Ph.D Thesis

Ph.D StudentBen-Sasson Ariel
SubjectVertical Organic Field Effect Transistors Realized by
Block-Copolymers Self-Assembly
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisors PROF. Nir Tessler
PROF. Gitti Frey
Full Thesis textFull thesis text - English Version


The Vertical Organic Field Effect Transistor (VOFET) architecture presents a radical rethinking of the traditional, lateral, one - aimed at defeating the trade-off between process complexity and electrical performance. VOFETs and lateral OFETs share the same functional layers, including: Gate, Gate dielectric, Source, Semiconductor, and Drain. But while the channel length L - perhaps the most important structural parameter - is determined in lateral devices by lithography processes, in the vertical design it is simply determined by the semiconductor layer thickness. Hence, L can be easily downscaled to the scale of a few tens of nm - counterbalancing the low mobility of organic semiconductors, doing away with complex lithography processes.

Our study proposed that the vertical alignment of the transistor’s three electrodes can work at the transistor’s benefit by nano-structuring the sandwiched electrode - rendering it electrically transparent. To that aim, this work begins with developing self-assembly processes using block-copolymers to determine patterns, shape and dimensions, of thin films and pattern transferring methods for nano-scale engineering of electric fields transparent electrodes. Through its implementation we provided a proof of concept for the Patterned Electrode-VOFET. We proceeded to develop an efficient computational modeling tool in MatLab environment, tailored for modeling organic electronic devices. Our combined theoretical experimental approach provided a clear physical picture of the non-standard VOFET operational modes, switching mechanisms, structure-operation link, and a set of second generation design guidelines.

To make our case for the VOFET as a viable candidate for complementary circuit technology we focused on n-type organic semiconductors, due to their lower mobility compared to p-type ones. Our study has shown that the vertical transistor can drive high current densities (>10mA cm-2), operate at low-power and low-voltages (<3V), operate as an electronic switch (On/Off ratio >4x104) at high frequency (~MHz), and possess a uniquely efficient ambipolar behavior. Additionally, we extended our study on transparent electrodes fabrication methods, introducing new techniques: photolithography cleanroom-compatible methods, and entirely self-assembly solution-processable ones that are compatible with printing technology. 

The VOFET presents a platform ripe for rich scientific research and at the same time has the potential to be put into practical use both for complementary circuit technology, and as a fundamental current-driving and light emitting element in flat panel displays. As it constitutes a completely different platform than that offered by lateral OFETs, it will not be long before further purposes are identified, beyond those mentioned in this work.