Influence of active layer thickness on the cut-off frequency of a-IGZO thin film transistors

Abstract

Design optimizations like channel length and gate/contact overlap length scaling are generally used for cut-off frequency (fT ) enhancement of thin film transistors (TFT). But channel length scaling leads to mobility degradation due to contact resistance (RC ). RC in TFTs is directly proportional to the transfer length (LT ), and is inversely proportional to the overlap length (LOV ). LT depends upon the metal/semiconductor interfacial resistivity, the vertical bulk resistance and the semiconductor sheet resistance. For the same interfacial contact properties, active layer thickness (Ta ) scaling can be effectively utilized for lowering the vertical bulk resistance thereby lowering LT and enhancing fT . To study the effect of thickness scaling on the bandwidth of TFT based amplifiers, a-IGZO thin film transistor of effective channel thickness of 70nm are first fabricated and characterized. Density of states model of this TFT is extracted and numerical simulations are used to study the influence of active layer thickness scaling. By varying the channel lengths from 10μm to 100μm, transfer line method of contact resistance estimation, is used to extract LT and RC of devices. As the active layer thickness is varied from 70μm to 30μm it is found that LT decreases from 1μm to 60 nm, due to lower vertical bulk resistance. This results in a lower RC and higher μeff, facilitating further scaling of LOV , leading to enhanced fT of the device.