Low-voltage and high-gain WSe₂ avalanche phototransistor with an out-of-plane WSe₂/WS₂ heterojunction

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.