Quantum Materials - 08

Abstract

An antiambipolar transistor (AAT) has a PN junction in the transistor channel and exhibits a Λ-shaped transfer curve [1]. This unique conduction property has realized innovative logic circuits such as multivalued logic circuits, reconfigurable two-input logic circuits, and multivalued logic-in-memory [1,2,3]. In this study, we extended these findings to neuromorphic device operations, where a floating-gate-type AAT (FG-AAT) was employed. Versatile synaptic operations were enabled by applying pulse voltages with various conditions (polarities, widths and intervals of pulses). Importantly, the potentiation and depression properties were attained under the input signals with consistent polarity. This result implies that reconfigurable synaptic operations are possible by controlling the charge accumulation in FG, leading to the development of new neuromorphic applications.
The device structure of the FG-AAT is shown in Fig. (a), where C8-BTBT (16 nm) and PhC2-BQQDI (8 nm) films were used as p-type and n-type transistor channels, respectively. Additionally, a Au/Cr film acted as the floating gate. A 10 nm-thick polystyrene (PS) film was formed as the surface coating layer. Fig. (b) shows the drain current–gate voltage (I_D–V_G) characteristics of the FG-AAT at the initial (black), hole-trapped (red), and electron-trapped (blue) states. In the initial state, a Λ-shaped transfer curve was observed with a peak voltage (V_peak) of 1.8 V. The V_peak was shifted from 1.8 V to 2.8 V (blue line) by applying a positive pulse voltage (V_G = 5 V, width = 10 s) due to the electron accumulation in FG. Conversely, a negative pulse voltage (V_G = –5 V, width = 10 s) decreased V_peak from 1.8 V to 0.7 V owing to the hole accumulation in FG. The overall V_peak shift reached 2.1 V. This feature was then applied to neuromorphic device operations. When a negative pulse voltage (V_G = –5 V, width = 100 ms) was applied, ID increased, indicating long-term potentiation. Significantly, ID was transited from increase to decrease by applying the negative pulse voltages consistently. This operation coincides with the long-term depression (Fig. (c)). Usually, such synaptic plasticity is attained by applying the pulse voltage with the opposite polarity. However, Λ-shaped transfer curve induced in AAT enables the continuous change from potentiation to depression even with the same pulse voltages. This finding has the potential to develop reconfigurable synaptic operations using FG-AATs.