1. An N-type organic thin film transistor, which comprises: a substrate; a gate electrode locating on the substrate; a gate-insulating layer covering the gate electrode, and the gate-insulating layer is made of silk protein; a buffering layer locating on the gate-insulating layer, and the buffering layer is made of pentacene; an N-type organic semiconductor layer locating on the buffering layer; and a source electrode and a drain electrode, wherein the N-type organic semiconductor layer, the buffering layer, the source electrode and the drain electrode are disposed over the gate dielectric layer.