# Recent Advances in Photodetectors Based on Two-Dimensional Material/Si Heterojunctions
## 2 Figures of Merit for Photodetectors
### Quantum Efficiency and Gain
Figure 1a illustrates that a portion of the photogenerated holes are trapped in defects, requiring electrons to be injected from the source electrode to maintain charge balance when photogenerated electrons are collected at the drain electrode. Therefore, more than one electron is collected for each photon, resulting in a gain. However, these gain mechanisms do not occur in a photodiode detector. Figure 1b shows that the photogenerated carriers can be separated and collected by electrodes due to the built-in electrical field. Although electrons have higher mobility than holes, electrons cannot be injected from a p-type semiconductor. This is because electrons are minority carriers in a p-type semiconductor, and the contact between the p-type semiconductor and the electrode allows for Ohmic contact for holes but blocking contact for electrons. Therefore, no gain is possible, even under reverse bias voltage, as shown in Figure 1c. The reverse bias voltage only enlarges the depletion region and the electrical field within it, increasing the light absorption volume and the carrier drift velocity without carrier multiplication. The traps in the depletion region cannot induce gain due to the blocking contact. Additionally, the traps can lead to the recombination of photogenerated electrons and holes, as shown in Figure 1d, resulting in a reduced photoresponse. The presence of charge traps at the interface is responsible for the observation of a link between zero and open-circuit voltage in the current-voltage curve under light illumination, and reduced short-circuit current and reduced open-circuit voltage [55]. In summary, the built-in electrical field, the enhanced built-in electrical field under reverse bias and the presence of charge traps in the depletion region of photodiodes cannot lead to carrier multiplication. Consequently, the responsivity of photodiodes should be less than or equal to the value calculated by Equation (6). To induce gain in photodiodes, alternative carrier multiplication mechanisms are required. One such mechanism is avalanche breakdown, which induces impact ionization, resulting in carrier multiplication and gain in the detector. This phenomenon is widely used in avalanche photodiodes [56]. The induction of avalanche breakdown requires a large reverse bias. Recently, it has been reported that photogenerated hot carriers with sufficient energy can also lead to carrier multiplication, resulting in an EQE greater than 100% [57]. It is important to note that the photo energy must significantly exceed the band gap energy to induce impact ionization. The emergence of 2D/Si heterojunction photodetectors brings about unique properties, including gain. The gain in graphene/Si heterojunctions is well-explained and will be discussed later. However, the gain mechanism remains unclear for semiconducting 2D material/Si heterojunctions. For instance, zero bias semiconducting 2D material/Si heterojunction photodetectors have exhibited remarkably high responsivity, far surpassing the value calculated by Equation (6) [58; 59; 60; 61]. Additionally, reverse biased semiconducting 2D material/Si heterojunctions without avalanche breakdown have also shown gain [62; 63; 64; 65; 66]. Therefore, further investigation into the gain mechanism in these heterojunctions is necessary.