Technology Description

In general, the performance of traditional organic field effect transistors (OFETs) is poor compared to inorganic transistors, as evidenced by the low current output and high operating voltages of OFETs. This relatively poor performance results from both the conventional design of organic OFETs and the inherent low mobility of organic semiconductor materials.

Conventional OFET device design puts practical limits on the channel length as well as the area cross section for the source–drain current. As shown in Figure 1, conventional OFETs have a lateral relationship between the source and drain. Under this construct the space between the source and the drain needs to be minimized in order to reduce voltage need and to increase current output of the OFETs. However, the fabrication techniques used to set the spacing between the source and drain, leave the two electrodes relatively far apart thereby practically limiting the performance capabilities of the conventional OFETs. Additionally, conventional OFETs require the gate to be located between the source and the drain. This architecture limits the cross-sectional area for current flow and therefore limiting current output.

ORFID's VOFET overcomes these limitations with its novel architecture. Shown in Figure 2, this device architecture provides a very short channel length (< 0.1 µm) between the source and drain and an extremely large cross-sectional area, allowing low operating voltages (less than 5 V) and high current outputs (up to 10 mA). The channel length is determined by the thickness of the deposited organic film and the device current is determined by the overlap of the source and drain. Moreover, the device operating mechanism is quite different from traditional transistors. This difference is due to the modulation of charge injection by induced charges near the source electrode.

Key Advantages

One area of interest to ORFID is disposable applications. Large surface area displays (signage, for example) AMOLEDs and printable RFID are examples. Disposability is synonymous with ultra-low cost. The VOFET's architecture and performance make it suitable for such applications.

The architecture of the VOFET enables two key electrical performance characteristics: (1) high working current and (2) low operating, voltage as demonstrated in the figure to the right. Both characteristics are a direct consequence of the short channel length and large area cross-section defined by the overlap of the source and drain electrodes. The voltage-current features of the VOFET are ideally suited for OLEDs display applications.

From a manufacturing perspective, the VOFET is a structure simple to fabricate. The channel is easily constructed by deposition of the organic layer while controlling its thickness. There is no need for lithography. Moreover, because of its unique vertical structure most of its electrical properties are insensitive to modest variations in X-Y dimensions making the device suitable for manufacture by both printing and vacuum deposition technologies at or near room temperature. Low-temperature processing is the key to manufacturing AMOLEDs on plastic substrates and printing technology is key to ultra low-cost manufacturing. ORFID’s VOFET architecture is amenable to such fabrication conditions and processes.

Other types of transistor technologies have the potential to meet display requirements for AMOLEDs. Amorphous silicon (a-Si) and low-temperature polysilicon (LTPS) have suitable performance characteristics for AMOLED displays. However, these materials are typically processed at temperatures that are too high for plastic substrates. Furthermore, silicon-based technologies are not amenable to wide area, ultra-low-cost fabrication techniques, such as printing.

Conventional OFETs are made at low temperature and some have been shown to have high performance capabilities. However, these devices are not amenable to low-cost fabrication processes. A considerable amount of technology development is needed to reach this point. Conventional OFETs typically operate at much higher voltages and much lower currents than the VOFET rendering them unsuitable as pixel drivers for OLEDs.