(A) systematic approach to identifying the nature of source contacts in organic field-effect transistor

Abstract

Performance of organic field-effect transistors (OFETs) is influenced by various factors such as the mobility of organic semiconductors, dielectric constant and thickness of gate dielectrics, channel geometry, dielectric/ semiconductor interfaces, and contact resistances at source and drain contacts. Recently, contact engineering and thus measurement of contact resistance are increasingly becoming vital as the channel resistance continues to reduce with the development of high-mobility materials. Previously reported methods for contact resistance measurement, however, had limitations, for example, in terms of separate source and drain extraction and/or gate voltage dependency, calling for an alternative method. The aim of this study is to present a new systematic methodology which overcomes such limitations. The study specifically focuses on the extraction of the source contact resistance, as the effect of the source contact resistance could particularly be detrimental. Based on device physics, it is shown that the gate voltage dependent source contact voltage Vss can be expressed by two values, namely the intrinsic and extrinsic field effect mobility, the former of which can be extracted from so-called transmission line method (TLM). Upon reorganization of data that are obtained from standard FET characterization, the current-voltage (I-VSS) characteristics of the source contact can be obtained whether the contact is limited by resistance, diode-like rectifying contact, or their combination. This theoretical derivation procedure is verified by simulation. Extraction of the source I-V curve is successfully accomplished for various source components, proving the validity of the proposed method. The study presents a new systematic approach to obtain not only the distinguished source contact resistance with gate voltage dependency, but also the I-V characteristics of such contacts, which can provide one with additional insights to its nature in a self-consistent manner.