ABSTRACT 2D transition metal dichalcogenides (TMDCs) are promising candidates for next‐generation nanoelectronics and optoelectronics. Yet, controlling layer number, stacking angle, and interfacial quality in van der Waals (vdW) heterostructures remains challenging, often limiting device performance and reproducibility. Homojunctions formed within a single 2D material can circumvent these issues, but their reliable fabrication and systematic exploration of exotic quantum phenomena remain elusive. Here, we report the fabrication and characterization of a thickness‐modulated lateral p–n homojunction from a single flake of Nb‐doped MoS 2 . This configuration suppresses interface traps without external interface engineering, enabling excellent and highly stable device performance. The diodes exhibit strong rectifying behavior with a rectification ratio of ≈10⁴ and a remarkably low ideality factor ( η = 1.23). Notably, we observe field‐dependent negative differential resistance (NDR) at low temperatures, offering unique prospects for unconventional electronic applications. The devices also achieve high photoresponsivity (1.09 × 10 3 A W −1 ), external quantum efficiency (2.16 × 10⁵%), and detectivity (7.5 × 10 10 Jones). Furthermore, electrical breakdown studies reveal avalanche multiplication at relatively low voltages, enabling high‐performance avalanche photodetectors. Overall, our results demonstrate a simple yet robust approach for probing carrier multiplication in 2D homojunction p–n diodes, underlining the broad potential of TMDCs in advanced optoelectronic and quantum device applications.