator gate contact form the basic TFT, while the amount and polarity of the polarization charge on the surface of the ferroelectric material of a second gate contact determines the pinchoff voltage of the TFT transfer characteristic. Measurements on experimental units demonstrate that the pinchoff voltage is adjustable over a sizable range, and that TFT transconductance changes in excess of 1000 to 1 can be obtained. The time required to change between different states of the TFT characteristic is limited by the switching time of the ferroelectric material which, in general, can be of the order of microseconds. Electrical instabilities in the transfer characteristics of the devices, however, may limit their practical circuit application. The instabilities are observed as a slow time variation of pinchoff voltage after a state has been established. Experimental units use triglycine sulfate for the ferroelectric material and tellurium-silicon monoxide thin film transistors.