11–13 For instance, n-type O-defective TiO 2 quantum dot-decorated p-type Ti-defective TiO 2 exhibits excellent performance in photoelectrochemical and photocatalytic hydrogen generation, holding great promise toward the application of p–n homojunctions in highly-efficient photocathodes for hydrogen generation. p–n homojunctions normally possess superior electronic and optoelectronic properties, because the excellent crystallographic alignment can eliminate the lattice mismatch and thus endow the material with perfect band alignment. 9,10 In comparison, p–n homojunction has been intensively explored as a new emerging configuration, which is based on a single semiconductor. 7,8 However, the large interfacial-trap density and uneven interfacial roughness degrades its device performance under optimal working condition. 4–6 A p–n heterojunction is formed by joining two different semiconductors, which can take advantage of the large variety of electronic properties available among different semiconductors to efficiently separate charge. 1–3 Up to now, p–n heterojunction and homojunction architectures have been widely applied in modern electronics. Introduction Semiconductor p–n junctions are the fundamental building blocks for electronic and optoelectronic devices. The present study gains some key insights into the importance of surface functionalization in constructing p–n homojunction electronic and optoelectronic devices based on monolayer SiGe. It is predicted that the built F4TCNQ-4K/SiGe FET can meet the requirements for high-performance (the high current density) and low-power (low subthreshold swing (SS)) applications, according to the International Technology Roadmap for Semiconductors in 2028. By spatially adsorbing F4TCNQ molecule and K atoms on the source and drain leads, respectively, we designed a p–n homojunction SiGe field-effect transistor (FET). Both of p-type 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-adsorbed and n-type 4 K-adsorbed SiGe systems show enhanced optical absorption in the infrared region, indicating their promising applications in infrared optoelectronic devices. It is found that the n-type doping effect of K atoms is stronger than that of benzyl viologen (BV) molecule on the surface of SiGe monolayer, owing to the strong covalent interaction. We tune the electronic properties of monolayer siligene (SiGe) into p/n-type via the covalent functionalization of electrophilic/nucleophilic dopants, using ab initio quantum transport calculations. P–n homojunctions are superior to p–n heterojunctions in constructing nanoscale functional devices, owing to the excellent crystallographic alignment.