A physically unclonable function (PUF) circuit includes a program control transistor, a program select transistor, a read select transistor, and a PUF bit storage transistor. The PUF bit storage transistor has a drain region coupled to the read select transistor, a source region coupled to a source line and the program select transistor, a channel region, a gate dielectric layer, and a gate electrode coupled to the program select transistor. The gate dielectric layer has a first portion formed on the drain region, a second portion formed on the source region, and a main portion formed on the channel region and between the first portion and the second portion, thicknesses of the first portion of the gate dielectric layer and the second portion of the gate dielectric layer being smaller than a thickness of the main portion of the gate dielectric layer.