Integrated circuit devices including vertically stacked field effect transistors

Abstract

An integrated circuit device includes: an active region extending in a first horizontal direction on a substrate; a first transistor at a first vertical level on the active region, the first transistor including a first source/drain region having a first conductive type; and a second transistor at a second vertical level that is higher than the first vertical level on the active region, the second transistor including a second source/drain region having a second conductive type and overlapping the first source/drain region in a vertical direction, wherein the first source/drain region and the second source/drain region have different sizes.