Provided is an image sensor having a heterojunction bipolar transistor (HBT) and a method of fabricating the same. The image sensor is fabricated by SiGe BiCMOS technology. In the image sensor, a PD employs a floating-base-type SiGe HBT. A floating base of the SiGe HBT produces a positive voltage with respect to a collector during an exposure process, and the HBT performs a reverse bipolar operation due to the positive voltage so that the collector and an emitter exchange functions. The SiGe HBT can sense an optical current signal and also amplify the optical current signal. The image sensor requires only three transistors in a pixel so that the degree of integration can increase. The image sensor has an improved sensitivity of signals in the short wavelength region and a sensing signal has excellent linearity such that both a sensing mechanism and control circuit are very simple.