Organic thin film transistors (OTFTs) have attracted considerable interest because of their flexibility, low cost, light weight and easy process over large areas. Such various merits gave promising application of OTFTs to electronic circuits, displays, and sensors.
Attempts to enhance the performance of OTFTs have mainly relied on the decrease of the channel length and the increase of the cross-sectional area for the larger operation currents. Vertical type-OTFT is one of the best alternatives to decrease the channel length and increase the channel width, effectively at the same time.
We fabricated the vertical junction transistor via µ-contact printing method combined with Langmuir-Schaefer technique for the fine patterning of three terminals, i.e., collector, emitter and base.First, we made the pattern of collector with Au-nanoparticles (Au-NPs) on the substrate using polymeric stamp. Subsequently, the sandwich structure consisting of the bottom organic layer of poly-(3-hexylthiophene) (P3HT), the base of Au-NPs, the top organic layer of P3HT, and the emitter of Au-NPs was fabricated. For the improvement of the Au-NP electrodes, the whole structure was thermally annealed in vacuum at 170 °C after formation of Au-NP patterns. The active channel was made of p-type P3HTs and the schottky contacts were formed at the emitter and collector regions by µ-contact printing techniques.
NPN like junction transistor, we made, consists of two schottky junction diodes connected in a back-to-back configuration by a shared anode region. OJT could control the larger current by a small bias current like a conventional transistor. The formation of the ohmic contacts in the base region enables the easy injection of holes through the emitter, acting as a NPN-type transistor. So the control of the contact properties was crucial to achieve the proper operation of organic junction transistors. By use of the soft-lithography techniques without any other conventional lithography, we could fabricate a vertical type junction transistor with low operating voltages of a few volts, but high current levels of a few milliamps. We will discuss the operation mechanism of such formed junction transistor in terms of the electrical contact between metal and semiconductor, in detail. |