Moiré heterostructures exhibit gate-controllable exotic electronic phenomena, which are promising for future electronic devices, but most phenomena are limited to cryogenic temperatures. Now, Yan and colleagues demonstrate a moiré-heterostructure-based neuromorphic transistor that enables reconfigurable synaptic response and adaptive learning at room temperature (Nature 624, 551–556; 2023).
The most interesting aspect of this moiré transistor is an emergent electronic ratcheting effect arising from the dynamical interplay between the two subsystems. The authors apply a varying voltage bias (VTG) at the top gate from 0 to ΔVTG and then back to 0 within a time interval. They record the drain current during the interval and extract the electron (hole) density (denoted as ne and nh) when VTG comes back to 0 (where VD is the drain voltage and VBG is the back-gate voltage). The electron (hole) density remains unchanged up to a threshold ΔVTG, but begins to increase linearly with larger ΔVTG (panel c). This can be understood as follows: when the top-gate voltage increases (forward sweeping), it adds the electron (hole) into the localized states formed by the moiré potential. The threshold indicates fully occupied localized states. Then, if the top-gate voltage continues to increase, the electron (hole) will be added to the conducting channel. The interesting point happens during the backward sweeping process (VTG decreases from ΔVTG to 0). The added localized electrons are removed from the moiré potential, but those in the conducting channel cannot be removed, resulting in a hysteresis behaviour in the drain current with respect to the sweeping voltage. This electron (hole) ratchet is similar to a mechanical ratchet that can move clockwise (anticlockwise) but cannot move anticlockwise (clockwise) (insets of panel c).
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