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The properties of photodetectors based on two-dimensional materials can be significantly enhanced by avalanche effect. However, a high avalanche breakdown voltage is needed to reach impact ionization, which leads to high power consumption. Here, we report the unique features of a low-voltage avalanche phototransistor formed by an in-plane WSe2 field effect transistor (FET) with an out-of-plane WSe2/WS2 P—N heterojunction (HJ FET). The avalanche breakdown voltage in the device can be decreased from −31 to −8.5 V when compared with that in WSe2 FET. The inherent mechanism is mainly related to the redistributed electric field in the WSe2 channel after the formation of the out-of-plane P—N heterojunction. When the bias voltage is −16.5 V, the photoresponsivity in the HJ FET is enhanced from 1.5 to 135 A/W, which is significantly higher than that in the WSe2 FET because of the obvious reduction of the avalanche breakdown voltage. Moreover, HJ FET shows a higher responsivity than WSe2 FET in the range of 400–1,100 nm under low bias voltage. This phenomenon is caused by accelerating electron—hole spatial separation in the heterojunction. These results indicate that the use of an WSe2 FET with an out-of-plane WSe2/WS2 heterojunction is ideal for high-performance photodetectors with low power consumption. [ABSTRACT FROM AUTHOR]

Recently, WS2 has been considered a promising photodetector material due to its benefits of atomic thickness, easily customized single-crystal van der Waals heterostructure, and efficient photon absorption. However, the photosensitivity obtained from the WS2 photodetector is still unsatisfactory. Therefore, this article proposes a WS2-based high sensitivity field-effect transistor (FET) based on avalanche multiplication. Under 532-nm illumination, WS2 FET has ultralow dark current ($10^{-{12}}$ A), high responsivity (74 A/W), and high detectivity ($1.45\times 10^{{13}}$ Jones). In addition, the avalanche breakdown voltage of the device is reduced by employing graphene as an electrode. Because the height of the Schottky barrier can be reduced by replacing the Au electrode with a graphene electrode, the lower Schottky barrier can result in higher electric field intensity in the WS2 channel. Meanwhile, the unique transfer and output characteristics are observed in WS2 FET with graphene electrode. The inherent mechanisms are discussed, considering the Fermi-level tenability of graphene flakes and ambipolar transport properties of WS2. These results open a promising door to realize the WS2 FET with low power consumption and ultrahigh photoresponsivity, which will be ideal for high-performance photodetectors. [ABSTRACT FROM AUTHOR]