The Trade-Off between Transconductance and Speed for Vertical Organic Electrochemical Transistors

The high transconductance gm of organic electrochemical transistors (OECTs) has received widespread attention and made OECTs a valid candidate for wearable sensor systems. However, the large transconductance is often accompanied by large switching time constants, tau, making the transconductance an imperfect benchmark. For a fair assessment, the ratio of transconductance to switching time constant gm tau$\frac{g_m}{\tau }$ has to be considered instead of any single parameter in isolation. One approach put forward to optimize OECTs is a vertical design, in which the channel length can be scaled into the sub-micrometer regime. Here, a new vertical device geometry is proposed, in which the active volume of the mixed conductor is confined to a small cavity between source and drain, yielding excellent performance and reproducibility. It is shown that this approach yields optimized gm tau$\frac{g_m}{\tau }$ ratios instead of maximized transconductance only. However, this scaling is effective only in a small range of device dimensions and requires careful optimization of the device to not be limited by parasitic effects such as excess volume of the mixed conductor or parasitic series resistances. Overall, the design considerations discussed here provide new guidelines to optimize OECTs, not only for high transconductance but for operation at high frequencies as well. The high transconductance of organic electrochemical transistors (OECTs) has made OECTs a valid candidate for wearable sensor systems. However, its large transconductance is often accompanied by large switching time constants, making the transconductance alone an imperfect benchmark. Here, the trade-off between transconductance and speed of OECTs is studied, and a vertical transistor design to optimize this balance is proposed. image