Charge-Modulated Extended Gate Organic Field Effect Transistor for Biosensing Applications

The interest in organic field effect transistors (OFETs) employed as a biosensing platform has grown in recent years, driven largely by the potential to create inexpensive, sensitive analytical devices with a wide range of chemical and biological sensing applications. A particularly promising architecture for these type of devices is the Charge-Modulated Organic Field-Effect Transistor (CM-OFET). In the CM-OFET, a control gate electrode is capacitively coupled to a floating gate and used to bias the OFET, eliminating the need for an additional, often macroscale, reference electrode. In addition, charge accumulated in a designated sensing region of the floating gate modulates the output source drain current, ISD, of the transistor, providing sensing activity that is spatially separated from the organic semiconductor layer.
Here, a CM-OFET based on solution processed Tips-pentacene as the organic semi-conductor that is both low cost and very simple to fabricate is reported. The CM-OFET biosensors fabricated here were predominantly based on the widely used Si/SiO2 substrates, where the degenerately doped Si acted as the gate electrode with a SiO2 dielectric layer. A limited number of Al/Al2O3 based CM-FETS are also presented. This thesis includes a detailed description of fabrication of these CM-OFET devices alongside a detailed discussion of the principle of operation, both as organic transistors and as analytical for monitoring pH and protein detection.
The thesis focusses primarily on the characteristics of CM-OFET devices based on the Si/SiO2 substrate. The fabrication of Si/SiO2 CM-OFETs was very simple, requiring only a single lithography or shadow evaporation stage. Despite the simplicity, the CM-OFETs reliably displayed electrical characteristics typical of organic field effect transistors. The electrical characteristics were reproducible with over 90% yield. However, the responses of the devices when tested for pH sensing and protein detection, were inconsistent and with large error. Further analysis of the CM-OFET architecture revealed limitations associated with the geometrical layout of the Si/SiO2 CM-OFET device may have caused this deficiency in sensing response.
A modified CM-OFET employing Al/Al2O3 as gate and gate dielectric layers was designed in which the geometry was optimized to maximise sensitivity to changes in charge within the sensing region. A process for the fabrication of the Al/Al2O3 CM-OFET was developed and the Al-based CM-OFETs were found to exhibit behaviour typical of an organic transistor, albeit with relatively lower source drain current compared to Si/SiO2 CM-OFET devices. Due to limited time, the sensitivity of the Al-based CM-OFET was not fully characterized. Further work regarding the enhancement of the device’s charge carrier mobility of the device and particularly, experimental investigation of the Al/Al2O3 CM-OFET for sensing applications is needed.