A transistor structure includes a source region and a drain region disposed in a substrate, extending along a first direction. A polysilicon layer is disposed over the substrate, extending along a second direction perpendicular to the first direction, wherein the polysilicon layer includes a first edge region, a channel region and a second edge region formed as a gate region between the source region and the drain region. The polysilicon layer has at least a first opening pattern at the first edge region having a first portion overlapping the gate region; and at least a second opening pattern at the second edge region having a second portion overlapping the gate region.