An adaptive thin film transistor

Abstract

A thin film transistor (TFT) is described and demonstrated 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 of polarity of the polarization charge on the surface of the ferroelectric material of a second gate contact determines the pinch-off voltage of the TFT transfer characteristic. The characteristic is adaptive because once the polarization charge has been changed by an applied potential to the ferroelectric gate contact it will remain at its new value even if the potential is removed. The time required to adapt the characteristic is limited by the switching time of the ferroelectric material which, in general, can be of the order of microseconds. Experimental adaptive thin film transistors have been fabricated using triglycine sulfate for the ferroelectric (single crystal wafer used as substrate), evaporated silicon monoxide for the insulator and evaporated tellurium (p-type) for the semiconductor material. Preliminary electrical measurements indicate that the pinch-off voltage is adaptable over a sizable range and as a consequence, TFT transconductance ratios in excess of 100 to 1 are possible for maximum positive and negative polarization states of the ferroelectric material.