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Organic electrochemical transistors (OECTs) are promising transducers for biointerfacing due to their high transconductance, biocompatibility, and availability in a variety of form factors. Most OECTs reported to date, however, utilize rather large channels, limiting the transistor performance and resulting in a low transistor density. This is typically a consequence of limitations associated with traditional fabrication methods and with 2D substrates. Here, the fabrication and characterization of OECTs with vertically stacked contacts, which overcome these limitations, is reported. The resulting vertical transistors exhibit a reduced footprint, increased intrinsic transconductance of up to 57 mS, and a geometry-normalized transconductance of 814 S m . The fabrication process is straightforward and compatible with sensitive organic materials, and allows exceptional control over the transistor channel length. This novel 3D fabrication method is particularly suited for applications where high density is needed, such as in implantable devices.

With a host of new materials being investigated as active layers in organic electrochemical transistors (OECTs), several advantageous characteristics can be utilized to improve transduction and circuit level performance for biosensing applications. Here, the subthreshold region of operation of one recently reported high performing OECT material, poly(2‐(3,3′‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)‐[2,2′‐bithiophen]‐5‐yl)thieno[3,2‐b]thiophene), p(g2T‐TT) is investigated. The material's high subthreshold slope (SS) is exploited for high voltage gain and low power consumption. An ≈5× improvement in voltage gain (A V) for devices engineered for equal output current and 370× lower power consumption in the subthreshold region, in comparison to operation in the higher transconductance (g m), superthreshold region usually reported in the literature, are reported. Electrophysiological sensing is demonstrated using the subthreshold regime of p(g2T‐TT) devices and it is suggested that operation in this regime enables low power, enhanced sensing for a broad range of bioelectronic applications. Finally, the accessibility of the subthreshold regime of p(g2T‐TT) is evaluated in comparison with the prototypical poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the role of material design in achieving favorable properties for subthreshold operation is discussed.

Organic electrochemical transistors (OECTs) are receiving a great deal of attention as transducers of biological signals due to their high transconductance. A ubiquitous property of these devices is the non-monotonic dependence of transconductance on gate voltage. However, this behavior is not described by existing models. Using OECTs made of materials with different chemical and electrical properties, we show that this behavior arises from the influence of disorder on the electronic transport properties of the organic semiconductor and occurs even in the absence of contact resistance. These results imply that the non-monotonic transconductance is an intrinsic property of OECTs and cannot be eliminated by device design or contact engineering. Finally, we present a model based on the physics of electronic conduction in disordered materials. This model fits experimental transconductance curves and describes strategies for rational material design to improve OECT performance in sensing applications.

Equation 6 of this manuscript was incorrect in the originally published version. The correct equation is:The overall results reported are unaffected by this error. CORRECTION

Organic electrochemical transistors (OECTs) are transistors that can have extrinsic transconductances as high as 400 S m , but they typically have response times on the order of 1 ms or longer. These response speeds are limited by ion transport. It is shown that OECTs can exceed the ionic response speed by a factor of 30 when operated in a high-speed bias regime.

We fabricated and theoretically investigated an add-drop filter using an on-chip whispering gallery mode (WGM) microtoroid resonator with ultra-high quality factor (Q) side coupled to two taper fibers, forming the bus and drop waveguides. The new device design incorporates silica side walls close to the microresonators which not only enable placing the coupling fibers on the same plane with respect to the microtoroid resonator but also provides mechanical stability, leading to an add-drop filter with high drop efficiency and improved robustness to environmental perturbations. We show that this new device can be thermally tuned to drop desired wavelengths from the bus without significantly affecting the drop efficiency, which is around 57%. Index Terms-Whispering gallery mode (WGM), microresonator, add-drop filter, microtoroid, thermal effects I. INTRODUCTION PTICAL add-drop filters (ADFs) that add or remove narrow band wavelengths of light from a broader optical signal being carried along a bus waveguide are fundamental building blocks of optical transmission and communication systems. They are key elements in multiplexers, modulators and optical switches. Thus, great efforts have been put in designing ADF architectures with improved efficiency, spectral selectivity (i.e., quality factor), and spectral tunability. The past two decades have seen significant progress in ADF designs ranging from all-fiber architectures [1]-[5] to photonic crystal (PhC) structures [6]-[8] and waveguide coupled whispering gallery mode (WGM) microresonators (e.g., microsphere, microring, microdisk, microtoroid, etc) [9]-[15].All-fiber systems relying on Bragg gratings accomplish high drop efficiency but have quality factors of only about ∼ 10 3 , which yields a wavelength selectivity of about 0.7nm (or equivalently a frequency selectivity of ∼87.7GHz) [2]- [5]. Another all-fiber scheme using a fiber taper coupled microfiber knot, which works as a resonator, has been reported to have a quality factor of Q ∼ 1.3 × 10 4 [1]. In a two-dimensional PhC with a complete photonic band-gap, nearly 100% drop efficiency and a Q close to 700 have been numerically demonstrated, however, with a very small overall transmission (∼ 15% of the incident power) [6]. A proposal using PhC ring resonator has promised more than 96% drop efficiency with 160 < Q < 10 3 . These PhC based ADFs are fabricated with very high

We encapsulated a high-quality (Q) factor optical whispering gallery mode (WGM) microtoroid resonator together with its side coupled fiber taper inside a low refractive index polymer, achieving a final Q higher than 10 7 . Packaging provides stable resonator-fiber taper coupling, long-term maintenance of high-Q, a protective layer against contaminants, and portability to microtoroid resonator based devices. We tested the robustness of the packaged device under various conditions and demonstrated its capability for thermal sensing.