  In the last decade, a 2-terminal passive circuit element called a memristor
has been developed for non-volatile resistive random access memory and has more
recently shown promise for neuromorphic computing. Compared to flash memory,
memristors have higher endurance, multi-bit data storage, and faster read/write
times. However, although 2-terminal memristors have demonstrated basic neural
functions, synapses in the human brain outnumber neurons by more than a factor
of 1000, which implies that multiterminal memristors are needed to perform
complex functions such as heterosynaptic plasticity. Previous attempts to move
beyond 2-terminal memristors include the 3-terminal Widrow-Hoff memistor and
field-effect transistors with nanoionic gates or floating gates, albeit without
memristive switching in the transistor. Here, we report the scalable
experimental realization of a multi-terminal hybrid memristor and transistor
(i.e., memtransistor) using polycrystalline monolayer MoS2. Two-dimensional
(2D) MoS2 memtransistors show gate tunability in individual states by 4 orders
of magnitude in addition to large switching ratios with high cycling endurance
and long-term retention of states. In addition to conventional neural learning
behavior of long-term potentiation/depression, 6-terminal MoS2 memtransistors
possess gate-tunable heterosynaptic functionality that is not achievable using
2-terminal memristors. For example, the conductance between a pair of two
floating electrodes (pre-synaptic and post-synaptic neurons) is varied by 10X
by applying voltage pulses to modulatory terminals. In situ scanning probe
microscopy, cryogenic charge transport measurements, and device modeling reveal
that bias-induced MoS2 defect motion drives resistive switching by dynamically
varying Schottky barrier heights.
