Engineering the Interplay Between Ion and Electron Transport for Low-Power Transistors and Memory

The interplay between ions and electrons governs the performance of devices as ubiquitous as batteries, and processes as common as the biochemistry essential for life.  Our group uses ions to control electron and hole transport in two-dimensional (2D) materials for the development of beyond-CMOS, low-power transistors, and a new type of graphene-based flash memory.  We demonstrate reconfigurable p- and n-type doping of a 2D field effect transistor (FET) based on the transition metal dichalcogenide (TMD), MoTe₂; the ON/OFF ratio exceeds 10⁵ and the subthreshold swing is 90 mV/decade (ACS Nano, Article ASAP, DOI: 10.1021/nn506521p).  These values are achieved by exploiting the temperature-dependent relationship between ion and polymer mobility of the electrolyte gate.  By doping the channel p- or n-type and lowering the device temperature below the glass transition temperature (T_g) of the electrolyte, the ions are effectively "locked" into place with doping densities exceeding 10¹³ cm^-2 for both electrons and holes.  The lock-in temperature is well below room temperature (~220 K) for PEO-based electrolytes making this approach an impractical gating strategy; however, our group has shown that the lock-in temperature can be increased to room temperature by increasing the T_g of the polymer electrolyte.  The formation and relaxation dynamics associated with p- and n-type doping can be described by a stretched exponential, providing a way to quantify the temperature-dependent doping retention time.  Field-driven ion transport is modeled using COMSOL multiphysics.  This work highlights the application of chemical engineering and polymer physics principles to the development of beyond-CMOS nanoelectronics. 

This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.

Figure:  (a) Cross-sectional schematic of electrolyte-gated MoTe₂ field effect transistor (FET) (b) Transfer characteristics showing ~90 mV/decade subthreshold swing (c) Normalized drain current versus time capturing the temperature-dependent relaxation of the ions away from the channel surface (d) Temperature dependent relaxation time from both experiments and modeling.