Vertical integration: a key concept for future flexible and printed electronics

This review aims at introducing a vertical integration approach as a promising new driver of field-effect transistor circuits and systems, which can overcome limitations of flexible and printed electronics. A large portion of this article is dedicated to systematic categorization and illustration of important and recent demonstrations of vertically integrated circuit building blocks incorporating organic materials, metal-oxide semiconductors, nanotubes, and nanosheets. Key features of each of these materials platforms and their fabrication issues are also broadly discussed. Then, common technical requirements for high-performance devices and circuits are critically evaluated in view of the specific geometrical and theoretical aspects of vertically connected complementary digital logic inverters. Prominent technological opportunities and challenges for the vertical and three-dimensional transistor integration are finally addressed to further motivate active multidisciplinary research on related materials, devices, and systems.