The interface between Si and graphene contains a large number of surface states pinning the surface Fermi level, leading to a strong leakage current noise. However, the low optical absorption of graphene (~ 2.3% for a single layer of graphene) is responsible for low responsivity. Graphene/Si heterojunction has high photoelectric conversion efficiency and thus has great potential to obtain high-performance photodetectors. Graphene-based photodetectors have been extensively studied in the past few years. So, graphene-based multifunctional photodetectors and terahertz modulators can overcome the above issue due to its miniaturization of electronic communication systems. Recently, graphene has been employed in the field of photodetectors and terahertz modulators due to its special energy band structure and ultra-high carrier mobility. Therefore, the development of a high-speed modulator that works at room temperature, small in size, and easy to process, is of great significance for the development and practicality of terahertz technology. Terahertz (THz) broadband modulator is a key device to realize the practical application of terahertz wave in communication, imaging, sensing and other application areas. Graphic AbstractĪn optoelectronic device such as a photodetector converts the light signal into an electrical signal and is widely used in optical communications, thermal imaging, environmental monitoring, biomedical imaging, etc. A similar of an electronic semiconductor diode is observed that only allows the passage of terahertz wave for negative bias and blocks the positive ones. However, almost no change is observed in the transmission of terahertz wave when a positive bias is applied. Besides, the addition of TiO 2 layer in this terahertz modulator continuously widens the carrier depletion region under negative bias, thereby realizing modulation of the terahertz wave, making the modulation depth up to 23% under − 15 V bias. So this ensures a higher optical gain of the device in the form of up to 3.6 A/W responsivities and 4 × 10 13 Jones detectability under 750 nm laser irradiation. It is found that photodetector in the “back-to-back” p-n-p energy band structure has a seriously unbalanced distribution of photogenerated carriers in the vertical direction when light is irradiated from the graphene side. In this paper, we have reported a multifunctional device from graphene/TiO 2/p-Si heterojunction, followed by its systematical analysis of optical response in a device under ultraviolet–visible-infrared band and transmission changes of terahertz waves in the 0.3–1.0 THz band under different bias voltages.