Structural Analysis in Two-Dimensional Heterojunction Photodiodes

Two-dimensional (2D) materials have attracted significant attention for van der Waals photodiodes because their electronic properties range from insulating and semimetallic to semiconducting behavior.In particular, transition metal dichalcogenides (TMDs) provide tunable band structures, strong light-matter interaction, and atomically thin layered geometries suitable for heterojunction engineering without strict lattice-matching constraints.This article reviews the material and structural factors governing fast photoresponse in 2D photodiodes.Representative 2D materials, including graphene, black phosphorus, hexagonal boron nitride, and TMDs, are discussed according to their functional roles in van der Waals optoelectronic devices.The discussion then focuses on the evolution of device architectures from lateral to semi-vertical and full-vertical geometries.Using simplified resistance and response-time models, this progression is interpreted as a structural strategy for alleviating the carrier-transit bottleneck associated with extended in-plane transport.Increasing verticalization shortens the carrier extraction distance, reduces diffusion-related delay, and improves drift-assisted collection.At the same time, practical performance remains influenced by capacitance, interfacial resistance, and contact selectivity.These results highlight the importance of jointly optimizing material properties and carrier-extraction geometry for high-speed 2D photodiodes.