In　this　paper,　the　frequency　responses　of　InP/InGaAs　vertical-incidence　p-i-n　photodiodes　with　various　optical　apertures　of　50-80　mu　m　are　investigated.　The　same　resonant　frequency　(fR)　of　approximately　1.02　GHz　was　observed　for　all　device　apertures.　There　are　two　conditions　that　can　lead　to　the　resonance　of　the　frequency　response.　Therefore,　an　equivalent　circuit　model　and　a　carrier　transport　model　were　used　to　fit　and　analyze　the　resonance　curve.　We　then　found　that　the　resonant　circuit　can　produce　frequency　response　oscillations,　but　the　peak　position　occurs　at　a　higher　frequency　and　the　radio-frequency　response　drops　more　rapidly,　which　does　not　match　the　measured　curve.　Therefore,　we　believe　that　carrier　transport　delay　is　the　main　cause　of　the　resonance.　Based　on　the　energy　bands　of　our　detectors　and　comparison　with　a　reference　device,　it　was　found　that　when　the　blocked　holes　at　the　InGaAsP/InGaAs　heterojunction　interface　tunnel　through　the　potential　barrier,　a　resonant　frequency　could　be　induced.　Using　our　material　structure　and　tunneling　model,　the　tunneling　frequency　of　the　holes　was　calculated　to　be　0.99　GHz,　which　matched　the　measured　resonant　frequency　value.　Our　resonant　models　provided　two　methods　to　improve　the　bandwidth　for　various　photodetectors　without　any　other　change　on　the　performance　characteristics.