In this paper, we establish the structure model of multilayer Graphene (MLG)/Mg ₂ Si/Si heterojunction photodetector (PD) based on the Technology CAD (TCAD). We further investigate the effect of Graphene layers on the photoelectric properties of MLG/Mg ₂ Si/Si heterojunction PD. The simulation results indicate that compared with Mg ₂ Si/Si PD, the photoelectric properties of MLG/Mg ₂ Si/Si heterojunction PD are enhanced. The responsivity is increased from 0.51 A/W to 0.84 A/W, the external quantum efficiency (EQE) increased from 70.68% to 107.29%, and the noise equivalent power (NEP) reduced from 7.07 × 10 ^–11 WHz ^–1/2 to 1.59 × 10 ^–11 WHz ^–1/2 . It is further shown that detectivity (D ^* ) is increased from 1.4 × 10 ¹⁰ Jones to 6.3 × 10 ¹⁰ Jones, and the on/off ratio is increased from 141.45 to 629. After adding Graphene layers, the maximum improvement rates of the responsivity, EQE, NEP, D ^* , and on/off ratio of heterojunction PDs improved by 65%, 52%, 77.5%, 350%, and 345%, respectively. Moreover, it is seen that the energy band of MLG/Mg ₂ Si/Si heterojunction PDs is shifted to the valence band, the reverse breakdown voltage and dark current are decreased. Our results indicate that the photocurrent in the spectral response is increased and the wavelength corresponding to the EQE peak is shifted towards red. A strong electric field is formed at the interface between MLG and Mg ₂ Si, which is beneficial to the separation of photogenic carriers. The simulation results indicate that the MLG/Mg ₂ Si/Si heterojunction PDs can detect visible light and NIR with a wavelength in the range of 400–1500 nm.

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