A semiconductor device includes: a 1st source/drain region; a 2nd source/drain region; a channel structure connecting the 1st source/drain region to the 2nd source/drain region; a gate structure configured to control the channel structure; a backside source/drain contact structure connected to a bottom surface of the 1st source/drain region; a backside isolation structure at a lower portion of the semiconductor device; and a 1st contact spacer on the backside source/drain contact structure, wherein the 1st contact spacer is configured to isolate the backside source/drain contact structure from another circuit element in the backside isolation structure.