Geometrical Structure and Interface Dependence of Bias Stress Induced Threshold Voltage Shift in C60-Based OFETs

The influence of the nature of interface between organic semiconductor and gate dielectric on bias stress electrical stability of n-type C-60-based organic field effect transistors (OFETs) was studied. The bias stress induced threshold voltage (Vth) shift was found to depend critically on the OFET device structure: the direction of Vth shift in top-gate OFETs was opposite to that in bottom-gate OFETs, while the use of the dual-gate OFET structure resulted in just very small variations in Vth. The opposite direction of Vth shift is attributed to the different nature of interfaces between C-60 semiconductor and Parylene dielectric in these devices. The Vth shift to more positive voltages upon bias stress in bottom-gate C-60-OFET was similar to that observed for other n-type semiconductors and rationalized by electron trapping in the dielectric or at the gate dielectric/C-60 interface. The opposite direction of Vth shift in top-gate C-60-OFETs is attributed to free radical species created in the course of Parylene deposition on the surface of C-60 during device fabrication, which produce plenty of hole traps. It was also realized that the dual-gate OFETs gives stable characteristics, which are immune to bias stress effects.