No “solid state” here: these transistors are organic and biocompatible

June 18, 2020 //By Bill Schweber
organic
Researchers created both organic, biocompatible transistors as well as mixed-conducting particulate material that can implement multiple, nonlinear, dynamic electronic functions.

We generally associate the term “semiconductor” with the phrase “solid state” as if they’re the same thing. That makes sense, given that our industry’s literal and figurative foundations are usually based on solid physical elements such as silicon, geranium, and gallium. Nonetheless, a whole world of “organic” transistors has attracted considerable attention and has little relation to those in the solid state.

Recently, a multidisciplinary research team at Columbia University developed ion-driven organic transistors that are designed to record individual neurons and perform real-time computation. These biocompatible devices could enable better diagnosis and monitoring of neurological diseases.

Professor Dion Khodagholy, who directs the university’s Translational NeuroElectronics Lab, noted “Instead of having large implants encapsulated in thick metal boxes to protect the body and electronics from each other, such as those used in pacemakers, and cochlear and brain implants, we could do so much more if our devices were smaller, flexible, and inherently compatible with our body environment.”

Silicon-based devices can’t function in the presence of ions and water; they need to be encapsulated in the body. Further, as electron-based devices, they don’t interact well with the ionic signals that the body’s cells use to communicate. Some organic materials have been developed to overcome these limits, but their electrical performance has been inadequate for the target applications.

In this project, the ionic conduction of organic materials is used to create ion-driven transistors from a composite of poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (designated as PEDOT:PSS) and polyethylenimine (PEI). These enhancement-mode, internal ion-gated organic electrochemical transistors (e-IGTs) have embedded mobile ions inside their channels (Fig. 1).
Since the ions don’t need to travel long distances to be a part of in the channel switching process, they can be switched on and off quickly and efficiently. This results in devices with high transconductance and fast speed that can be independently gated to create scalable conformable integrated circuits.


Fig. 1: Structure and steady-state characteristics of IGT: (A) Schematic illustration of IGT cross-section and wiring diagram for device operation (top); d-sorbitol creates an ion reservoir, maintaining mobile ions (green) that can travel within the channel. PEDOT-rich regions are shown in light blue and PSS lamellas (flakes) in white (bottom). (B) Optical micrograph displaying the top view of an individual transistor (scale bar, 20 µm). Inset shows a cross-sectional scanning electron microscopy (SEM) image acquired at a tilt angle of 30°. Ion membrane (light red), channel (light blue), and Au contacts for gate (G) and source (S; beige) are visible (scale bar, 5 µm). (C) Output characteristics (ID − VD) of IGT device (L = 5 µm, W = 500 µm) for gate voltage (VG) varying from 0 V (top curve) to +0.6 V (bottom curve) with a step of +0.1 V; color intensity corresponds to VG amplitude.(D) Transfer curve for VD = −0.6 V (black), and the corresponding transconductance (orange), |gmmax| = 32.30 mS. (Source: Columbia University)


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