A semiconductor device includes a transistor. The transistor includes a gate electrode, a channel layer, a gate dielectric layer, a first source/drain region and a second source/drain region and a first spacer. The channel layer is disposed on the gate electrode. The gate dielectric layer is located between the channel layer and the gate electrode. The first source/drain region and the second source/drain region are disposed on the channel layer at opposite sides of the gate electrode, and at least one of the first and second source/drain regions includes a first portion and a second portion between the first portion and the gate electrode. The first spacer is disposed on the channel layer. The first spacer is disposed on a first sidewall of the second portion of the at least one of the first and second source/drain regions, and the first portion is disposed on the first spacer.