Image sensor having vertical, transfer, reset, source follower, and select transistors vertically aligned over the photodiode

An image sensor is disclosed. The image sensor includes a semiconductor substrate, a plurality of pillars protruding from the semiconductor substrate, and spaced from each other, a spacer layer on the semiconductor substrate and a sidewall of each of the plurality of pillars, a plurality of gate structures on the spacer layer, and a plurality of unit pixels arranged in a matrix form. The first unit pixel includes a first photodiode (PD) formed in the semiconductor substrate, a first pillar, a second pillar and a third pillar of the plurality of pillars, and a first gate structure and a second gate structure of the plurality of gate structures. Each of the first pillar and the second pillar includes a first channel region and a first drain region on the first channel region. The third pillar is not surround by any gate structure of the plurality of gate structures.