(A) study on organic vertical field-effect transistors and their application to phototransistors

Abstract

Organic field-effect transistors (OFETs) offer a promising prospect for future electronics in various emerging fields such as flexible displays, sensor arrays, and memory devices due to the versatility of organic semiconductors. Even with the great advances for OFETs over the last few decades, the inherent low electrical mobility of organic semiconductors is one of the difficulties with respect to the real-life application of OFETs. In order to overcome the limitation of OFETs, as well as the development of high mobility organic semiconductors, numerous studies have intensively focused on improving device performance through the channel length scaling of OFETs. In this dissertation, organic vertical field-effect transistors (OVFETs) are investigated as a device with a novel architecture that can solve some of the limitations found in conventional OFETs. Although there have been many types of studies dedicated to the development of OVFETs, they often lack in detailed understanding of the operation principles and identification of key device parameters. As an effort to address these issues, 2D device simulation is adopted to elucidate the operation principles of OVFETs. Furthermore, we modify the OVFET architecture to incorporate bulk-heterojunction active layers and thus realize organic vertical phototransistors (OVPTs). In particular, source-contact limited behavior of OVFETs is utilized in a way that source contact resistance can be modulated via photocarrier generation, yielding high responsivity with a significant contrast from the dark reference signal. As a result, the proposed device shows high responsivity of ca. $10^3$ A/W in the visible region ($\lamda = 535 nm$) and on/off ratio up to $10^6$ with low operation voltage (< 10 V). This study will open a path for the design of high-performance OVFETs with a novel architecture and extends the feasibility of OVFETs in wide range applications.