Very recently, stacked two-dimensional materials have been studied, focusing on the van der Waals interaction at their stack junction interface. Here, we report field effect transistors (FETs) with stacked transition metal dichalcogenide (TMD) channels, where the heterojunction interface between two TMDs appears useful for nonvolatile or neuromorphic memory FETs. A few nanometer-thin WSe2 and MoTe2 flakes are vertically stacked on the gate dielectric, and bottom p-MoTe2 performs as a channel for hole transport. Interestingly, the WSe2/MoTe2 stack interface functions as a hole trapping site where traps behave in a nonvolatile manner, although trapping/detrapping can be controlled by gate voltage (V GS). Memory retention after high V GS pulse appears longer than 10000 s, and the Program/Erase ratio in a drain current is higher than 200. Moreover, the traps are delicately controllable even with small V GS, which indicates that a neuromorphic memory is also possible with our heterojunction stack FETs. Our stack channel FET demonstrates neuromorphic memory behavior of ∼94% recognition accuracy.

We study a low voltage short pulse operating multilevel memory based on van der Waals heterostack (HS) n-MoSe2/n-MoS2 channel field-effect transistors (FETs). Our HS memory FET exploited the gate voltage (VGS)-induced trapping/de-trapping phenomena for Program/Erase functioning, which was maintained for long retention times owing to the existence of heterojunction energy barrier between MoS2 and MoSe2. More interestingly, trapped electron density was incrementally modulated by the magnitude or cycles of a pulsed VGS, enabling the HS device to achieve multilevel long-term memory. For a practical demonstration, five different levels of drain current were visualized with multiscale light emissions after our memory FET was integrated into an organic light-emitting diode pixel circuit. In addition, our device was applied to a synapse-imitating neuromorphic memory in an artificial neural network. We regard our unique HS channel FET to be an interesting and promising electron device undertaking multifunctional operations related to the upcoming fourth industrial revolution era.

Band‐like transport behavior of H‐doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low‐temperature electrical measurements, where MoTe2, WSe2, and MoS2 are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n‐type conduction and their mobility increases without losing on‐state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p‐ or n‐type. Density functional theory calculations show that H‐doped MoTe2, WSe2, and MoS2 have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H‐doped TMD channels is metallic showing band‐like transport rather than thermal hopping. These results indicate that H‐doped TMD FETs are practically useful even at low‐temperature ranges.

Homojunction PN and PIN diodes based on 2D transition metal dichalcogenide (TMD) MoTe2 are reported in this work. Up to date, for PN junction diodes, type II‐based heterojunction diodes are mainly seen in report, but homojunction PN diodes using 2D‐layered materials are still rare although they enable seamless integration. Recently, hydrogen (H)‐doped n‐type MoTe2, achieved via atomic layer deposition (ALD) on top of a p‐type MoTe2 surface, was reported. Consequently, a lateral homojunction PN diode was realized by H‐doping. In fact, MoTe2‐based devices with a thickness on the order of nanometers can be applied for short‐wave infrared (SWIR) detection in the range of ≈1300 nm, a wavelength that Si‐based devices cannot properly address. Here, a seamless MoTe2 homojunction PIN diode is demonstrated, achieving the detection of visible to 1300 nm SWIR photons. This thin MoTe2 initially forms a PN junction by selective H‐doping, but a PIN diode is even obtained using two split gates. Compared to the PN diode mode, the PIN mode greatly enhances the photoresponse in the visible to 1300 nm wavelength range because of the increased built‐in electric field. The Franz–Keldysh effect is regarded strongly responsible for the effective absorption of 1300 nm SWIR photons in MoTe2. It is anticipated that this development may support Si photodetectors for integration on Si devices.