(Invited) High-Frequency Operation of Vertical Organic Field-Effect Transistors

Abstract

Organic thin-film transistors (OTFT) are attracting great interest in view of flexible electronics enabling future revolutionizing applications such as flexible displays or wearable electronics. Due to the poor intrinsic charge carrier transport in organic semiconductor thin-films, the main target of development in the last two decades was to find suited semiconductor materials offering a higher charge carrier mobility. However, although record values of clear beyond 10cm²/Vs have been demonstrated, key transistor parameters such as the transconductance or transition frequency have not been improved substantially. In consequence, the performance of today's OTFTs is still too low for many demanding electronic applications. In the last couple of years we have develop various architectures for vertical organic transistor offering superior current driving capabilities compared to state-of-the-art horizontal organic TFTs. Within this 3-dimensional VOFET geometry, the channel length is not defined by the applied patterning technique but rather by the thickness of the organic layers, typically <300nm, separating the vertically stacked source and drain electrodes. This ultra-short channel configuration allows for superior current driving, however, it is usually accompanied by undesired short-channel effects such as loss of saturation and an increased off-current level. In my presentation I introduce the VOFET concept and highlight the development of optimized transistor geometries leading to superior performance with an on/off ratio>10⁷, transition frequency approaching 100MHz and excellent operational stability. In the second part of the contribution I focus on the potential of VOFETs for active matrix OLED (AMOLED) display driving. Comparing typical TFT design guidelines for vertical and horizontal TFTs, it can be deduced that the VOFET design leads to a considerable reduction of area consumption within a pixel which ultimately enables a higher display resolution and a lower power consumption. The concept of vertical transistors is not restricted to organic semiconductors. Hence, I discuss in the third part of my presentation the potential of high mobility metal-oxide semiconductors for vertical thin-film transistors for GHz operation.