[SOLVED] why in APD PIN most multiplication happens in intrinsic region????

[stackprogramer]

hi,my question is about APD detector:
why in APD PIN most multiplication happens in intrinsic region????and a little in n or p region?
a reason that i think is big resistance in intrinsic region

 

[stackprogramer]

finally i conclude that when multiplication is happened that electric field is high,and in this region is true this point.