Ultrathin Indium Tin Oxide Accumulation Mode Electrolyte‐Gated Transistors for Bioelectronics

Abstract

Electrolyte‐gated field effect transistors and electrochemical transistors have emerged as powerful components for bioelectronic sensors and biopotential recording devices. A set of parameters must be considered when developing devices to amplify weak electrophysiological signals. These include maximum transconductance values, cut‐off frequencies, and large on/off current ratios. Organic polymer‐based devices have recently dominated the field, especially when considering flexibility as a key factor. Oxide semiconductors may also offer these features, as well as advantages like higher mobility. Herein, flexible, ultrathin, indium tin oxide (ITO) electrolyte‐gated transistors are reported. These accumulation‐mode devices combine n‐type operation with µe = 9.5 cm2 Vs−1, high transconductance (gm = 44 mS), and on/off ratios (105) as well as optically transparent layouts. While oxides are normally considered brittle, mechanically flexible ITO layers are obtained by room temperature deposition of amorphous layers onto parylene C. This process results in low strain, producing devices that survive bending. ITO electrochemically degrades, however, with cycling. To overcome this, the surface is passivated with high dielectric constant inert capping layers of Ta2O5 or Ta2O5/AlN. This greatly improves stability while preserving low gate voltages. Based on their overall performance, ITO‐based EGFETs are promising for bioelectronics.