Reliability of n-Type Organic Field Effect Transistors

The long time operational stability of C60 based n-type organic field effect transistors (OFETs) was investigated. The changes in the device characteristics were monitored under different conditions of bias stress up to 3000 hours. By measuring several cycles of measurements of transfer and output characteristics, the long time stability of C60 based OFETs and their reproducibility was documented. The major instability of the threshold voltage, was caused by trapping of charges in the active layer or at the interface of semiconductor and dielectric layer. The role of dielectric layers was quantified by choosing three different dielectric layers. It was found that the bias stress induced charges can be trapped in the active layer as well as in the dielectric layer. The charge trapping in the active layer happened ten times faster as compared to the trapping of charges in dielectric layers. The use of appropriate dielectric layers in C60 based OFETs increases the bias stress stability up to 55%. Furthermore, it was shown, that the fabrication of electrically stress stable devices is possible by using C60 layers grown at higher substrate temperature resulting in larger grain sizes.