The present application provides an electrostatic discharge guard structure for photonic platform based photodiode systems. In particular this application provides a photodiode assembly comprising: a photodiode (such as a Si or SiGe photodiode); a waveguide (such as a silicon waveguide); and a guard structure, wherein the guard structure comprises a diode, extends about all or substantially all of the periphery of the Si or SiGe photodiode and allows propagation of light from the silicon waveguide into the Si or SiGe photodiode.