Organic Channel FET Device Used as pH Sensor
Brian Dance, Contributing Editor -- Semiconductor International, 6/1/2001
Carmen Bartic and researchers at IMEC (Leuven, Belgium) have developed organic field effect transistors (OFETs) for use as sensing devices for charge detection in aqueous media. In their OFET, the silicon nitride above the organic channel is in direct contact with the solution being tested. The electrochemical potential developed at the interface between the solution and the gate dielectric determines the current passing through the organic semiconductor channel of the device.
Such OFETs may be compared with conventional ion-sensitive FETs (ISFETs) produced with CMOS technology on a silicon substrate. ISFETs have long-term stability problems and are not easy to manufacture. However, ISFETs offer the advantages of small dimensions, good high- to low- impedance conversion and the possibility of multi-ion sensing. Organic materials may offer processing advantages over semiconductors, such as the use of a relatively cheap glass or polymer substrate and lower-temperature processing. If OFETs can be produced cheaply, they may be used as disposable biosensors in health applications, such as the monitoring of body fluids, where reuse is undesirable.
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| Two layers each of silicon dioxide and silicon nitride were deposited sequentially on each side of the wafer, and the silicon was completely removed above the channel area. (Source: IMEC) |
The IMEC researchers employed a silicon wafer purely as a mechanical support that takes no part in device operation. In future devices, the silicon will be replaced by a plastic substrate for reduced cost. Two layers of silicon dioxide and two layers of silicon nitride were deposited sequentially on both sides of the wafer. The silicon was completely removed above the channel area (Figure).
In the absence of a metallic gate, there was only a thin membrane over the channel. The thickness of the oxide (470 nm) and of the nitride (150 nm) was chosen for reduced mechanical stress after etching. The silicon nitride was the sensitive gate dielectric material. It also formed a stopping layer for the anisotropic etching that was used to remove the silicon from this area, leaving a 1 × 1 mm sensitive area.
The 100 nm thick gold source and drain interdigitated electrodes formed ohmic contacts onto the conduction channel. The finger width and the spacing between two fingers were both 10 µm. These electrodes were deposited with a high width-to-length ratio because the organic semiconductor that formed the channel had a low conductivity. The electrodes were covered with a layer of poly(3-hexylthiophene) (P3HT) deposited by spin coating from a 0.8% by weight solution in chloroform. This behaves as a p-type semiconductor with a hole mobility of 10-2 cm-2/Vsec. It was selected for its good solubility, ease of processing and good environmental and thermal stability. The conducting channel formed at the interface between the dielectric and the organic semiconductor. The device was encapsulated in an epoxy material.
The electrical characteristics were measured at room temperature in buffer solutions with pH values of 2 to 10. A silver/silver chloride reference electrode was also placed in the buffer to form a three-terminal device biased like a conventional FET in the common source circuit. The reference electrode acted like a metallic gate in the organic transistor.
If the drain-to-source potential was kept constant at -10 V and the reference voltage -2 V, the drain current varied linearly with pH over a range of 4-10. The drain current changed by 20-30 nA per unit of pH change. This rather low output sensitivity resulted from the inferior semiconducting properties of the P3HT organic material relative to those of crystalline semiconductors. Low carrier mobilities result in low values of transconductance.
The researchers say that the device transconductance can be optimized for better characteristics by improving parameters such as carrier mobility, electrode geometry and gate dielectric capacitance. The use of materials of higher dielectric constant and a greater density of surface sites will improve both the organic transistor action and the pH sensitivity.
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