We　report　high-speed　large-area　silicon　photodetectors　(PDs)　that　were　fabricated　on　silicon-on-insulator　(SOI)　substrates.　These　coplanar　interdigital　PDs　were　fabricated　with　finger　spacing　in　the　0.5-　5\mu　\text{m}　range.　One　of　the　detectors,　which　had　an　area　of　2300\mu　\text{m}＜^＞{{2}}　,　achieved　a　bandwidth　as　high　as　14.1　GHz　under　a　bias　voltage　of　-10　V　at　a　wavelength　of　850　nm.　Meanwhile,　the　device　with　the　maximum　area　of　4300\mu　\text{m}＜^＞{{2}}　also　achieved　a　7.9-GHz　bandwidth,　suggesting　our　detectors　are　highly　suitable　for　high-speed　850-nm　optical　receiver　and,　especially,　for　large　area　receiving　applications.　We　established　a　new　transition　time　and　resistor-capacitor　(RC)　equivalent　circuit　model　to　analyze　and　calculate　the　3-dB　frequency　of　the　coplanar　interdigital　PD;　the　model　results　were　well-matched　with　the　measurement　results.