A novel lateral Ge $\mathrm{/}$Si avalanche photodiode without a charge region is investigated herein using device physical simulation. High field is provided by the band-gap barrier and build-in field at the Ge $\mathrm{/}$Si interface in the vertical direction. Modulating the Si mesa thickness ( $\mathrm{0}\mathrm{-}\mathrm{0.4}\mathsf{}\mu \mathsf{}\mathrm{\text{m}}$) and impurity concentration of the intrinsic Si substrate ( $\mathrm{1}\mathrm{\times }{\mathrm{10}}^{\mathrm{16}}\mathrm{-}\mathrm{4}\mathrm{\times }{\mathrm{10}}^{\mathrm{16}}\mathsf{}{\mathrm{\text{cm}}}^{\mathrm{-}\mathrm{3}}$) strengthens the electric field confinement in the substrate region and provides a high avalanche multiplication in the Si region. When the Si mesa thickness is $\mathrm{0.3}\mathsf{}\mu \mathsf{}\mathrm{\text{m}}$, and the impurity concentration of the Si substrate is $\mathrm{2}\mathrm{\times }{\mathrm{10}}^{\mathrm{16}}\mathsf{}{\mathrm{\text{cm}}}^{\mathrm{-}\mathrm{3}}$, the Lateral Avalanche PhotoDiode (LAPD) exhibits a peak gain of $\mathrm{246}$ under $\mathrm{1.55}\mathsf{}\mu \mathsf{}\mathrm{\text{m}}$ incident light power of $\mathrm{-}\mathrm{22.2}\mathsf{}\mathrm{\text{dBm}}$, which increases with decreasing light intensity.