One important technical hurdle that has to be overcome for using organic transistors in radio-frequency identification tags is for these devices to operate at rf frequencies (typically 13.56MHz) in the front end. It was long thought that organic transistors are too slow for this. In this letter we show that organic transistor based full-wave rectifier circuits utilizing pentacene, a p-channel organic semiconductor, can operate at this frequency with a useful efficiency. In order to achieve such high-frequency operation, we make use of the nonquasistatic state of the transistors.
In this work we announce a ferroelectric charge transfer device that uses a high-k buffer layer underneath the ferroelectric layer as a tunnel oxide, where the high electric field required for Fowler-Nordheim charge tunneling through the buffer layer is provided by the polarized ferroelectric. This device exhibits data retention of 10 years at room temperature while retaining the properties of fast write and low voltage operation. Since the electric field across a buffer layer due to the ferroelectric polarization surface charge is ∼E = Pr ε 0 εox then with P r on the order of 10 µC/cm 2 and ε ox for a high k dielectric is ∼20, we have an electric field of 5.6 MV/cm which is sufficient to generate significant tunnel currents across a high-k buffer layer. Charges tunneling from the Si substrate are trapped in the interface between the high-k buffer and the ferroelectric and only detrap thermally; this leads to the long data retention observed on these devices. Despite the fact that charge transfer is accomplished by tunneling, a relatively slow process, this device can be programmed or erased in only the time it takes to switch the ferroelectric, which is of the order of nanoseconds.
One of the potential application areas for organic and polymers transistors is in radiofrequency identification (RFID) tags. One of the key components of an RFID tag is the front-end rectifier that must rectify a 13.56 MHz AC signal received from a resonant tuned antenna. The rectifier supplies operating power to the tag. Organic transistor circuits have hitherto not operated at this high frequency. We show that by operating pentacene transistors in the non-quasi-static (NQS) regime such operating speeds can be achieved in rectifier circuits. The circuits were fabricated on flexible plastic substrates and employed a solution-cast dielectric. The pentacene mobilities are in the range 0.1-1.5 cm2/V-s. The channel lengths of the transistors are in the range 2-4 µm. Full-wave NQS mode rectifiers were measured to have voltage rectification efficiency in excess of 28% at 14 MHz, demonstrating that such circuits can be used in RFID tags. These circuits operated successfully at speeds up to 20 MHz.
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