We investigate the photocurrent generation by two-photon absorption effect in silicon microdisk resonators integrated with p-n diodes. The photocurrent is quite dependent on the p-n junction position with respect to the whispering gallery mode. Avalanche gain increases significantly when the bias exceeds -19 V, leading to considerable enhancement of photocurrent. At -22 V bias with on-chip optical power of 44.7 μW, the responsivity exceeds 1 A/W with an avalanche gain of 188 while the dark current is more than 50 times lower than the photocurrent.